On Oracle

Tuesday, February 20, 2018

SYS.LOADER_DIR_OBJS Query: Privileges and Performance

Oracle Query On External Table and Data Pump hit performance problem when missing "Create/Drop Any Directory" Privileges. The triggered slow query is on SYS.LOADER_DIR_OBJS located in an Oracle internal package.

Oracle MOS has one matching Documentation:
Slow Query On External Table If User Executing the Select Does Not Have "Create/Drop Any Directory" Privileges (Doc ID 2034938.1)

  Symptoms
    . Query against external tables perform slowly
    . User executing the select does not have "create any directory" or "drop any directory" privileges
    . The slowest query from the 10046 trace is the following recursive SQL:
        SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS;
      
  Solution
    1. The solution is to gather statistics for the KZSROROL column as follows:
         exec dbms_stats.gather_table_stats(null,'X$KZSRO',method_opt=>'for columns KZSROROL',no_invalidate => false);
    2. Afterwards to make sure that these statistics will not get deleted by future runs of 
       DBMS_STATS.GATHER_FIXED_OBJECTS_STATS command, lock the statistics:
         exec dbms_stats.lock_table_stats('SYS','X$KZSRO');

This performance problem is worth of studying because it is directly related to System Privileges.

We will set up one System privilege and one Object privilege. Toggling both privileges, make 5 different tests:

  Test-1: Object privilege: YES; System privilege: YES
  Test-2: Object privilege: YES; System privilege: NO
  Test-3: Object privilege: NO;  System privilege: YES
  Test-4: Object privilege: NO;  System privilege: NO
  Test-5: Object privilege: NO;  System privilege: NO, and SQL Patch

The first two Tests are querying one external table; the next three are querying SYS.LOADER_DIR_OBJS. Test-5 is same as Test-4, but with SQL Patch applied.

Oracle 12.1 introduced SQL Patch (unofficial, undocumented) to tune packaged SQL Statements. Although it is possible to manipulate xplan with SQL Patch, but it cannot detour Oracle security to influence query result.

There is another related MOS Docu: Diagnosing a High Number of Executions Against Table SYS.DIR$ (Doc ID 2230171.1)

Note: All tests are done in Oracle 12.1.0.2 on Solaris.

1. Test Setup


sqlplus admin/admin123@TESTDB

drop user ksun cascade;

create user ksun identified by test123;

grant select any table, create any table, drop any table, create session, execute any procedure to ksun;

grant create any directory to ksun;
--revoke create any directory from ksun; 

create or replace directory test_dir as '/oradata/app/oracle/admin/testdb/io';

grant read, write on directory test_dir to ksun;
--revoke read, write on directory test_dir from ksun; 

exec dbms_stats.unlock_table_stats('SYS','X$KZSRO');

exec dbms_stats.gather_table_stats('SYS','X$KZSRO',method_opt=>'for columns KZSROROL',no_invalidate => false);

exec dbms_stats.lock_table_stats('SYS','X$KZSRO');

drop table testt1_ext;

create table testt1_ext
  organization external
    (type oracle_datapump 
     default directory test_dir 
     location ('testt1_ext.txt'))
  as select 321 x from dual;

2. Test-1: Object privilege: YES; System privilege: YES


sqlplus admin/admin123@TESTDB

grant read, write on directory test_dir to ksun;
grant create any directory to ksun;

select * from sys.dba_sys_privs where grantee = 'KSUN';

  GRAN PRIVILEGE              ADMIN_OPTION COMMON
  ---- ---------------------- ------------ ------
  KSUN CREATE TABLE           NO           NO
  KSUN EXECUTE ANY PROCEDURE  NO           NO
  KSUN DROP ANY TABLE         NO           NO
  KSUN CREATE ANY TABLE       NO           NO
  KSUN CREATE ANY DIRECTORY   NO           NO
  KSUN SELECT ANY TABLE       NO           NO
  KSUN CREATE SESSION         NO           NO

select grantee, table_name, privilege, type from sys.dba_tab_privs where grantee = 'KSUN';

  GRANTEE TABLE_NAME PRIVILEGE TYPE
  ------- ---------- --------- ---------
  KSUN    TEST_DIR   READ      DIRECTORY
  KSUN    TEST_DIR   WRITE     DIRECTORY

The query takes less than 1 second.


sqlplus ksun/test123@TESTDB

select sql_id, executions, rows_processed, buffer_gets, elapsed_time, cpu_time, last_load_time   --,v.* 
 from v$sql v where sql_id = 'f5ra7dru5fk5n';
 
  SQL_ID        EXECUTIONS ROWS_PROCESSED BUFFER_GETS ELAPSED_TIME   CPU_TIME LAST_LOAD_TIME
  ------------- ---------- -------------- ----------- ------------ ---------- -------------------
  f5ra7dru5fk5n          5           5660       32729       198104     198283 2018-02-19/08:38:40 

select * from testt1_ext;
  
           X
  ----------
         321
  
1 row selected.
Elapsed: 00:00:00.03

select sql_id, executions, rows_processed, buffer_gets, elapsed_time, cpu_time, last_load_time   --,v.* 
 from v$sql v where sql_id = 'f5ra7dru5fk5n';
 
   SQL_ID        EXECUTIONS ROWS_PROCESSED BUFFER_GETS ELAPSED_TIME   CPU_TIME LAST_LOAD_TIME
  ------------- ---------- -------------- ----------- ------------ ---------- -------------------
  f5ra7dru5fk5n          6           6792       37343       217328     217563 2018-02-19/08:38:40

(delta BUFFER_GETS=37343-32729=4,614, ELAPSED_TIME=217328-198104=19,224)

alter session set tracefile_identifier = 'ksun_trc_test1';
exec dbms_monitor.session_trace_enable(waits=>true, binds=>false, plan_stat=>'all_executions');
select * from testt1_ext;
exec dbms_monitor.session_trace_disable;

********************************************************************************

SQL ID: f5ra7dru5fk5n Plan Hash: 1161458834

SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch     1133      0.05       0.05          0       3474          0        1132
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total     1135      0.05       0.05          0       3474          0        1132

Rows (avg) Row Source Operation
---------- ---------------------------------------------------
      1132 VIEW  LOADER_DIR_OBJS (cr=4614 pr=0 pw=0 time=53950 us cost=1189 size=2381728 card=1132)
      1132  UNION-ALL  (cr=4614 pr=0 pw=0 time=53093 us)
      1132   NESTED LOOPS  (cr=4614 pr=0 pw=0 time=51662 us cost=1131 size=99528 card=1131)
      1132    NESTED LOOPS  (cr=3482 pr=0 pw=0 time=48085 us cost=1131 size=99528 card=1131)
      1132     FIXED TABLE FULL X$DIR (cr=1140 pr=0 pw=0 time=39002 us cost=0 size=65598 card=1131)
      1132     INDEX RANGE SCAN I_OBJ1 (cr=2342 pr=0 pw=0 time=3597 us cost=1 size=0 card=1)(object id 1591123)
      1132    TABLE ACCESS BY INDEX ROWID OBJ$ (cr=1132 pr=0 pw=0 time=2996 us cost=1 size=30 card=1)
         1     FIXED TABLE FULL X$KZSPR (cr=0 pr=0 pw=0 time=29 us cost=0 size=7 card=1)
         0   HASH GROUP BY (cr=0 pr=0 pw=0 time=411 us cost=58 size=105 card=1)
         0    FILTER  (cr=0 pr=0 pw=0 time=29 us)
         0     NESTED LOOPS  (cr=0 pr=0 pw=0 time=0 us cost=57 size=105 card=1)
         0      NESTED LOOPS  (cr=0 pr=0 pw=0 time=0 us cost=57 size=47 card=1)
         0       HASH JOIN RIGHT SEMI (cr=0 pr=0 pw=0 time=0 us cost=56 size=17 card=1)
         0        VIEW  VW_NSO_1 (cr=0 pr=0 pw=0 time=0 us cost=0 size=60 card=15)
         0         FIXED TABLE FULL X$KZSRO (cr=0 pr=0 pw=0 time=0 us cost=0 size=60 card=15)
         0        TABLE ACCESS FULL OBJAUTH$ (cr=0 pr=0 pw=0 time=0 us cost=56 size=120471 card=9267)
         0       TABLE ACCESS BY INDEX ROWID BATCHED OBJ$ (cr=0 pr=0 pw=0 time=0 us cost=1 size=30 card=1)
         0        INDEX RANGE SCAN I_OBJ1 (cr=0 pr=0 pw=0 time=0 us cost=1 size=0 card=1)(object id 1591123)
         0      FIXED TABLE FIXED INDEX X$DIR (ind:1) (cr=0 pr=0 pw=0 time=0 us cost=0 size=58 card=1)
         1     FIXED TABLE FULL X$KZSPR (cr=0 pr=0 pw=0 time=27 us cost=0 size=7 card=1)

********************************************************************************

3. Test-2: Object privilege: YES; System privilege: NO


sqlplus admin/admin123@TESTDB

grant read, write on directory test_dir to ksun;
revoke create any directory from ksun;

The query takes about 17 seconds because of subquery TABLE FULL X$KZSRO.


sqlplus ksun/test123@TESTDB

select sql_id, executions, rows_processed, buffer_gets, elapsed_time, cpu_time, last_load_time   --,v.* 
 from v$sql v where sql_id = 'f5ra7dru5fk5n';
 
   SQL_ID        EXECUTIONS ROWS_PROCESSED BUFFER_GETS ELAPSED_TIME   CPU_TIME LAST_LOAD_TIME
  ------------- ---------- -------------- ----------- ------------ ---------- -------------------
  f5ra7dru5fk5n          6           6792       37343       217328     217563 2018-02-19/08:38:40

select * from testt1_ext;

           X
  ----------
         321

1 row selected.
Elapsed: 00:00:17.93

select sql_id, executions, rows_processed, buffer_gets, elapsed_time, cpu_time, last_load_time   --,v.* 
 from v$sql v where sql_id = 'f5ra7dru5fk5n';

  SQL_ID        EXECUTIONS ROWS_PROCESSED BUFFER_GETS ELAPSED_TIME   CPU_TIME LAST_LOAD_TIME
  ------------- ---------- -------------- ----------- ------------ ---------- -------------------
  f5ra7dru5fk5n          7           6793      109986     18108955   18104124 2018-02-19/08:38:40

(delta BUFFER_GETS=109986-37343=72,643, ELAPSED_TIME=18108955-217328=17,891,627)

alter session set max_dump_file_size = UNLIMITED;
alter session set tracefile_identifier = 'ksun_trc_test2';
exec dbms_monitor.session_trace_enable(waits=>true, binds=>false, plan_stat=>'all_executions');
select * from testt1_ext;
alter session set session_cached_cursors = 0;
exec dbms_monitor.session_trace_disable;

alter session set session_cached_cursors = 200;

********************************************************************************

SQL ID: f5ra7dru5fk5n Plan Hash: 1161458834

SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        2      0.03       0.03          0        235          0           0
Execute      2      0.00       0.00          0          0          0           0
Fetch        4     38.11      38.10          0      13750          0           2
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        8     38.15      38.13          0      13985          0           2

Rows (avg) Row Source Operation
---------- ---------------------------------------------------
         1 VIEW  LOADER_DIR_OBJS (cr=77167 pr=0 pw=0 time=19800468 us cost=1189 size=2381728 card=1132)
         1  UNION-ALL  (cr=77167 pr=0 pw=0 time=19800465 us)
         0   NESTED LOOPS  (cr=5776 pr=0 pw=0 time=124426 us cost=1131 size=99528 card=1131)
      1132    NESTED LOOPS  (cr=5747 pr=0 pw=0 time=125176 us cost=1131 size=99528 card=1131)
      1132     FIXED TABLE FULL X$DIR (cr=5664 pr=0 pw=0 time=120445 us cost=0 size=65598 card=1131)
      1132     INDEX RANGE SCAN I_OBJ1 (cr=83 pr=0 pw=0 time=1644 us cost=1 size=0 card=1)(object id 1591123)
         0    TABLE ACCESS BY INDEX ROWID OBJ$ (cr=29 pr=0 pw=0 time=964 us cost=1 size=30 card=1)
         0     FIXED TABLE FULL X$KZSPR (cr=0 pr=0 pw=0 time=30 us cost=0 size=7 card=1)
         1   HASH GROUP BY (cr=71391 pr=0 pw=0 time=19676026 us cost=58 size=105 card=1)
         2    FILTER  (cr=71391 pr=0 pw=0 time=19675276 us)
         2     NESTED LOOPS  (cr=71391 pr=0 pw=0 time=19675254 us cost=57 size=105 card=1)
     32313      NESTED LOOPS  (cr=6763 pr=0 pw=0 time=363426 us cost=57 size=47 card=1)
     32313       HASH JOIN RIGHT SEMI (cr=196 pr=0 pw=0 time=65194 us cost=56 size=17 card=1)
         2        VIEW  VW_NSO_1 (cr=0 pr=0 pw=0 time=10 us cost=0 size=60 card=15)
         2         FIXED TABLE FULL X$KZSRO (cr=0 pr=0 pw=0 time=6 us cost=0 size=60 card=15)
     43515        TABLE ACCESS FULL OBJAUTH$ (cr=196 pr=0 pw=0 time=45666 us cost=56 size=120471 card=9267)
     32313       TABLE ACCESS BY INDEX ROWID BATCHED OBJ$ (cr=6567 pr=0 pw=0 time=227682 us cost=1 size=30 card=1)
     32313        INDEX RANGE SCAN I_OBJ1 (cr=4851 pr=0 pw=0 time=118484 us cost=1 size=0 card=1)(object id 1591123)
         2      FIXED TABLE FIXED INDEX X$DIR (ind:1) (cr=64628 pr=0 pw=0 time=19318193 us cost=0 size=58 card=1)
         0     FIXED TABLE FULL X$KZSPR (cr=0 pr=0 pw=0 time=16 us cost=0 size=7 card=1)

********************************************************************************

4. Test-3: Object privilege: NO; System privilege: YES


sqlplus admin/admin123@TESTDB

revoke read, write on directory test_dir from ksun; 
grant create any directory to ksun;

Make the test:


sqlplus ksun/test123@TESTDB

SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS

  NAME       PATH                                READ   WRITE  EXECUTE
  ---------- ----------------------------------- ------ ------ -------
  TEST_DIR   /oradata/app/oracle/admin/testdb/io TRUE   TRUE   TRUE

1 row selected.
Elapsed: 00:00:00.04

The select returns 1 row, and takes less than 1 second.

Here the xplan:


sqlplus admin/admin123@TESTDB

select * from table(dbms_xplan.display_cursor('f5ra7dru5fk5n', null, 'ADVANCED ALLSTATS LAST'));

SQL_ID  f5ra7dru5fk5n, child number 0
-------------------------------------
SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS
 
Plan hash value: 1161458834
 
--------------------------------------------------------------------------------------------------------------
| Id  | Operation                                 | Name            | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |
--------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                          |                 |        |       |  1189 (100)|          |
|   1 |  VIEW                                     | LOADER_DIR_OBJS |   1132 |  2325K|  1189   (1)| 00:00:01 |
|   2 |   UNION-ALL                               |                 |        |       |            |          |
|   3 |    NESTED LOOPS                           |                 |   1131 | 99528 |  1131   (0)| 00:00:01 |
|   4 |     NESTED LOOPS                          |                 |   1131 | 99528 |  1131   (0)| 00:00:01 |
|   5 |      FIXED TABLE FULL                     | X$DIR           |   1131 | 65598 |     0   (0)|          |
|*  6 |      INDEX RANGE SCAN                     | I_OBJ1          |      1 |       |     1   (0)| 00:00:01 |
|*  7 |     TABLE ACCESS BY INDEX ROWID           | OBJ$            |      1 |    30 |     1   (0)| 00:00:01 |
|*  8 |      FIXED TABLE FULL                     | X$KZSPR         |      1 |     7 |     0   (0)|          |
|   9 |    HASH GROUP BY                          |                 |      1 |   105 |    58   (2)| 00:00:01 |
|* 10 |     FILTER                                |                 |        |       |            |          |
|  11 |      NESTED LOOPS                         |                 |      1 |   105 |    57   (0)| 00:00:01 |
|  12 |       NESTED LOOPS                        |                 |      1 |    47 |    57   (0)| 00:00:01 |
|* 13 |        HASH JOIN RIGHT SEMI               |                 |      1 |    17 |    56   (0)| 00:00:01 |
|  14 |         VIEW                              | VW_NSO_1        |     15 |    60 |     0   (0)|          |
|  15 |          FIXED TABLE FULL                 | X$KZSRO         |     15 |    60 |     0   (0)|          |
|* 16 |         TABLE ACCESS FULL                 | OBJAUTH$        |   9267 |   117K|    56   (0)| 00:00:01 |
|  17 |        TABLE ACCESS BY INDEX ROWID BATCHED| OBJ$            |      1 |    30 |     1   (0)| 00:00:01 |
|* 18 |         INDEX RANGE SCAN                  | I_OBJ1          |      1 |       |     1   (0)| 00:00:01 |
|* 19 |       FIXED TABLE FIXED INDEX             | X$DIR (ind:1)   |      1 |    58 |     0   (0)|          |
|* 20 |      FIXED TABLE FULL                     | X$KZSPR         |      1 |     7 |     0   (0)|          |
--------------------------------------------------------------------------------------------------------------

5. Test-4: Object privilege: NO; System privilege: NO

Revoke both privileges:


sqlplus admin/admin123@TESTDB

revoke read, write on directory test_dir from ksun; 
revoke create any directory from ksun;

Make the test:


sqlplus ksun/test123@TESTDB

SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS

no rows selected
Elapsed: 00:00:12.49

no rows selected, and takes about 12 seconds.

Here the xplan (same as Test-3):


sqlplus admin/admin123@TESTDB

select * from table(dbms_xplan.display_cursor('f5ra7dru5fk5n', null, 'ADVANCED ALLSTATS LAST'));

SQL_ID  f5ra7dru5fk5n, child number 0
-------------------------------------
SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS
 
Plan hash value: 1161458834
 
-----------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                 | Name            | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |  OMem |  1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                          |                 |        |       |  1189 (100)|          |       |       |          |
|   1 |  VIEW                                     | LOADER_DIR_OBJS |   1132 |  2325K|  1189   (1)| 00:00:01 |       |       |          |
|   2 |   UNION-ALL                               |                 |        |       |            |          |       |       |          |
|   3 |    NESTED LOOPS                           |                 |   1131 | 99528 |  1131   (0)| 00:00:01 |       |       |          |
|   4 |     NESTED LOOPS                          |                 |   1131 | 99528 |  1131   (0)| 00:00:01 |       |       |          |
|   5 |      FIXED TABLE FULL                     | X$DIR           |   1131 | 65598 |     0   (0)|          |       |       |          |
|*  6 |      INDEX RANGE SCAN                     | I_OBJ1          |      1 |       |     1   (0)| 00:00:01 |       |       |          |
|*  7 |     TABLE ACCESS BY INDEX ROWID           | OBJ$            |      1 |    30 |     1   (0)| 00:00:01 |       |       |          |
|*  8 |      FIXED TABLE FULL                     | X$KZSPR         |      1 |     7 |     0   (0)|          |       |       |          |
|   9 |    HASH GROUP BY                          |                 |      1 |   105 |    58   (2)| 00:00:01 |   831K|   831K|          |
|* 10 |     FILTER                                |                 |        |       |            |          |       |       |          |
|  11 |      NESTED LOOPS                         |                 |      1 |   105 |    57   (0)| 00:00:01 |       |       |          |
|  12 |       NESTED LOOPS                        |                 |      1 |    47 |    57   (0)| 00:00:01 |       |       |          |
|* 13 |        HASH JOIN RIGHT SEMI               |                 |      1 |    17 |    56   (0)| 00:00:01 |  2293K|  2293K|  709K (0)|
|  14 |         VIEW                              | VW_NSO_1        |     15 |    60 |     0   (0)|          |       |       |          |
|  15 |          FIXED TABLE FULL                 | X$KZSRO         |     15 |    60 |     0   (0)|          |       |       |          |
|* 16 |         TABLE ACCESS FULL                 | OBJAUTH$        |   9267 |   117K|    56   (0)| 00:00:01 |       |       |          |
|  17 |        TABLE ACCESS BY INDEX ROWID BATCHED| OBJ$            |      1 |    30 |     1   (0)| 00:00:01 |       |       |          |
|* 18 |         INDEX RANGE SCAN                  | I_OBJ1          |      1 |       |     1   (0)| 00:00:01 |       |       |          |
|* 19 |       FIXED TABLE FIXED INDEX             | X$DIR (ind:1)   |      1 |    58 |     0   (0)|          |       |       |          |
|* 20 |      FIXED TABLE FULL                     | X$KZSPR         |      1 |     7 |     0   (0)|          |       |       |          |
-----------------------------------------------------------------------------------------------------------------------------------------

6. Test-5: Object privilege: NO; System privilege: NO, and SQL Patch

Since the recursive query is located in Oracle internal packaged application, SQL Patch is used to add hints.

One similar test was done by Blog: Slow external table access.

Revoke both privileges and create SQL Patch:


sqlplus admin/admin123@TESTDB

revoke read, write on directory test_dir from ksun; 
revoke create any directory from ksun;

begin
  sys.dbms_sqldiag_internal.i_create_patch(
    sql_text  => 'SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS'
   ,hint_text => 'CARDINALITY(@"SEL$2" "D"@"SEL$2", 2) CARDINALITY(@"SEL$2" "O"@"SEL$2", 3) '||
                 'CARDINALITY(@"SEL$0C7B6CF6" "D"@"SEL$6" 4) '||
                 'USE_HASH(@"SEL$2" "D"@"SEL$2" O@SEL$2) USE_HASH(@"SEL$0C7B6CF6" "D"@"SEL$6")'
   ,name      => 'SQLPATCH123');
end;
/

select name, created, status from dba_sql_patches where name in ('SQLPATCH123');

  NAME        CREATED             STATUS
  ----------- ------------------- -------
  SQLPATCH123 9-FEB-2018 11:15:22 ENABLED

select dbms_lob.substr(comp_data, 80, 1) hint, signature from sys.SQLOBJ$DATA t;

  HINT                                                                                         SIGNATURE
  -------------------------------------------------------------------------------- ---------------------
  <outline_data><hint><![CDATA[CARDINALITY(@"SEL$2" "D"@"SEL$2", 2) CARDINALITY(@"    516452634368408982
  <outline_data><hint><![CDATA[      IGNORE_OPTIM_EMBEDDED_HINTS                     9073911181602513588

alter system flush shared_pool;

Make the test:


sqlplus ksun/test123@TESTDB

SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS;

no rows selected
Elapsed: 00:00:00.29

Although query is fast (less than 1 second), but no rows selected. Therefore, it is not possible to bypass Oracle Security by SQL Patch.

Xplan confirms that the SQL Patch was applied (also indicated in Note: SQL patch "SQLPATCH123") because it is different with that of Test-4.


sqlplus admin/admin123@TESTDB

select * from table(dbms_xplan.display_cursor('f5ra7dru5fk5n', null, 'ADVANCED ALLSTATS LAST'));

SQL_ID  f5ra7dru5fk5n, child number 0
-------------------------------------
SELECT NAME, PATH, READ, WRITE, EXECUTE FROM SYS.LOADER_DIR_OBJS
 
Plan hash value: 153645881
 
------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                  | Name            | E-Rows |E-Bytes| Cost (%CPU)| E-Time   |  OMem |  1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                           |                 |        |       |  3522 (100)|          |       |       |          |
|   1 |  VIEW                                      | LOADER_DIR_OBJS |      3 |  6312 |  3522   (1)| 00:00:01 |       |       |          |
|   2 |   UNION-ALL                                |                 |        |       |            |          |       |       |          |
|*  3 |    HASH JOIN                               |                 |      2 |   176 |  3515   (0)| 00:00:01 |   964K|   964K| 1297K (0)|
|   4 |     FIXED TABLE FULL                       | X$DIR           |      2 |   116 |     0   (0)|          |       |       |          |
|*  5 |     INDEX FAST FULL SCAN                   | I_OBJ2          |      3 |    90 |  3515   (0)| 00:00:01 |       |       |          |
|*  6 |      FIXED TABLE FULL                      | X$KZSPR         |      1 |     7 |     0   (0)|          |       |       |          |
|   7 |    HASH GROUP BY                           |                 |      1 |   105 |     7  (15)| 00:00:01 |   831K|   831K|          |
|*  8 |     FILTER                                 |                 |        |       |            |          |       |       |          |
|*  9 |      HASH JOIN SEMI                        |                 |      1 |   105 |     6   (0)| 00:00:01 |  1927K|  1186K| 2361K (0)|
|  10 |       NESTED LOOPS                         |                 |      1 |   101 |     6   (0)| 00:00:01 |       |       |          |
|  11 |        NESTED LOOPS                        |                 |      4 |   352 |     4   (0)| 00:00:01 |       |       |          |
|  12 |         FIXED TABLE FULL                   | X$DIR           |      4 |   232 |     0   (0)|          |       |       |          |
|  13 |         TABLE ACCESS BY INDEX ROWID BATCHED| OBJ$            |      1 |    30 |     1   (0)| 00:00:01 |       |       |          |
|* 14 |          INDEX RANGE SCAN                  | I_OBJ1          |      1 |       |     1   (0)| 00:00:01 |       |       |          |
|* 15 |        INDEX RANGE SCAN                    | I_OBJAUTH1      |      1 |    13 |     1   (0)| 00:00:01 |       |       |          |
|  16 |       VIEW                                 | VW_NSO_1        |     15 |    60 |     0   (0)|          |       |       |          |
|  17 |        FIXED TABLE FULL                    | X$KZSRO         |     15 |    60 |     0   (0)|          |       |       |          |
|* 18 |      FIXED TABLE FULL                      | X$KZSPR         |      1 |     7 |     0   (0)|          |       |       |          |
------------------------------------------------------------------------------------------------------------------------------------------
 
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
 
   1 - SET$1        / LOADER_DIR_OBJS@SEL$1
   2 - SET$1       
   3 - SEL$2       
   4 - SEL$2        / D@SEL$2
   5 - SEL$2        / O@SEL$2
   6 - SEL$C8360722 / X$KZSPR@SEL$5
   7 - SEL$0C7B6CF6
  12 - SEL$0C7B6CF6 / D@SEL$6
  13 - SEL$0C7B6CF6 / O@SEL$6
  14 - SEL$0C7B6CF6 / O@SEL$6
  15 - SEL$0C7B6CF6 / OA@SEL$6
  16 - SEL$3C459230 / VW_NSO_1@SEL$0C7B6CF6
  17 - SEL$3C459230 / X$KZSRO@SEL$7
  18 - SEL$2F35337B / X$KZSPR@SEL$10

Note
-----
   - SQL patch "SQLPATCH123" used for this statement

After above test, drop used SQL patch:


sqlplus admin/admin123@TESTDB

exec dbms_sqldiag.drop_sql_patch('SQLPATCH123');

6. Discussions

In summary:

    With System privilege (Test-1 and Test-3), queries on external table and SYS.LOADER_DIR_OBJS are fast.
    Without System privilege (Test-2 and Test-4), queries are slow.
    Without System privilege (Test-5), but with SQL Patch, query is fast, but no rows selected.

Therefore, above 5 Tests showed that query result and performance depend on faultless System privilege.

Actually, the query on SYS.LOADER_DIR_OBJS:


  select name, path, read, write, execute from sys.loader_dir_objs;

is a recursive statement when accessing external table:


  select * from testt1_ext;

The performance is caused by the recursive statement.

In case of missing System privilege, "SQL Patch" can be used to manipulate its xplan, and makes it fast. But the result is different in comparing to query with System privilege.

Looking the difference of SQL Trace xplan between Test-1 (fast) and Test-2 (slow), we can see that subquery on X$KZSRO in fast xplan returns 0 rows, but in slow xplan returns 2 rows, and hence generates expensive NESTED LOOPS.

Here the definition of SYS.LOADER_DIR_OBJS:


create or replace force view sys.loader_dir_objs (name, path, read, write, execute) bequeath definer as
  select o.name, d.os_path, 'TRUE', 'TRUE', 'TRUE'
    from sys.obj$ o, sys.x$dir d
   where     o.obj# = d.obj#
         and (   o.owner# = uid
              or exists (select null from v$enabledprivs where priv_number in (-177, -178)))
  union all
    select o.name,
           d.os_path,
           decode (sum (decode (oa.privilege#, 17, 1, 0)), 0, 'FALSE', 'TRUE'),
           decode (sum (decode (oa.privilege#, 18, 1, 0)), 0, 'FALSE', 'TRUE'),
           decode (sum (decode (oa.privilege#, 12, 1, 0)), 0, 'FALSE', 'TRUE')
      from sys.obj$ o, sys.x$dir d, sys.objauth$ oa
     where     o.obj# = d.obj#
           and oa.obj# = o.obj#
           and oa.privilege# in (12, 17, 18)
           and oa.grantee# in (select kzsrorol from x$kzsro)
           and not (   o.owner# = uid
                    or exists (select null from v$enabledprivs where priv_number in (-177, -178)))
  group by o.name, d.os_path;

Wednesday, January 24, 2018

Oracle Logical Read: Current Gets Access Path and Cost

Oracle Logical Reads (Buffer Gets) fetch data from Buffer Cache (Memory) in two different Modes: Current Mode Get (DB Block Gets) and Consistent Mode Get (Consistent Gets). Blocks in Consistent Mode are the memory versions at the POINT IN TIME the query started, whereas Blocks in Current Mode are the versions at CURRENT TIME (right now). Each block can have multiversion Clones in Consistent Mode, but maximum one single version in Current Mode.

Following the discussions of Blog: Oracle Physical Read Access Path and Cost, and Oracle Logical Read: Consistent Gets Access Path and Cost, now is the time to look into second part of Logical Reads: Current Mode Gets.

We will use SQL Trace Event 10046 to display the statistics, and dtrace script: dtracelio.d published in Blog: Dynamic tracing of Oracle logical I/O to show block read details and sequence. Ideally Oracle could provide one Event to trace Current Mode Get, which is similar to Event 10200 ("consistent read buffer status") for Consistent Mode Get.

Blog: Dynamic tracing of Oracle logical I/O: part 2. Dtrace LIO v2 is released includes a detail description of Oracle internal functions:

Consistent gets:
    kcbgtcr   – Kernel Cache Buffer Get Consistent Read. This is general entry point for consistent read.
    kcbldrget – Kernel Cache Buffer Load Direct-Read Get. The function performing direct-path read. 
                Interesting detail: 10.2 the function kcbldrget is called just after kcbgtcr, 
                in 11.2 by (from) kcbgtcr.

Current gets (db block gets):
    kcbgcur – Kernel Cache Buffer Get Current Read
    kcbget  – Kernel Cache Buffer Get Buffer. This is analogue of kcbgcur function, 
              as I observed this function is called for index branch and leaf blocks
    kcbnew  – Kernel Cache Buffer New Buffer
    kcblnb (kcblnb_dscn in 11.2) – Kernel Cache Buffer Load New Buffer. 
              The function performing direct-path load. 
              Decoding of block coordinates is not supported for this function 
              in the current version of DTraceLIO. 
              The parameters such as object_id, data_object_id will be shown as 0 (zero)

Note: All tests are done in Oracle 12.1.0.2 on Solaris with 6 physical processors.

1. Test Setup


drop tablespace ksun_tts  including contents and datafiles;
drop tablespace ksun_its1 including contents and datafiles;
drop tablespace ksun_its2 including contents and datafiles;

create tablespace ksun_tts  datafile '/oratestdb/oradata/testdb/ksun_tts.dbf'  size 200m online;
create tablespace ksun_its1 datafile '/oratestdb/oradata/testdb/ksun_its1.dbf' size 200m online;
create tablespace ksun_its2 datafile '/oratestdb/oradata/testdb/ksun_its2.dbf' size 200m online;

drop table ksun_tab;

create table ksun_tab tablespace ksun_tts 
    as select trunc(level/1000) type_id, mod(level, 100) id1, trunc(mod(level, 1000)/100) id2, -1234 dummy 
  from dual connect by level <= 4099;

create unique index ksun_tab#u#1 on ksun_tab (type_id, id1, id2, dummy) tablespace ksun_its1;
create unique index ksun_tab#u#2 on ksun_tab (type_id, id2, id1)        tablespace ksun_its2;

exec dbms_stats.gather_table_stats(null, 'KSUN_TAB', cascade=>true);

select name, ts# from v$tablespace  where name like '%KSUN%' or ts# in (2, 1999);
  --UNDO         2
  --U1        1999
  --KSUN_TTS  2231
  --KSUN_ITS1 2232
  --KSUN_ITS2 2233

select rfile#, name, v.* from  v$datafile v where name like '%ksun%';
  --917 /oratestdb/oradata/testdb/ksun_tts.dbf
  --931 /oratestdb/oradata/testdb/ksun_its1.dbf
  --101 /oratestdb/oradata/testdb/ksun_its2.dbf

select object_name, object_id from dba_objects where object_name like 'KSUN_TAB%' or object_id in (1577349);
  --RMTAB$       1577349
  --KSUN_TAB     2286270
  --KSUN_TAB#U#1 2286271
  --KSUN_TAB#U#2 2286272
  
select segment_name, blocks, v.* from dba_segments v where segment_name like 'KSUN_TAB%';
  --KSUN_TAB     16
  --KSUN_TAB#U#1 24
  --KSUN_TAB#U#2 16

select 'KSUN_TAB', count(distinct dbms_rowid.rowid_relative_fno (rowid)||'-'||dbms_rowid.rowid_block_number (rowid)) used_blocks from KSUN_TAB;
  --KSUN_TAB     11

select 'KSUN_TAB#U#1', count(distinct dbms_rowid.rowid_block_number(sys_op_lbid (2286271, 'L', t.rowid))) used_blocks from KSUN_TAB t;
  --KSUN_TAB#U#1 14

select 'KSUN_TAB#U#2', count(distinct dbms_rowid.rowid_block_number(sys_op_lbid (2286272, 'L', t.rowid))) used_blocks from KSUN_TAB t;
  -- KSUN_TAB#U#2 11

-- list different functions used to perform different types of logical I/O
select indx, kcbwhdes from x$kcbwh where indx in (799, 1060, 1062, 1066, 1153, 1199);
  --799   ktewh26:  kteinpscan
  --1060  kdiwh15:  kdifxs
  --1062  kdiwh17:  kdifind
  --1066  kdiwh22:  kdifind
  --1153  kdiwh169: skipscan
  --1199  kddwh01:  kdddel

2. Tests

We will perform 3 Tests, the first one uses 2 different indexes, the second uses only one index, and the last one uses the same index twice.

2.1 Test_1


--******************************* SQL Trace *******************************--
delete /*+ test_1 use_concat index_ss(@DEL$1 T@DEL$1 ksun_tab#u#2) 
  index_ss(@DEL$1_2 T@DEL$1_2 ksun_tab#u#1) */ ksun_tab t where 99 in (id2, id1)

-- rollback; -- make test repeatable

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0         12        211          41
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.00       0.00          0         12        211          41

Rows (avg) Row Source Operation
---------- ---------------------------------------------------
         0 DELETE  KSUN_TAB (cr=12 pr=0 pw=0 time=1569 us)
        41  CONCATENATION  (cr=12 pr=0 pw=0 time=356 us)
         0   INDEX SKIP SCAN KSUN_TAB#U#2 (cr=6 pr=0 pw=0 time=82 us cost=3 size=14 card=1)(object id 2286272)
        41   INDEX SKIP SCAN KSUN_TAB#U#1 (cr=6 pr=0 pw=0 time=228 us cost=8 size=574 card=41)(object id 2286271)


--******************************* dtrace *******************************--
SQL> delete /*+ test_1 use_concat index_ss(@DEL$1 T@DEL$1 ksun_tab#u#2) 
       index_ss(@DEL$1_2 T@DEL$1_2 ksun_tab#u#1) */ ksun_tab t where 99 in (id2, id1);
   41 rows deleted.

     -- rollback; -- make test repeatable


$ > dtracelio.d 23585

Dynamic tracing of Oracle logical I/O v2.1 by Alexander Anokhin ( http://alexanderanokhin.wordpress.com )

  108075: 108075 kcbgtcr(0xFFFFFD7FFAE17D58,0,1153,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1153 exam: 0
  124990:   4336 kcbgtcr(0xFFFFFD7FFAE17D58,0,1153,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1153 exam: 0
    --Repeated lines removed--
    
  424351:   4783 kcbgtcr(0xFFFFFD7FFA0EF830,0,1153,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1153 exam: 0
  428303:   2234 kcbgtcr(0xFFFFFD7FFA0EF830,0,1153,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1153 exam: 0
    --Repeated lines removed--
    
  511748:  21448 kcbgcur(0xFFFFFD7FFAED0EB0,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  524191:   3355 kcbgcur(0xFFFFFD7FFAED0EB0,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  658937:   5220 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  663567:   2134 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  695708:  18720 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  707668:   2415 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  745953:   5864 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  750657:   2105 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  759713:   4981 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  764240:   1989 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  
  784175:   3267 kcbgcur(0xFFFFFD7FFAED0EB0,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  789056:   2245 kcbgcur(0xFFFFFD7FFAED0EB0,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  806713:   2546 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  811388:   2215 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  818717:   3387 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  823620:   2283 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  833734:   2543 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  838017:   1949 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  843481:   2253 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  847875:   1988 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  --Repeated lines removed (40 Repeats)--    


Pick one line:  
  524191:   3355 kcbgcur(0xFFFFFD7FFAED0EB0,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
You can see here:
  timestamp  - 524191, 
  delta time - 3355 (from last timestamp to current timestamp),
  kcbgcur(0xFFFFFD7FFAED0EB0,2,1199,0) – it is function call with 3 arguments,
  tsn: 2231        – a tablespace number, ts# from v$tablespace for KSUN_TTS,
  rdba: 0xe5400083 – a relative dba (data block address)
  (917/131)        – file 917 (ksun_tts.dbf) block 131
  obj: 2286270     – dictionary object number, object_id from dba_objects for KSUN_TAB,
  dobj: 2286270    – data object number, data_object_id from dba_objects for KSUN_TAB,
  where: 1199      – location from function (kcbgcur in this case) was executed. This is INDX from x$kcbwf for "kddwh01: kdddel".

===================== Logical I/O Summary (grouped by object/function) ==============
 function    stat   object_id   data_object_id   mode_held   where     bufs     calls
--------- ------- ----------- ---------------- ----------- ------- -------- ---------
  kcbgcur      cu           0               -1           2      53        2         2
  kcbgcur      cu           0               -1           2      88        2         2
  kcbgcur      cu           0               -1           2      86        4         4
   kcbnew      cu           0               -1                  47        4         4
  kcbgtcr      cr     2286271          2286271                1153       12        12
  kcbgtcr      cr     2286272          2286272                1153       12        12
  kcbgcur      cu     2286270          2286270           2    1199       82        82
  kcbgcur      cu     2286271          2286271           1    1062       82        82
  kcbgcur      cu     2286272          2286272           1    1062       82        82
   kcbget      cu     2286271          2286271           2    1066       82        82
   kcbget      cu     2286272          2286272           2    1066       82        82
=====================================================================================

================================= Logical I/O Summary (grouped by object) ==========================================
 object_id  data_object_id       lio        cr    cr (e)    cr (d)        cu    cu (d) ispnd (Y) ispnd (N)   pin rls
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
         0              -1        12         0         0         0        12         0         0         0         4
   2286270         2286270        82         0         0         0        82         0         0         0        82
   2286271         2286271       176        12         0         0       164         0         0         0        12
   2286272         2286272       176        12         0         0       164         0         0         0        12
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
     total                       446        24         0         0       422         0         0         0       110
====================================================================================================================

Legend
  lio      : logical gets (cr + cu)
  cr       : consistent gets
  cr (e)   : consistent gets - examination
  cr (d)   : consistent gets direct
  cu       : db block gets
  cu (d)   : db block gets direct
  ispnd (Y): buffer is pinned count
  ispnd (N): buffer is not pinned count
  pin rls  : pin releases

2.2 Test_2


--******************************* SQL Trace *******************************--
delete /*+ test_2 index(t ksun_tab#U#2)  */ ksun_tab t where 99 in (id2, id1)

-- rollback; -- make test repeatable

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0         12         75          41
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.00       0.00          0         12         75          41

Rows (avg) Row Source Operation
---------- ---------------------------------------------------
         0 DELETE  KSUN_TAB (cr=12 pr=0 pw=0 time=1025 us)
        41  INDEX FULL SCAN KSUN_TAB#U#2 (cr=12 pr=0 pw=0 time=405 us cost=6 size=574 card=41)(object id 2286272)


--******************************* dtrace *******************************--
SQL > delete /*+ test_2 index(t ksun_tab#U#2) */ ksun_tab t where 99 in (id2, id1);
   41 rows deleted.
   
      -- rollback; -- make test repeatable
   
$ > dtracelio.d 23585

Dynamic tracing of Oracle logical I/O v2.1 by Alexander Anokhin ( http://alexanderanokhin.wordpress.com )

  233462:  22641 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  245054:   2829 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  365187:   2913 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  369681:   1947 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  404103:   2826 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  408598:   2029 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  429972:   2233 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  434528:   2212 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  453201:   6922 kcbgtcr(0xFFFFFD7FFAED1730,0,1060,0) [tsn: 2233 rdba: 0x19400085 (101/133) obj: 2286272 dobj: 2286272] where: 1060 exam: 0
  458011:   2301 kcbgtcr(0xFFFFFD7FFAED1730,0,1060,0) [tsn: 2233 rdba: 0x19400085 (101/133) obj: 2286272 dobj: 2286272] where: 1060 exam: 0 
  479314:   2777 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  484221:   2266 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  506878:   2334 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  511463:   2202 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  532201:   2334 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  536500:   1965 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  556552:   2244 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  560690:   1834 kcbgcur(0xFFFFFD7FFAED9270,2,1199,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  577493:   3208 kcbgtcr(0xFFFFFD7FFAED1730,0,1060,0) [tsn: 2233 rdba: 0x19400086 (101/134) obj: 2286272 dobj: 2286272] where: 1060 exam: 0
  581798:   2276 kcbgtcr(0xFFFFFD7FFAED1730,0,1060,0) [tsn: 2233 rdba: 0x19400086 (101/134) obj: 2286272 dobj: 2286272] where: 1060 exam: 0
    --Repeated lines removed--
  
  1786824:   5448 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  1791226:   2020 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  1830860:  17280 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  1841407:   2737 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  1885701:   3609 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  1890635:   1952 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  1904651:   3127 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2233 rdba: 0x19400085 (101/133) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  1909925:   2340 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2233 rdba: 0x19400085 (101/133) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
   --Repeated lines removed--   
     
     
  2202041:   5066 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  2206643:   2018 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  2220344:   5196 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  2224948:   2136 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  2242122:   2441 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  2246527:   2011 kcbgcur(0xFFFFFD7FFFDF3200,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  2255942:   2484 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2232 rdba: 0xe8c0008a (931/138) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  2260392:   1987 kcbget(0xFFFFFD7FFFDF3200,2,1066,0) [tsn: 2232 rdba: 0xe8c0008a (931/138) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
   --Repeated lines removed--   


===================== Logical I/O Summary (grouped by object/function) ==============
 function    stat   object_id   data_object_id   mode_held   where     bufs     calls
--------- ------- ----------- ---------------- ----------- ------- -------- ---------
  kcbgcur      cu           0               -1           2      53        2         2
  kcbgcur      cu           0               -1           2      86        2         2
  kcbgcur      cu           0               -1           2      88        2         2
  kcbgtcr      cr     2286272          2286272                1298        2         2
  kcbgtcr      cr     2286272          2286272                1299        2         2
   kcbnew      cu           0               -1                  47        2         2
  kcbgcur      cu     2286271          2286271           1    1062        8         8
   kcbget      cu     2286271          2286271           2    1066        8         8
  kcbgtcr      cr     2286272          2286272                1060       20        20
  kcbgcur      cu     2286272          2286272           1    1062       22        22
   kcbget      cu     2286272          2286272           2    1066       22        22
  kcbgcur      cu     2286270          2286270           2    1199       82        82
=====================================================================================

================================= Logical I/O Summary (grouped by object) ==========================================
 object_id  data_object_id       lio        cr    cr (e)    cr (d)        cu    cu (d) ispnd (Y) ispnd (N)   pin rls
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
         0              -1         8         0         0         0         8         0         0         0         2
   2286270         2286270        82         0         0         0        82         0         0         0        82         
   2286271         2286271        16         0         0         0        16         0         0         0         0
   2286272         2286272        68        24         2         0        44         0         0         2        18
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
     total                       174        24         2         0       150         0         0         2       102
====================================================================================================================

2.3 Test_3


--******************************* SQL Trace *******************************--
delete /*+ test_3 use_concat index_ss(@DEL$1_1 T@DEL$1 ksun_tab#u#2) 
  index_ffs(@DEL$1_2 T@DEL$1_2 ksun_tab#u#2) */ ksun_tab t where 99 in (id2, id1)


call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0         23        211          41
Fetch        0      0.00       0.00          0          0          0           0
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        2      0.00       0.00          0         23        211          41

ows (avg)  Row Source Operation
---------  ---------------------------------------------------
        0  DELETE  KSUN_TAB (cr=23 pr=0 pw=0 time=1180 us)
       41   CONCATENATION  (cr=23 pr=0 pw=0 time=398 us)
        0    INDEX SKIP SCAN KSUN_TAB#U#2 (cr=6 pr=0 pw=0 time=72 us cost=3 size=14 card=1)(object id 2286272)
       41    INDEX FAST FULL SCAN KSUN_TAB#U#2 (cr=17 pr=0 pw=0 time=242 us cost=5 size=574 card=41)(object id 2286272)
           

--******************************* dtrace *******************************--
  131208:   3788 kcbgtcr(0xFFFFFD7FFFDE5230,0,798,0) [tsn: 0 rdba: 0x5133b8 (1/1127352) obj: 1577349 dobj: 1577349] where: 798 exam: 0
  181969:   3256 kcbgtcr(0xFFFFFD7FFFDE4E40,0,798,0) [tsn: 0 rdba: 0x5133b8 (1/1127352) obj: 1577349 dobj: 1577349] where: 798 exam: 0
    --Repeated lines removed -- 
    
  2537995:   2950 kcbgcur(0xFFFFFD7FFA0FB028,2,1199,0) [tsn: 2231 rdba: 0xe5400085 (917/133) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  2543063:   2038 kcbgcur(0xFFFFFD7FFA0FB028,2,1199,0) [tsn: 2231 rdba: 0xe5400085 (917/133) obj: 2286270 dobj: 2286270] where: 1199 mode_held: 2
  2557738:   2324 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  2562180:   2074 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2232 rdba: 0xe8c00083 (931/131) obj: 2286271 dobj: 2286271] where: 1062 mode_held: 1
  2569550:   3515 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  2575512:   2290 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2232 rdba: 0xe8c00087 (931/135) obj: 2286271 dobj: 2286271] where: 1066 mode_held: 2
  2584580:   2492 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  2589073:   2002 kcbgcur(0xFFFFFD7FFFDF4660,1,1062,0) [tsn: 2233 rdba: 0x19400083 (101/131) obj: 2286272 dobj: 2286272] where: 1062 mode_held: 1
  2594662:   2312 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2233 rdba: 0x19400086 (101/134) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
  2599288:   1901 kcbget(0xFFFFFD7FFFDF4660,2,1066,0) [tsn: 2233 rdba: 0x19400086 (101/134) obj: 2286272 dobj: 2286272] where: 1066 mode_held: 2
    --Repeated lines removed (40 Repeats)--   

===================== Logical I/O Summary (grouped by object/function) ==============
 function    stat   object_id   data_object_id   mode_held   where     bufs     calls
--------- ------- ----------- ---------------- ----------- ------- -------- ---------
  kcbgcur      cu           0               -1           2      53        2         2
  kcbgcur      cu           0               -1           2      88        2         2
  kcbgtcr      cr     1577349          1577349                 799        2         2
  kcbgtcr      cr     2286272          2286272                 798        2         2
  kcbgcur      cu           0               -1           2      86        4         4
  kcbgtcr      cr     1577349          1577349                 798        4         4
  kcbgtcr      cr     2286272          2286272                 799        4         4
  kcbgtcr      cr     2286272          2286272                 800        4         4
   kcbnew      cu           0               -1                  47        4         4
  kcbgtcr      cr     2286272          2286272                1153       12        12
  kcbgtcr      cr     2286272          2286272                1118       24        24
  kcbgcur      cu     2286270          2286270           2    1199       82        82
  kcbgcur      cu     2286271          2286271           1    1062       82        82
  kcbgcur      cu     2286272          2286272           1    1062       82        82
   kcbget      cu     2286271          2286271           2    1066       82        82
   kcbget      cu     2286272          2286272           2    1066       82        82
=====================================================================================

================================= Logical I/O Summary (grouped by object) ==========================================
 object_id  data_object_id       lio        cr    cr (e)    cr (d)        cu    cu (d) ispnd (Y) ispnd (N)   pin rls
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
   1577349         1577349         6         6         0         0         0         0         0         0         6
         0              -1        12         0         0         0        12         0         0         0         4
   2286270         2286270        82         0         0         0        82         0         0         0        82
   2286271         2286271       164         0         0         0       164         0         0         0         0
   2286272         2286272       210        46         0         0       164         0         0         0        46
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
     total                       474        52         0         0       422         0         0         0       138
====================================================================================================================

3. Discussions

Note that Dtrace Script statistics are double counted, which will be discussed in next Section: Dtrace Script double counted Statistics.

(a). Looking execution statistics of all 3 Tests, which delete the same 41 rows:

         Test    Consistent Gets  Current Gets
         ------  ---------------  ------------
         Test_1               12           211
         Test_2               12            75
         Test_3               23           211
         
     the majority of Logical Read is generated from Current Gets.

(b). Test_1 makes 211 Current Mode Gets, whereas Test_2 does 75 Current Mode Gets.

     In Test_1, dtrace shows that each Current Get(kcbgcur) of KSUN_TAB (obj: 2286270) 
     follows by two Current Gets (kcbgcur and kcbget) of KSUN_TAB#U#1 (obj: 2286271) 
     and two Current Gets (kcbgcur and kcbget) of KSUN_TAB#U#2 (obj: 2286272). 
     41 deleted Rows results in:
     
           41 (KSUN_TAB)  + 2*41 (KSUN_TAB#U#1) + 2*41 (KSUN_TAB#U#2) = 205 Current Gets
           
     plus 4 Current Gets (kcbgcur) and 2 Current Gets (kcbnew) of UNDO Segments (dobj: -1). 
     So in total, 205 + 4 + 2 = 211.
     
     In Test_2, at first, 41 Current Gets (kcbgcur) of KSUN_TAB (obj: 2286270) for 41 deleted Rows, 
     then 22 Current Gets (kcbgcur and kcbget) of KSUN_TAB#U#2 (obj: 2286272), 
     and 8  Current Gets (kcbgcur and kcbget) of KSUN_TAB#U#1 (obj: 2286271):
     
           41 (KSUN_TAB)  + 22 (KSUN_TAB#U#2) + 8 (KSUN_TAB#U#1) = 71 Current Gets
           
     plus 3 Current Gets (kcbgcur) and 1 Current Gets (kcbnew) of UNDO Segments (dobj: -1). 
     So in total, 71 + 3 + 1 = 75.

(c). SQL Trace xplan contains only Consistent Mode Gets ("cr=xxx"), but not Current Mode Gets.
     However, Current Mode Gets is included in xplan statistics under Column current.

(d). Test_3 makes 211 Current Mode Gets, same as Test_1 although only one single Index: 
     KSUN_TAB#U#2 is involved in xplan.
     
     Dtrace shows that it follows the same access pattern as Test_1 to read both KSUN_TAB#U#1 
     and KSUN_TAB#U#2 in Current Mode, but SQL Trace is not able to point out KSUN_TAB#U#1 Gets.

(e). Test_3 SQL Trace published 23 Consistent Mode Gets (query=23), but Dtrace shows 26 (=52/2). 
     The 3 (=26-23) is to read RMTAB$ (object_id = 1577349), which are not disclosed in SQL Trace.
     (See MOS: Bug 20853821 - HIgh number of executions for queries on rmtab$ (Doc ID 20853821.8))
     
     Therefore, SQL Trace is not able to disclose all Logical Reads.
     
     Probably they stem from recursive calls, which can be visible in Sqlplus Autotrace.

4. Dtrace Script double counted Statistics

Comparing dtracelio.d output with SQL Trace:

    Test1: SQL Trace  query = 12 current = 211
           Dtrace     cr    = 24 cu      = 422    
           
    Test2: SQL Trace  query = 12 current =  75
           Dtrace     cr    = 24 cu      = 150

it seems that Dtrace double counts cr and cu (also verified with Sqlplus Autotrace, v$sql.buffer_gets, and sys.dbms_sqltune.report_sql_monitor). In all tests, we purposely deleted and selected odd number of rows (41), but all number in dtrace Summary are even number.

We can observe this double counting when comparing 3 different Traces: one Sql Trace, one 10200 trace, and one dtrace for the same query.

At first, a small dtrace command shows us that each kcbgtcr is called by ktrget2, so in 10200 Trace, there is almost a 1-to-1 mapping between kcbgtcr and ktrget2.

SQL Trace shows that the query made cr=15 on KSUN_TAB#U#2 (object id 2286272), and 28-15=13 on KSUN_TAB (object id 2286270). dtracelio.d shows that cr=30 on object_id=2286272, cr=26 on object_id= 2286270.

10200 Trace contains two rows for each ktrget2 (which is immediate parent of kcbgtcr) call, first is "started", second is "completed", which probably caused this double counting.


--*************************** kcbgtcr is called by ktrget2 ***************************--
SQL > select/*+ test_2 index(t ksun_tab#U#2) */ * from ksun_tab t where 99 in (id2, id1);

$ dtrace -n 'pid$target::kcbgtcr:entry {ustack(2, 0); @[pid, ustack(2, 0)] = count();}' -p 23585
    oracle`kcbgtcr
    oracle`ktrgtc2+0x332
    
    oracle`kcbgtcr
    oracle`ktrgtc2+0x332
    
    oracle`kcbgtcr
    oracle`ktrget2+0x2b7    
  --53 Repeated lines removed-- 
    
    oracle`kcbgtcr
    oracle`ktrgtc2+0x332
      2
      
    oracle`kcbgtcr
    oracle`ktrget2+0x2b7
     54


--******************************* SQL Trace *******************************--
select /*+ test_2 index(t ksun_tab#U#2) */ * from ksun_tab t where 99 in (id2, id1)

call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        4      0.00       0.00          0         28          0          41
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        6      0.00       0.00          0         28          0          41

Rows (avg)  Row Source Operation
----------  ---------------------------------------------------
        41  TABLE ACCESS BY INDEX ROWID BATCHED KSUN_TAB (cr=28 pr=0 pw=0 time=49 us cost=7 size=574 card=41)
        41   INDEX FULL SCAN KSUN_TAB#U#2 (cr=15 pr=0 pw=0 time=636 us cost=6 size=0 card=41)(object id 2286272)


--******************************* 10200 Trace *******************************--
alter session set tracefile_identifier = 'test_2_10200_trace';
alter session set events '10200 trace name context forever,level 10';
  select/*+ test_2 index(t ksun_tab#U#2) */ * from ksun_tab t where 99 in (id2, id1);
alter session set events '10200 trace name context off'; 
 
   ktrget2(): started   for block <0x08b7 : 0xe5400083> objd: 0x0022e2be (KSUN_TAB)
   ktrget2(): completed for block <0x08b7 : 0xe5400083> objd: 0x0022e2be (KSUN_TAB)
       
   ktrget2(): started   for block <0x08b9 : 0x1940008e> objd: 0x0022e2c0 (KSUN_TAB#U#2)
   ktrget2(): completed for block <0x08b9 : 0x1940008e> objd: 0x0022e2c0 (KSUN_TAB#U#2)
 --Repeated lines removed-- 
 
 
--******************************* dtrace *******************************--
SQL > select/*+ test_2 index(t ksun_tab#U#2) */ * from ksun_tab t where 99 in (id2, id1);
   
$ > dtracelio.d 23585

  1365716:  10444 kcbgtcr(0xFFFFFD7FFAEE4798,0,1061,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1061 exam: 0
  1372023:   2458 kcbgtcr(0xFFFFFD7FFAEE4798,0,1061,0) [tsn: 2233 rdba: 0x19400084 (101/132) obj: 2286272 dobj: 2286272] where: 1061 exam: 0
  1399863:   2673 kcbgtcr(0xFFFFFD7FFA0FA518,0,1007,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1007 exam: 0
  1403855:   2116 kcbgtcr(0xFFFFFD7FFA0FA518,0,1007,0) [tsn: 2231 rdba: 0xe5400083 (917/131) obj: 2286270 dobj: 2286270] where: 1007 exam: 0
  1457916:   5590 kcbgtcr(0xFFFFFD7FFAEE4798,0,1060,0) [tsn: 2233 rdba: 0x19400085 (101/133) obj: 2286272 dobj: 2286272] where: 1060 exam: 0
  1462033:   2347 kcbgtcr(0xFFFFFD7FFAEE4798,0,1060,0) [tsn: 2233 rdba: 0x19400085 (101/133) obj: 2286272 dobj: 2286272] where: 1060 exam: 0
  1493378:   2493 kcbgtcr(0xFFFFFD7FFA0FA518,0,1007,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1007 exam: 0
  1497338:   2096 kcbgtcr(0xFFFFFD7FFA0FA518,0,1007,0) [tsn: 2231 rdba: 0xe5400084 (917/132) obj: 2286270 dobj: 2286270] where: 1007 exam: 0    
   --Repeated lines removed-- 
     
===================== Logical I/O Summary (grouped by object/function) ==============
 function    stat   object_id   data_object_id   mode_held   where     bufs     calls
--------- ------- ----------- ---------------- ----------- ------- -------- ---------
  kcbgtcr      cr     2286272          2286272                1298        2         2
  kcbgtcr      cr     2286272          2286272                1299        2         2
  kcbgtcr      cr     2286272          2286272                1061        6         6
  kcbgtcr      cr     2286272          2286272                1060       20        20
  kcbgtcr      cr     2286270          2286270                1007       26        26
=====================================================================================

================================= Logical I/O Summary (grouped by object) ==========================================
 object_id  data_object_id       lio        cr    cr (e)    cr (d)        cu    cu (d) ispnd (Y) ispnd (N)   pin rls
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
         0               0         0         0         0         0         0         0         0         2         0
   2286270         2286270        26        26         0         0         0         0       130        32        26
   2286272         2286272        30        30         2         0         0         0         0         8        28
---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------
     total                        56        56         2         0         0         0       130        42        54
====================================================================================================================

5. Performance Test

Run above 3 Tests 10000 times and watch their statistics difference.


alter system flush shared_pool; 

declare
 p_cnt number := 10000;
begin
  for i in 1..p_cnt loop
    delete /*+ test_1 use_concat index_ss(@DEL$1 T@DEL$1 ksun_tab#u#2) index_ss(@DEL$1_2 T@DEL$1_2 ksun_tab#u#1) */ 
      ksun_tab t where 99 in (id2, id1);
    rollback;
  end loop;
  
  for i in 1..p_cnt loop
    delete /*+ test_2 index(t ksun_tab#U#2)  */ ksun_tab t where 99 in (id2, id1);
    rollback;
  end loop;  
  
  for i in 1..p_cnt loop
    delete /*+ test_3 use_concat index_ss(@DEL$1_1 T@DEL$1 ksun_tab#u#2) index_ffs(@DEL$1_2 T@DEL$1_2 ksun_tab#u#2) */ 
      ksun_tab t where 99 in (id2, id1);
    rollback;
  end loop;  
end;
/

select substr(sql_text, 1, 18) sql_text, buffer_gets, round(buffer_gets/10000) buffer_gets_per_exec
      ,rows_processed, physical_read_bytes, physical_write_bytes, elapsed_time, cpu_time, sql_id  --, v.*
from v$sql v where lower(sql_text) like 'delete%test%' order by v.sql_text;

SQL_TEXT          BUFFER_GETS BUFFER_GETS_PER_EXEC ROWS_PROCESSED PHYSICAL_READ_BYTES PHYSICAL_WRITE_BYTES ELAPSED_TIME CPU_TIME SQL_ID
----------------- ----------- -------------------- -------------- ------------------- -------------------- ------------ -------- -------------
DELETE /*+ test_1     2210823                  221         410000             1146880            210026496      7534494  7400730 g2267fmk9u39r
DELETE /*+ test_2      870403                   87         410000              786432            210026496      7802519  7755552 5xqavr92xbvw7
DELETE /*+ test_3     2320261                  232         410000              401408            105013248      9445604  9320234 66dw2ur1uakhn

6. dtracelio.d

Dtrace script is appended in the Blog for a quick reference. The script is extended with time stamp and time delta similar to truss (-d, -D options) prefixed in each output line.


#!/usr/sbin/dtrace -CZs

/* Add time stamp and time delta similar to truss (-d, -D options) */

/********************************************************************************************/
/*                                                                                          */
/*   File Name:    dtracelio.d                                                              */
/*   Version:      2.1                                                                     */
/*   Authors:      Alexander Anokhin ( http://alexanderanokhin.com )                        */
/*                 Andrey Nikolaev   ( http://andreynikolaev.wordpress.com )                */
/*   Dated:        Mar 2012                                                                 */
/*   Purpose:      The script shows calls of the functions                                  */
/*                 performing logical I/O in Oracle.                                        */
/*                                                                                          */
/*   Usage:        dtracelio.d  [show_each_call] [interval]                            */
/*                 PID: unix process ID                                                     */
/*                 show_each_call - Bitmask determining how functions calls will be shown   */
/*                                  1st bit - logical I/O functions                         */
/*                                  2nd bit - buffer pinning                                */
/*                                  Examples:                                               */
/*                                    0: output of each call will be disabled               */
/*                                    1: logical I/O functions will be shown                */
/*                                    3: logical I/O and buffer pinning functions           */
/*                                       will be shown                                      */
/*                                  Default value: 1                                        */
/*                 interval - Specifies the number of seconds over which                    */
/*                            Summary form with cumulative figures will be shown.           */
/*                            Works only when show_each_call is disabled.                   */
/*                                                                                          */
/*   Other:        Some bits of info in 8103.1                                              */
/*                 Note that data structures definitions are not full                       */
/*                 in current version of the script                                         */
/*                                                                                          */
/********************************************************************************************/

#pragma D option quiet
#pragma D option defaultargs
#pragma D option switchrate=10Hz

/*      0. Several standard oratype.h declarations */

typedef unsigned long long ub8; /* unsigned int of length 8 */
typedef unsigned int       ub4;
typedef unsigned short     ub2;
typedef unsigned char      ub1;
typedef int       sb4;

/*  */
typedef sb4 kobjd;
typedef sb4 kobjn;
typedef ub4 ktsn;
typedef ub4 krdba;

/* definition from MOS note 8103.1 */

typedef struct kdbafr /* full relative dba */ {
    ktsn tsn_kdbafr;  /* a tablespace number */
    krdba dba_kdbafr; /* a relative dba */ } kdbafr;

typedef struct ktid /* relative dba + objd */ {
    struct kdbafr dbr_ktid; /* a relative dba */
    kobjd objd_ktid; /* data object number */
    kobjn objn_ktid; /* dictionary object number */ } ktid;

typedef struct kcbds
{
    struct ktid kcbdstid; /* full relative DBA plus object number */
    /* Here unknown (yet ;-)) part of the structure */ } kcbds;

BEGIN
{
    show_each_call      = ($$2 == NULL) ? 1:$2;
    tick_time_interval  = ($$3 == NULL) ? 0:$3;
    trace_logical_io    = 1;
    trace_buf_pinning   = 1;
    show_lio_calls      = show_each_call & 1;
    show_buf_pins_calls = show_each_call & 2;
    printf("\nDynamic tracing of Oracle logical I/O v2.1 by Alexander Anokhin ( http://alexanderanokhin.wordpress.com )\n\n"); 
    t0 = 0;
}

pid$1::kcbgtcr:entry,
pid$1::kcbgcur:entry,
pid$1::kcbnew:entry,
pid$1::kcbrls:entry,
pid$1::kcbispnd:entry,
pid$1::kcbldrget:entry,
pid$1::kcbget:entry
{
    blk         = ((kcbds *) copyin(arg0, sizeof(kcbds)));
    tsn         = blk->kcbdstid.dbr_ktid.tsn_kdbafr;
    rdba        = blk->kcbdstid.dbr_ktid.dba_kdbafr;
    objd        = blk->kcbdstid.objd_ktid;
    objn        = blk->kcbdstid.objn_ktid;
    rdba_file   = rdba >> 22; /* for smallfile tablespaces */
    rdba_block  = rdba & 0x3FFFFF;
    t0 = (t0 == 0) ? timestamp:t0; t1 = timestamp;
}



pid$1::kcbgtcr:entry
/trace_logical_io/
{
    stat       = "cr";
    where      = arg2&0xFFFF;
    mode_held  = "";

    @func[probefunc,stat,objn,objd,mode_held,where] = count();
    @bufs[probefunc,stat,objn,objd,mode_held,where] = sum(1);

    @obj_cr[objn,objd] = sum(1);
    @total_cr = sum(1);

    @obj_cr_exam[objn,objd] = sum(arg1);
    @total_cr_exam = sum(arg1);

    @obj_lio[objn,objd] = sum(1);
    @total_lio = sum(1);
}

pid$1::kcbgtcr:entry
/trace_logical_io && show_lio_calls/
{   t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;    /* Elapsed nanosecond from beginning: Elapsed delta from last printf */
    printf("%s(0x%X,%d,%d,%d) [tsn: %d rdba: 0x%x (%d/%d) obj: %d dobj: %d] where: %d exam: %d\n",probefunc,arg0,arg1,arg2,arg3,tsn, rdba,rdba_file,rdba_block,objn, objd, where, arg1); }


pid$1::kcbldrget:entry
/trace_logical_io/
{
    stat       = "cr (d)";
    where      = 0;
    mode_held  = "";

    @func[probefunc,stat,objn,objd,mode_held,where] = count();
    @bufs[probefunc,stat,objn,objd,mode_held,where] = sum(1);

    @obj_cr_d[objn,objd] = sum(1);
    @total_cr_d = sum(1);

    @obj_cr[objn,objd] = sum(-1);
    @total_cr = sum(-1);

  /* commented because incremented in kcbgtcr yet
    @obj_lio[objn,objd] = sum(1);
    @total_lio = sum(1);
  */
}

pid$1::kcbldrget:entry
/trace_logical_io && show_lio_calls/
{   t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%s(0x%X,%d,%d,%d)\n",probefunc,arg0,arg1,arg2,arg3);
}


pid$1::kcbgcur:entry,
pid$1::kcbget:entry
/trace_logical_io/
{
    stat       = "cu";
    where      = arg2&0xFFFF;
    mode_held  = lltostr(arg1);

    @func[probefunc,stat,objn,objd,mode_held,where] = count();
    @bufs[probefunc,stat,objn,objd,mode_held,where] = sum(1);

    @obj_cu[objn,objd] = sum(1);
    @total_cu  = sum(1);

    @obj_lio[objn,objd] = sum(1);
    @total_lio = sum(1);
}

pid$1::kcbgcur:entry,
pid$1::kcbget:entry
/trace_logical_io && show_lio_calls/
{   t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%s(0x%X,%d,%d,%d) [tsn: %d rdba: 0x%x (%d/%d) obj: %d dobj: %d] where: %d mode_held: %s\n",probefunc,arg0,arg1,arg2,arg3,tsn, rdba,rdba_file,rdba_block,objn, objd, where, mode_held); }

pid$1::kcbnew:entry
/trace_logical_io/
{
    stat       = "cu";
    where      = arg2&0xFFFF;
    mode_held  = "";
    blocks     = arg1;

    @func[probefunc,stat,objn,objd,mode_held,where] = count();
    @bufs[probefunc,stat,objn,objd,mode_held,where] = sum(blocks);

    @obj_cu[objn,objd] = sum(blocks);
    @total_cu = sum(blocks);

    @obj_lio[objn,objd] = sum(blocks);
    @total_lio = sum(blocks);
}

pid$1::kcbnew:entry
/trace_logical_io && show_lio_calls/
{   t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%s(0x%X,%d,%d,%d) [tsn: %d rdba: 0x%x (%d/%d) obj: %d dobj: %d] where: %d mode_held: %s blocks: %d\n",probefunc,arg0,arg1,arg2,arg3,tsn, rdba,rdba_file,rdba_block,objn, objd, where, mode_held, blocks); }

pid$1::kcblnb:entry, pid$1::kcblnb_dscn:entry /trace_logical_io/ {
    stat = "cu (d)";
    @func[probefunc,stat,0,0,"",0] = count();
    @bufs[probefunc,stat,0,0,"",0] = sum(1);

    @obj_cu[objn,objd] = sum(1);
    @total_cu = sum(1);

    @obj_cu_d[objn,objd] = sum(1);
    @total_cu_d = sum(1);

    @obj_lio[objn,objd] = sum(1);
    @total_lio = sum(1);
}



pid$1::kcblnb:entry, pid$1::kcblnb_dscn:entry /trace_logical_io && show_lio_calls/ {
    t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%s(0x%X,%d,%d,%d)\n",probefunc,arg0,arg1,arg2,arg3);
}



/* buffer pinning */

pid$1::kcbispnd:entry
/trace_buf_pinning && show_buf_pins_calls/ {
    t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%s(0x%X,%d,%d,%d) [tsn: %d rdba: 0x%x (%d/%d) obj: %d dobj: %d]: ",probefunc,arg0,arg1,arg2,arg3,tsn, rdba,rdba_file,rdba_block,objn,objd);
}

pid$1::kcbispnd:return
/trace_buf_pinning && arg1 == 0/
{
    @obj_is_not_pnd[objn,objd] = sum(1);
    @total_is_not_pnd = sum(1);
}

pid$1::kcbispnd:return
/trace_buf_pinning && arg1 == 1/
{
    @obj_is_pnd[objn,objd] = sum(1);;
    @total_is_pnd = sum(1);
}

pid$1::kcbispnd:return
/trace_buf_pinning && show_buf_pins_calls/ {
    t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%d\n",arg1);
}

pid$1::kcbrls:entry
/trace_buf_pinning/
{
    @obj_pinrls[objn,objd] = sum(1);
    @total_pinrls = sum(1);
}

pid$1::kcbrls:entry
/trace_buf_pinning && show_buf_pins_calls/ {
    t2 = timestamp; printf("%10d: %6d ", t2-t0, t2-t1); t1 = t2;
    printf("%s(0x%X,%d,%d,%d) [tsn: %d rdba: 0x%x (%d/%d) obj: %d dobj: %d]\n",probefunc,arg0,arg1,arg2,arg3,tsn, rdba,rdba_file,rdba_block,objn,objd);
}


tick-1sec
/show_each_call == 0 && tick_time_interval > 0 && ++ticks >= tick_time_interval/ {
    printf("\n\n");
    printf("\n================================= Logical I/O Summary (grouped by object) ==========================================\n");
    printf(" object_id  data_object_id       lio        cr    cr (e)    cr (d)        cu    cu (d) ispnd (Y) ispnd (N)   pin rls\n");
    printf("---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------\n");
    printa("%10d %15d %9@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d\n", 
            @obj_lio, @obj_cr, @obj_cr_exam, @obj_cr_d, @obj_cu, @obj_cu_d, @obj_is_pnd, @obj_is_not_pnd, @obj_pinrls);
    printf("---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------\n");
    printa("     total %25@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d\n", @total_lio, @total_cr, @total_cr_exam, @total_cr_d, @total_cu, @total_cu_d, @total_is_pnd, @total_is_not_pnd, @total_pinrls);
    printf("====================================================================================================================\n");
    ticks = 0;
}



END
{
    printf("\n===================== Logical I/O Summary (grouped by object/function) ==============\n");
    printf(" function    stat   object_id   data_object_id   mode_held   where     bufs     calls\n");
    printf("--------- ------- ----------- ---------------- ----------- ------- -------- ---------\n");
    printa("%9s %7s %11d %16d %11s %7d %@8d %@9d\n", @bufs, @func);
    printf("=====================================================================================\n");
}

END
{
    printf("\n================================= Logical I/O Summary (grouped by object) ==========================================\n");
    printf(" object_id  data_object_id       lio        cr    cr (e)    cr (d)        cu    cu (d) ispnd (Y) ispnd (N)   pin rls\n");
    printf("---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------\n");
    printa("%10d %15d %9@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d\n", 
            @obj_lio, @obj_cr, @obj_cr_exam, @obj_cr_d, @obj_cu, @obj_cu_d, @obj_is_pnd, @obj_is_not_pnd, @obj_pinrls);
    printf("---------- --------------- --------- --------- --------- --------- --------- --------- --------- --------- ---------\n");
    printa("     total %25@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d %9@d\n", @total_lio, @total_cr, @total_cr_exam, @total_cr_d, @total_cu, @total_cu_d, @total_is_pnd, @total_is_not_pnd, @total_pinrls);
    printf("====================================================================================================================\n");
    printf("\n");
    printf("Legend\n");
    printf("lio      : logical gets (cr + cu)\n");
    printf("cr       : consistent gets\n");
    printf("cr (e)   : consistent gets - examination\n");
    printf("cr (d)   : consistent gets direct\n");
    printf("cu       : db block gets\n");
    printf("cu (d)   : db block gets direct\n");
    printf("ispnd (Y): buffer is pinned count\n");
    printf("ispnd (N): buffer is not pinned count\n");
    printf("pin rls  : pin releases\n");
}

Oracle Logical Read: Consistent Gets Access Path and Cost

Oracle Logical Reads (Buffer Gets) fetch data from Buffer Cache (Memory) in two different Modes: Consistent Mode Get (Consistent Gets) and Current Mode Get (DB Block Gets). Blocks in Consistent Mode are the memory versions at the POINT IN TIME the query started, whereas Blocks in Current Mode are the versions at CURRENT TIME (right now). Each block can have multiversion Clones in Consistent Mode, but maximum one single version in Current Mode.

This Blog will discuss Consistent Gets, and next Blog will talk about DB Block Gets.

At first, we test Consistent Gets in 4 different Access Paths and measure the Cost in terms of Event 10200 ("consistent read buffer status"), then demonstrate 'latch: cache buffers chains' in row-by-row slow processing.

Note: All tests are done in Oracle 12.1.0.2 on AIX, Solaris, Linux with 6 physical processors.

1. Test Setup

Pick the same Test Code from Blog: Hot Block Identification and latch: cache buffers chains.


drop table test_tab;
              
create table test_tab 
  INITRANS   26           -- prevent Segments ITL Waits and Deadlocks
as select level id, rpad('ABC', 10, 'X') val from dual connect by level <= 5;

alter table test_tab minimize records_per_block;

truncate table test_tab;

insert into test_tab select level id, rpad('ABC', 10, 'X') val from dual connect by level <= 100;

commit;

drop index test_tab#u1;

create unique index test_tab#u1 on k.test_tab (id) 
  pctfree    90
  initrans   26        -- prevent Segments ITL Waits and Deadlocks
;

exec dbms_stats.gather_table_stats(null, 'TEST_TAB', cascade => TRUE);

select block, to_char(block, 'xxxxxxxx') block_hex, count (*) cnt 
  from (select rowid rid, 
               dbms_rowid.rowid_object (rowid) object, 
               dbms_rowid.rowid_relative_fno (rowid) fil, 
               dbms_rowid.rowid_block_number (rowid) block, 
               dbms_rowid.rowid_row_number (rowid) ro, 
               t.* 
          from test_tab t 
         order by fil, block, ro) 
 group by block 
 order by block, cnt desc;

     BLOCK BLOCK_HEX        CNT
  -------- --------- ----------
   3070331    2ed97b          5
   3070332    2ed97c          5
   3070333    2ed97d          5
   3070334    2ed97e          5
   3070335    2ed97f          5
   3072640    2ee280          5
   3072641    2ee281          5
   3072642    2ee282          5
   3072643    2ee283          5
   3072644    2ee284          5
   3072645    2ee285          5
   3072646    2ee286          5
   3072647    2ee287          5
   3072649    2ee289          5
   3072650    2ee28a          5
   3072651    2ee28b          5
   3072652    2ee28c          5
   3072653    2ee28d          5
   3072654    2ee28e          5
   3072655    2ee28f          5
 20 rows selected. 
  
select object_name, object_id from dba_objects where object_name = 'TEST_TAB#U1';  

  OBJECT_NAME  OBJECT_ID
  ----------- ----------
  TEST_TAB#U1    2260907

select block#, to_char(block#, 'xxxxxxxx') block#_hex
      ,count(*) rows_per_block
from (
  select dbms_rowid.rowid_block_number(sys_op_lbid (2260907, 'L', t.rowid))  block#
        ,t.*
    from test_tab t
   where id is not null
)
group by block#
order by block#; 

    BLOCK# BLOCK#_HE ROWS_PER_BLOCK
  -------- --------- --------------
   3072660    2ee294              5
   3072661    2ee295              5
   3072662    2ee296              5
   3072663    2ee297              5
   3072664    2ee298              5
   3072665    2ee299              5
   3072666    2ee29a              5
   3072667    2ee29b              5
   3072668    2ee29c              5
   3072669    2ee29d              5
   3072670    2ee29e              5
   3072671    2ee29f              5
   3072673    2ee2a1              5
   3072674    2ee2a2              5
   3072675    2ee2a3              5
   3072676    2ee2a4              5
   3072677    2ee2a5              5
   3072678    2ee2a6              5
   3072679    2ee2a7              5
   3072680    2ee2a8              5
 20 rows selected

2. Buffer Read Access Path Tests

Trace Event 10200 shows all consistent read Blocks and their sequence. We will demonstrate the number of consistent read Blocks with different access path.

At first, display Table and Index information:


column object_name format a12
column object_id_hex format a20
column data_object_id_hex format a20

select object_name, object_id, data_object_id, 
       to_char(object_id, 'xxxxxxxx') object_id_hex, to_char(data_object_id, 'xxxxxxxx') data_object_id_hex
 from dba_objects where object_name in ('TEST_TAB', 'TEST_TAB#U1');


 OBJECT_NAME  OBJECT_ID  DATA_OBJECT_ID  OBJECT_ID_HEX  DATA_OBJECT_ID_HEX
 ----------- ---------- --------------- -------------- -------------------
 TEST_TAB       2260905         2260906         227fa9              227faa
 TEST_TAB#U1    2260907         2260907         227fab              227fab

2.1 USER ROWID without using Index

First collect ROWID for test, then make the test:


select /*+ user_rowid_no_index */ t.*, rowid rid from test_tab t 
where id in (1, 2, 3, 4, 5, 6, 7) order by t.id;

  ID VAL        RID
 --- ---------- ------------------
   1 ABCXXXXXXX AAIn+qAAAAALtl7AAA
   2 ABCXXXXXXX AAIn+qAAAAALtl7AAB
   3 ABCXXXXXXX AAIn+qAAAAALtl7AAC
   4 ABCXXXXXXX AAIn+qAAAAALtl7AAD
   5 ABCXXXXXXX AAIn+qAAAAALtl7AAE
   6 ABCXXXXXXX AAIn+qAAAAALtl8AAA
   7 ABCXXXXXXX AAIn+qAAAAALtl8AAB

alter session set events '10200 trace name context forever,level 10';

select /*+ user_rowid_no_index */ * from test_tab t 
where id in (1, 2, 3, 4, 5, 6, 7)
  and rowid in 
      ('AAIn+qAAAAALtl7AAA'
      ,'AAIn+qAAAAALtl7AAB'
      ,'AAIn+qAAAAALtl7AAC'
      ,'AAIn+qAAAAALtl7AAD'
      ,'AAIn+qAAAAALtl7AAE'
      ,'AAIn+qAAAAALtl8AAA'
      ,'AAIn+qAAAAALtl8AAB');

alter session set events '10200 trace name context off';

Rows (1st) Rows (avg) Rows (max)  Row Source Operation (0 Branch Block, 0 Leaf Blocks, 3 Table Blocks)
---------- ---------- ----------  ---------------------------------------------------
         7          7          7  INLIST ITERATOR  (cr=3 pr=0 pw=0 time=130 us)
         7          7          7   TABLE ACCESS BY USER ROWID TEST_TAB (cr=3 pr=0 pw=0 time=261 us cost=1 size=14 card=1)

ktrget2(): started for block  
 <0x07cf : 0x002ed97b> objd: 0x00227faa       <-- Table  Block 1
 <0x07cf : 0x002ed97b> objd: 0x00227faa       <-- Table  Block 1
 <0x07cf : 0x002ed97c> objd: 0x00227faa       <-- Table  Block 2

2.2 INDEX RANGE SCAN with INDEX ROWID BATCHED


alter session set events '10200 trace name context forever,level 10';

select /*+ index_range_scan */ * from test_tab t where id between 1 and 7;

alter session set events '10200 trace name context off';

Row Source Operation (1 Branch Block, 3 Leaf Blocks, 3 Table Blocks)
---------------------------------------------------
TABLE ACCESS BY INDEX ROWID BATCHED TEST_TAB (cr=7 pr=0 pw=0 time=210 us cost=3 size=98 card=7)
 INDEX RANGE SCAN TEST_TAB#U1 (cr=4 pr=0 pw=0 time=552 us cost=2 size=0 card=7)(object id 2260907)

ktrgtc2(): started for block
  <0x07cf : 0x002ee293> objd: 0x00227fab   <-- Branch Block 
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ed97b> objd: 0x00227faa       <-- Table  Block 1
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ed97b> objd: 0x00227faa       <-- Table  Block 1
  <0x07cf : 0x002ee295> objd: 0x00227fab     <-- Leaf  Block 2
  <0x07cf : 0x002ed97c> objd: 0x00227faa       <-- Table  Block 2

2.3 INDEX UNIQUE SCAN


select /*+ index_uniqe_scan */ * from test_tab t where id in (1, 2, 3, 4, 5, 6, 7);

Row Source Operation (2 Branch Block, 8 Leaf Blocks, 3 Table Blocks)
---------------------------------------------------
INLIST ITERATOR  (cr=13 pr=0 pw=0 time=218 us)
 TABLE ACCESS BY INDEX ROWID TEST_TAB (cr=13 pr=0 pw=0 time=812 us cost=3 size=98 card=7)
  INDEX UNIQUE SCAN TEST_TAB#U1 (cr=10 pr=0 pw=0 time=585 us cost=2 size=0 card=7)(object id 2260907)
  
ktrgtc2(): started for block  
  <0x07cf : 0x002ee293> objd: 0x00227fab   <-- Branch Block
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ed97b> objd: 0x00227faa       <-- Table  Block 1
  <0x07cf : 0x002ee293> objd: 0x00227fab   <-- Branch Block
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ed97b> objd: 0x00227faa       <-- Table  Block 1
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ee294> objd: 0x00227fab     <-- Leaf  Block 1
  <0x07cf : 0x002ee295> objd: 0x00227fab     <-- Leaf  Block 2
  <0x07cf : 0x002ed97c> objd: 0x00227faa       <-- Table  Block 2
  <0x07cf : 0x002ee295> objd: 0x00227fab     <-- Leaf  Block 2

2.4 INDEX ROWID GET

We can construct a pipelined functions to force INDEX ROWID GETs in a given frequency.


create type number_tab as table of number;
/

create or replace function id_pipelined(p_cnt number, p_sleep_seconds number := 0) 
return number_tab pipelined as
begin
  for i in 1..p_cnt loop
    if p_sleep_seconds > 0 then
      dbms_lock.sleep(p_sleep_seconds);  -- purposed delay, specially before producing first row
    end if;
    pipe row(i);
  end loop;
  return;
  exception
    when no_data_needed then
      raise;
    when others then
      dbms_output.put_line('others Handler');
      raise;
end;
/

select /*+ leading(c) cardinality(c 10) indx(t test_tab#u1) index_rowid_pipelined */ * 
  from table(cast (id_pipelined(7, 1.3) as number_tab)) c, test_tab t
 where c.column_value = t.id;
  
Row Source Operation (2 Branch Block, 8 Leaf Blocks, 7 Table Blocks)
---------------------------------------------------
NESTED LOOPS  (cr=17 pr=0 pw=0 time=253 us cost=35 size=160 card=10)
 NESTED LOOPS  (cr=10 pr=0 pw=0 time=7802413 us cost=35 size=160 card=10)
  COLLECTION ITERATOR PICKLER FETCH ID_PIPELINED (cr=0 pr=0 pw=0 time=7801325 us cost=30 size=20 card=10)
  INDEX UNIQUE SCAN TEST_TAB#U1 (cr=10 pr=0 pw=0 time=823 us cost=1 size=0 card=1)(object id 2260907)
 TABLE ACCESS BY INDEX ROWID TEST_TAB (cr=7 pr=0 pw=0 time=395 us cost=1 size=14 card=1)
  
ktrgtc2(): started for block 
 <0x07cf : 0x002ee293> objd: 0x00227fab  <-- Branch Block 
 <0x07cf : 0x002ee294> objd: 0x00227fab    <-- Leaf  Block 1
 <0x07cf : 0x002ed97b> objd: 0x00227faa      <-- Table  Block 1
 <0x07cf : 0x002ee293> objd: 0x00227fab  <-- Branch Block 
 <0x07cf : 0x002ee294> objd: 0x00227fab    <-- Leaf  Block 1
 <0x07cf : 0x002ed97b> objd: 0x00227faa      <-- Table  Block 1
 <0x07cf : 0x002ee294> objd: 0x00227fab    <-- Leaf  Block 1
 <0x07cf : 0x002ed97b> objd: 0x00227faa      <-- Table  Block 1
 <0x07cf : 0x002ee294> objd: 0x00227fab    <-- Leaf  Block 1
 <0x07cf : 0x002ed97b> objd: 0x00227faa      <-- Table  Block 1
 <0x07cf : 0x002ee294> objd: 0x00227fab    <-- Leaf  Block 1
 <0x07cf : 0x002ed97b> objd: 0x00227faa      <-- Table  Block 1
 <0x07cf : 0x002ee294> objd: 0x00227fab    <-- Leaf  Block 1
 <0x07cf : 0x002ee295> objd: 0x00227fab    <-- Leaf  Block 2
 <0x07cf : 0x002ed97c> objd: 0x00227faa      <-- Table  Block 2
 <0x07cf : 0x002ee295> objd: 0x00227fab    <-- Leaf  Block 2
 <0x07cf : 0x002ed97c> objd: 0x00227faa      <-- Table  Block 2

3. Discussions

3.1 Access Path

Above 4 tests showed:

  USER ROWID:         3 Blocks (0 Branch Block, 0 Leaf Blocks, 3 Table Blocks)
  INDEX RANGE SCAN:   7 Blocks (1 Branch Block, 3 Leaf Blocks, 3 Table Blocks)
  INDEX UNIQUE SCAN: 13 Blocks (2 Branch Block, 8 Leaf Blocks, 3 Table Blocks)
  INDEX ROWID GET:   17 Blocks (2 Branch Block, 8 Leaf Blocks, 7 Table Blocks)

The fourth test is the most expensive one because pipelined function is a row-by-row processing. The consumer starts processing only after producer produces it (the principle of "pipelined"). In the above example, consumer waits for "p_sleep_seconds" when receiving first row from producer. In the real applications, during "p_sleep_seconds", any modifications can occur, and in order to maintain Read Consistency from query start SCN, Oracle should use UNDO to clone Buffer, hence, 'latch: cache buffers chains' (see Demo in next Section).

In the aspect of Disk Read, row-by-row access is fulfilled by "db file sequential read" instead of more efficient multiblock (batch) "db file scattered read" or "db file parallel read" (to be discussed in next Blog: Oracle DB File Read Access Path and Cost).

Oracle 12c Database Data Cartridge Developer's Guide Section: 13 Using Pipelined and Parallel Table Functions wrote:

Rows from a collection returned by a table function can also be pipelined; this means that they are iteratively returned as they are produced, instead of being returned in a single batch after all processing of the table function's input is completed.

Pipelining enables a table function to return rows faster and can reduce the memory required to cache a table function's results.

It looks like that Pipelined function is a reflection of consuming row-by-row produced data as a memory (PGA) optimization of collection processing if intermediate result can be aggregated and consumed rows can be forgotten.

By the way, Pipelined table functions can be implemented in two ways: native PL/SQL and interface. In the above example, we use the native PL/SQL approach.

Oracle is a RDBMS based on relational model, where "relation" means "tabular", in mathematics, a set of objects (rows).

Blog Ask TOM: Slow processing on database using merge declared:
SQL is about sets, the best way to make SQL perform is to think SETS, not procedurally

The first test with "USER ROWID" is the cheapest fetching approach. In real application, it can be implemented by first caching ROWID of previously fetched ROWs, and subsequently, make the direct table fetch, and therefore bypass INDEX access. (See: Blog: Hot Block Identification and latch: cache buffers chains Section: "5. Index Hot Block Application Fix")

3.2 'latch: cache buffers chains' on Index Blocks

Index is made of a part of table columns. Usually it is more condensed than table, i.e, each Branch/Leaf Block can contain more rows than table Block. In other words, the same index Block is more frequently touched than table Block, hence more prone to 'latch: cache buffers chains' when multi sessions concurrently access it.

4. 'latch: cache buffers chains' Demo


create or replace procedure index_rowid_pipelined_get(p_cnt number, p_sleep_seconds number := 0) as
begin
  for c in (
    select /*+ leading(c) cardinality(c 10) indx(t test_tab#u1) index_rowid_pipelined */ * 
    from table(cast (id_pipelined(p_cnt, p_sleep_seconds) as number_tab)) c, test_tab t
    where c.column_value = t.id)
  loop
    null;
  end loop;
end;
/

create or replace procedure index_rowid_pipelined_get_jobs(p_job_cnt number, p_cnt number, p_sleep_seconds number := 0) as
  l_job_id pls_integer;
begin
  for i in 1.. p_job_cnt loop
    dbms_job.submit(l_job_id, 'begin while true loop index_rowid_pipelined_get('||p_cnt||', '||p_sleep_seconds||'); end loop; end;');
  end loop;
  commit;
end;    
/

Launch 6 Jobs on a UNIX with 6 physical Processors:


exec index_rowid_pipelined_get_jobs(6, 90, 0);

All Blocks in V$BH have status 'xcur'.

When modifying Table and no commit/rollback:


update test_tab set id = -id where abs(id) between 3 and 12;   -- Without commit/rollback

We can immediately observe 'latch: cache buffers chains' or 'buffer busy waits':


select * from v$session_wait where event in ('latch: cache buffers chains') order by event, sid;

select * from  v$session_wait_history where event in ('latch: cache buffers chains') order by event, sid;

In case of 'latch: cache buffers chains', one guess is that when

  V$SESSION_WAIT.state = 'WAITED SHORT TIME' (less than one centisecond), V$LATCH.Spin_Gets increased
  V$SESSION_WAIT.state = 'WAITED KNOWN TIME' (more than one centisecond), V$LATCH.Sleeps increased
  no entries in V$SESSION_WAIT, but V$BH.status='cr' Block Read,          V$LATCH.immediate_gets increased

but it is not clear how to confirm it.

By querying v$bh, we can see Consistent Read Copy of Blocks (cloned) marked as 'cr':


select d.object_name, data_object_id, b.status, b.* 
  from v$bh b, dba_objects d
 where b.objd = d.data_object_id
   and object_name in ('TEST_TAB', 'TEST_TAB#U1')
   --and b.status not in ('free')
   and b.status in ('xcur', 'cr')
order by b.status, object_name, block#;

In this case, Event 10201 ("consistent read undo application") can be used to trace CR Reads constructed by applying UNDO Blocks.

To stop all Jobs, execute following "clean_jobs".


-------------- clean_jobs --------------

create or replace procedure clean_jobs as
begin
  for c in (select job from dba_jobs) loop
    begin
       dbms_job.remove (c.job);
    exception when others then null;
    end;
    commit;
  end loop;

  for c in (select d.job, d.sid, (select serial# from v$session where sid = d.sid) ser 
              from dba_jobs_running d) loop
    begin
      execute immediate
             'alter system kill session '''|| c.sid|| ',' || c.ser|| ''' immediate';
      dbms_job.remove (c.job);
    exception when others then null;
    end;
    commit;
  end loop;
  
  -- select * from dba_jobs;
  -- select * from dba_jobs_running;
end;
/

-- exec clean_jobs; -- stop all jobs