February 2, 2010
(Back to the Previous Post in the Series)
In the first blog article in this series, we explored several methods for generating execution plans, specifcally, we saw how to:
- Display the actual (TYPICAL) execution plan for a SQL statement with estimated cardinality and costs.
- Display the actual (ALLSTATS LAST) execution plan for a SQL statement using the GATHER_PLAN_STATISTICS hint to compare the estimated cardinality with the actual number of rows returned from each operation in the plan, and obtain the actual final execution time (timing of the intermediate steps might not be close to actual time).
- Display the actual (ALLSTATS LAST) execution plan for a SQL statement with the STATISTICS_LEVEL parameter set to ALL at the session level to obtain the final execution time and close estimates for each of the intermediate steps (in some cases, a significant increase in execution time was experienced).
- Compare the changes in the actual execution plan when changing the OPTIMIZER_MODE parameter between ALL_ROWS, FIRST_ROWS_1, and CHOOSE. Note that setting the OPTIMIZER_MODE to FIRST_ROWS_1 may have caused the optimizer to under-estimate the number of rows that will be returned, as shown in the execution plan.
- Display a possibly correct execution plan with SQL*Plus’ AUTOTRACE and with EXPLAIN PLAN, and explore some of the problems that might be encountered when relying on those plans.
- See the effects of bind peeking on an execution plan.
- Retrieve the execution plan by setting event 10132.
- Read the execution plan directly from a 10046 trace file.
- Process the trace file with TKPROF, and one of the potential problems caused by telling TKPROF to generate explain plans.
The first blog article in this article series used the 64 bit version of Oracle Database 10.2.0.2 (with OPTIMIZER_FEATURES_ENABLE set to 10.2.0.1). It might be interesting to compare the output of the 64 bit version of Oracle 11.2.0.1 with that of Oracle 10.2.0.2 (note that the server running 11.2.0.1 is a bit faster than the server running 10.2.0.2). Comparing the outputs might answer the question of whether or not there is a bug in Oracle 10.2.0.2, for instance, relating the the cardinality estimates displayed in execution plans when the OPTIMIZER_MODE was set to FIRST_ROWS_1 – if the same output is present in 11.2.0.1, that probably means that the optimizer is working as designed. Obviously, this article will use the same table definition for T1, T2, T3, and T4 that were specified in the first article.
First, we will generate an execution plan for a SQL statement with the OPTIMIZER_MODE set to ALL_ROWS:
SET LINESIZE 150 ALTER SESSION SET OPTIMIZER_MODE='ALL_ROWS'; SELECT T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10; C1 C2 --- ---------- 1 1AAAAAAAAA 2 2AAAAAAAAA 3 3AAAAAAAAA 4 4AAAAAAAAA 5 5AAAAAAAAA 6 6AAAAAAAAA 7 7AAAAAAAAA 8 8AAAAAAAAA 9 9AAAAAAAAA 10 10AAAAAAAA
The typical way to display an execution plan for a SQL statement that was just executed is as follows (note that the third parameter, the FORMAT parameter, could have also been set to NULL to produce the same result):
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'TYPICAL'));
SQL_ID 9dq71tc7vasgu, child number 0
-------------------------------------
SELECT T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE
T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10
Plan hash value: 985299768
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 10219 (100)| |
|* 1 | HASH JOIN | | 9 | 2394 | 10219 (1)| 00:02:03 |
|* 2 | TABLE ACCESS FULL | T3 | 10 | 50 | 10214 (1)| 00:02:03 |
| 3 | TABLE ACCESS BY INDEX ROWID| T1 | 10 | 2610 | 4 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | SYS_C0017307 | 10 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."C1"="T3"."C1")
2 - filter(("T3"."C1"<=10 AND "T3"."C1">=1))
4 - access("T1"."C1">=1 AND "T1"."C1"<=10)
In the above, note that the calculated cost of the execution plan is 10219 (NOWORKLOAD CPU statistics). Now, a little more experimentation, setting OPTIMIZER_MODE to FIRST_ROWS (Oracle assumes that only the first row will be retrieved, and the rest will likely be discarded):
ALTER SESSION SET OPTIMIZER_MODE='FIRST_ROWS_1';
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN 1 AND 10;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'TYPICAL'));
SQL_ID 9dq71tc7vasgu, child number 1
-------------------------------------
SELECT T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE
T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10
Plan hash value: 2322422896
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 3073 (100)| |
| 1 | NESTED LOOPS | | | | | |
| 2 | NESTED LOOPS | | 2 | 532 | 3073 (1)| 00:00:37 |
|* 3 | TABLE ACCESS FULL | T3 | 7 | 35 | 3066 (1)| 00:00:37 |
|* 4 | INDEX UNIQUE SCAN | SYS_C0017307 | 1 | | 1 (0)| 00:00:01 |
| 5 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 261 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(("T3"."C1"<=10 AND "T3"."C1">=1))
4 - access("T1"."C1"="T3"."C1")
filter(("T1"."C1"<=10 AND "T1"."C1">=1))
In the above, notice that by changing the optimizer mode from ALL_ROWS to FIRST_ROWS_1 a new execution plan was created (the child number has increased by 1 and the Plan hash value has changed) that now uses two nested loops joins (first joining the index for table T1 with the table T3), rather than a hash join, and the calculated cost has decreased to 3073. You might be wondering why Oracle did not pick this execution plan with the nested loops joins, rather than the hash join when the OPTIMIZER_MODE was set to ALL_ROWS, since this plan has a lower calculated cost – note that Oracle 11.2.0.1 is estimating the same number of rows to be returned as Oracle 10.2.0.2. But, we really do not know anything about performance from just looking at the above plans. So, let’s repeat the test again, changing the SQL statement so that we are able to pass in ‘ALLSTATS LAST’ as the format parameter for the DBMS_XPLAN call:
ALTER SESSION SET OPTIMIZER_MODE='ALL_ROWS';
SELECT /*+ GATHER_PLAN_STATISTICS */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN 1 AND 10;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID ddnbt67ftu9ds, child number 0
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10
Plan hash value: 985299768
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.07 | 37170 | | | |
|* 1 | HASH JOIN | | 1 | 9 | 10 |00:00:00.07 | 37170 | 1452K| 1452K| 1365K (0)|
|* 2 | TABLE ACCESS FULL | T3 | 1 | 10 | 10 |00:00:00.01 | 37164 | | | |
| 3 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 10 | 10 |00:00:00.01 | 6 | | | |
|* 4 | INDEX RANGE SCAN | SYS_C0017307 | 1 | 10 | 10 |00:00:00.01 | 4 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."C1"="T3"."C1")
2 - filter(("T3"."C1"<=10 AND "T3"."C1">=1))
4 - access("T1"."C1">=1 AND "T1"."C1"<=10)
The query completed in 0.07 seconds, retrieved 10 rows, and used 1,365 * 1,024 bytes of memory during the in-memory hash join. No blocks were read from disk (the query was previously executed, and the table and index blocks managed to remain in the buffer cache).
ALTER SESSION SET OPTIMIZER_MODE='FIRST_ROWS_1';
SELECT /*+ GATHER_PLAN_STATISTICS */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN 1 AND 10;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID ddnbt67ftu9ds, child number 1
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10
Plan hash value: 2322422896
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.01 | 37189 |
| 1 | NESTED LOOPS | | 1 | | 10 |00:00:00.01 | 37189 |
| 2 | NESTED LOOPS | | 1 | 2 | 10 |00:00:00.01 | 37179 |
|* 3 | TABLE ACCESS FULL | T3 | 1 | 7 | 10 |00:00:00.01 | 37165 |
|* 4 | INDEX UNIQUE SCAN | SYS_C0017307 | 10 | 1 | 10 |00:00:00.01 | 14 |
| 5 | TABLE ACCESS BY INDEX ROWID| T1 | 10 | 1 | 10 |00:00:00.01 | 10 |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(("T3"."C1"<=10 AND "T3"."C1">=1))
4 - access("T1"."C1"="T3"."C1")
filter(("T1"."C1"<=10 AND "T1"."C1">=1))
As shown above, with the FIRST_ROWS_1 optimizer mode, the query completed in 0.01 seconds.
The GATHER_PLAN_STATISTICS hint is helpful because it allows DBMS_XPLAN to output the actual execution statistics for the previous SQL statement. If you do not want to use that hint, it is also possible to change the STATISTICS_LEVEL to ALL at the session level level, as the following demonstrates:
ALTER SESSION SET OPTIMIZER_MODE='ALL_ROWS';
ALTER SESSION SET STATISTICS_LEVEL='ALL';
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN 1 AND 10;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID 9dq71tc7vasgu, child number 2
-------------------------------------
SELECT T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE
T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10
Plan hash value: 985299768
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.07 | 37170 | | | |
|* 1 | HASH JOIN | | 1 | 9 | 10 |00:00:00.07 | 37170 | 1452K| 1452K| 1424K (0)|
|* 2 | TABLE ACCESS FULL | T3 | 1 | 10 | 10 |00:00:00.07 | 37164 | | | |
| 3 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 10 | 10 |00:00:00.01 | 6 | | | |
|* 4 | INDEX RANGE SCAN | SYS_C0017307 | 1 | 10 | 10 |00:00:00.01 | 4 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."C1"="T3"."C1")
2 - filter(("T3"."C1"<=10 AND "T3"."C1">=1))
4 - access("T1"."C1">=1 AND "T1"."C1"<=10)
Note that the change to the STATISTICS_LEVEL caused another hard parse for the SQL statement, and the child number is now listed as 2. For some reason, the amount of memory required for the hash join has increased slightly. If you look closely, you will also notice that the execution time remained the same before and after the changing of the STATISTICS_LEVEL parameter.
Let’s try again with the FIRST_ROWS_1 value specified for OPTIMIZER_MODE.
ALTER SESSION SET OPTIMIZER_MODE='FIRST_ROWS_1';
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN 1 AND 10;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID 9dq71tc7vasgu, child number 3
-------------------------------------
SELECT T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE
T1.C1=T3.C1 AND T1.C1 BETWEEN 1 AND 10
Plan hash value: 2322422896
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.07 | 37189 |
| 1 | NESTED LOOPS | | 1 | | 10 |00:00:00.07 | 37189 |
| 2 | NESTED LOOPS | | 1 | 2 | 10 |00:00:00.07 | 37179 |
|* 3 | TABLE ACCESS FULL | T3 | 1 | 7 | 10 |00:00:00.07 | 37165 |
|* 4 | INDEX UNIQUE SCAN | SYS_C0017307 | 10 | 1 | 10 |00:00:00.01 | 14 |
| 5 | TABLE ACCESS BY INDEX ROWID| T1 | 10 | 1 | 10 |00:00:00.01 | 10 |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter(("T3"."C1"<=10 AND "T3"."C1">=1))
4 - access("T1"."C1"="T3"."C1")
filter(("T1"."C1"<=10 AND "T1"."C1">=1))
Note again that there was a hard parse, and the child number increased by 1. This time, rather than completing in 0.01 seconds, the query required 0.07 seconds.
Let’s try again, this time using bind variables rather than constants (literals):
ALTER SESSION SET OPTIMIZER_MODE='ALL_ROWS';
ALTER SESSION SET STATISTICS_LEVEL='TYPICAL';
VARIABLE N1 NUMBER
VARIABLE N2 NUMBER
EXEC :N1:=1
EXEC :N2:=10
SELECT /*+ GATHER_PLAN_STATISTICS */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID cvq22z77c8fww, child number 0
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2
Plan hash value: 744086632
-----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.07 | 37170 | | | |
|* 1 | FILTER | | 1 | | 10 |00:00:00.07 | 37170 | | | |
|* 2 | HASH JOIN | | 1 | 9 | 10 |00:00:00.07 | 37170 | 1452K| 1452K| 1424K (0)|
|* 3 | TABLE ACCESS FULL | T3 | 1 | 10 | 10 |00:00:00.01 | 37164 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 10 | 10 |00:00:00.01 | 6 | | | |
|* 5 | INDEX RANGE SCAN | SYS_C0017307 | 1 | 10 | 10 |00:00:00.01 | 4 | | | |
-----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
2 - access("T1"."C1"="T3"."C1")
3 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1">=:N1 AND "T1"."C1"<=:N2)
Same execution plan as we saw earlier, with the same cardinality estimates due to bind variable peeking, except now there is a FILTER operation AT ID 1 with a predicate that states that the N1 bind variable must be less than or equal to the N2 bind variable – this is an automatically generated predicate caused by the BETWEEN syntax in the SQL statement.
Now we change the value of the second bind variable to a much larger value:
EXEC :N2:=500000
SELECT /*+ GATHER_PLAN_STATISTICS */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID cvq22z77c8fww, child number 0
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2
Plan hash value: 744086632
--------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 500K|00:00:08.01 | 123K| 12985 | | | |
|* 1 | FILTER | | 1 | | 500K|00:00:08.01 | 123K| 12985 | | | |
|* 2 | HASH JOIN | | 1 | 9 | 500K|00:00:07.72 | 123K| 12985 | 15M| 3723K| 23M (0)|
|* 3 | TABLE ACCESS FULL | T3 | 1 | 10 | 500K|00:00:00.11 | 37164 | 0 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 10 | 500K|00:00:06.72 | 86064 | 12985 | | | |
|* 5 | INDEX RANGE SCAN | SYS_C0017307 | 1 | 10 | 500K|00:00:00.43 | 34211 | 634 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
2 - access("T1"."C1"="T3"."C1")
3 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1">=:N1 AND "T1"."C1"<=:N2)
In the above, notice that there was no hard parse (same SQL_ID and child number as we saw earlier), and the E-Rows column is the same for the two DBMS_XPLAN outputs. The Used-Tmp column was not displayed (11.2.0.1 had a larger value for PGA_AGGREGATE_TARGET specified). Let’s repeat the test with the altered OPTIMIZER_MODE:
EXEC :N2:=10
ALTER SESSION SET OPTIMIZER_MODE='FIRST_ROWS_1';
SELECT /*+ GATHER_PLAN_STATISTICS */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID cvq22z77c8fww, child number 1
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2
Plan hash value: 1340904442
--------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.01 | 37189 |
|* 1 | FILTER | | 1 | | 10 |00:00:00.01 | 37189 |
| 2 | NESTED LOOPS | | 1 | | 10 |00:00:00.01 | 37189 |
| 3 | NESTED LOOPS | | 1 | 2 | 10 |00:00:00.01 | 37179 |
|* 4 | TABLE ACCESS FULL | T3 | 1 | 7 | 10 |00:00:00.01 | 37165 |
|* 5 | INDEX UNIQUE SCAN | SYS_C0017307 | 10 | 1 | 10 |00:00:00.01 | 14 |
| 6 | TABLE ACCESS BY INDEX ROWID| T1 | 10 | 1 | 10 |00:00:00.01 | 10 |
--------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
4 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1"="T3"."C1")
filter(("T1"."C1"<=:N2 AND "T1"."C1">=:N1))
EXEC :N2:=500000
SELECT /*+ GATHER_PLAN_STATISTICS */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID cvq22z77c8fww, child number 1
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2
Plan hash value: 1340904442
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 500K|00:00:03.77 | 782K| 1 |
|* 1 | FILTER | | 1 | | 500K|00:00:03.77 | 782K| 1 |
| 2 | NESTED LOOPS | | 1 | | 500K|00:00:03.52 | 782K| 1 |
| 3 | NESTED LOOPS | | 1 | 2 | 500K|00:00:02.00 | 282K| 1 |
|* 4 | TABLE ACCESS FULL | T3 | 1 | 7 | 500K|00:00:00.39 | 70498 | 0 |
|* 5 | INDEX UNIQUE SCAN | SYS_C0017307 | 500K| 1 | 500K|00:00:00.95 | 212K| 1 |
| 6 | TABLE ACCESS BY INDEX ROWID| T1 | 500K| 1 | 500K|00:00:00.86 | 500K| 0 |
-----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
4 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1"="T3"."C1")
filter(("T1"."C1"<=:N2 AND "T1"."C1">=:N1))
3.77 seconds to execute the SQL statement with OPTIMIZER_MODE set to FIRST_ROWS_1 and 8.01 seconds with OPTIMIZER_MODE set to ALL_ROWS (and 0.01 seconds compared to 0.07 seconds for the execution retrieving 10 rows). Obviously, this means that we should be running with OPTIMIZER_MODE set to FIRST_ROWS_1 if we want to optimize performance, right? Well, in short No. Maybe this will be something that will be investigated in a later blog article.
Now, turning to the unanalyzed tables. We modify the original SQL statement using bind variables to point to the two tables without up-to-date statistics on the tables and their indexes:
EXEC :N2:=10
ALTER SESSION SET OPTIMIZER_MODE='ALL_ROWS';
SELECT /*+ GATHER_PLAN_STATISTICS */
T4.C1,
SUBSTR(T2.C2,1,10) C2
FROM
T4,
T2
WHERE
T2.C1=T4.C1
AND T2.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID 94bv1jwkzcc38, child number 0
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T4.C1, SUBSTR(T2.C2,1,10) C2
FROM T4, T2 WHERE T2.C1=T4.C1 AND T2.C1 BETWEEN :N1 AND :N2
Plan hash value: 2134503202
--------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.01 | 37169 | 37128 | | | |
|* 1 | FILTER | | 1 | | 10 |00:00:00.01 | 37169 | 37128 | | | |
|* 2 | HASH JOIN | | 1 | 10 | 10 |00:00:00.01 | 37169 | 37128 | 711K| 711K| 1116K (0)|
| 3 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 10 | 10 |00:00:00.01 | 4 | 1 | | | |
|* 4 | INDEX RANGE SCAN | SYS_C0017308 | 1 | 10 | 10 |00:00:00.01 | 3 | 0 | | | |
|* 5 | TABLE ACCESS FULL | T4 | 1 | 1855 | 10 |00:00:00.01 | 37165 | 37127 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
2 - access("T2"."C1"="T4"."C1")
4 - access("T2"."C1">=:N1 AND "T2"."C1"<=:N2)
5 - filter(("T4"."C1">=:N1 AND "T4"."C1"<=:N2))
Note
-----
- dynamic sampling used for this statement (level=2)
The only change here is that Oracle is now estimating that 10 rows will be returned rather than the 9 we saw earlier, and the note below the predicate information heading that states that dynamic sampling at level 2 was used, oh – and the order of the row sources directly below the HASH JOIN line in the plan has changed just like on Oracle 10.2.0.2 (is this a problem?). Maybe dynamic sampling will be a topic for another blog article, but the topic is discussed in various books and articles on the Internet.
ALTER SESSION SET OPTIMIZER_MODE='FIRST_ROWS_1';
SELECT /*+ GATHER_PLAN_STATISTICS */
T4.C1,
SUBSTR(T2.C2,1,10) C2
FROM
T4,
T2
WHERE
T2.C1=T4.C1
AND T2.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID 94bv1jwkzcc38, child number 1
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T4.C1, SUBSTR(T2.C2,1,10) C2
FROM T4, T2 WHERE T2.C1=T4.C1 AND T2.C1 BETWEEN :N1 AND :N2
Plan hash value: 2134503202
--------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.01 | 37169 | 37129 | | | |
|* 1 | FILTER | | 1 | | 10 |00:00:00.01 | 37169 | 37129 | | | |
|* 2 | HASH JOIN | | 1 | 10 | 10 |00:00:00.01 | 37169 | 37129 | 711K| 711K| 1147K (0)|
| 3 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 10 | 10 |00:00:00.01 | 4 | 1 | | | |
|* 4 | INDEX RANGE SCAN | SYS_C0017308 | 1 | 10 | 10 |00:00:00.01 | 3 | 0 | | | |
|* 5 | TABLE ACCESS FULL | T4 | 1 | 1855 | 10 |00:00:00.01 | 37165 | 37128 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
2 - access("T2"."C1"="T4"."C1")
4 - access("T2"."C1">=:N1 AND "T2"."C1"<=:N2)
5 - filter(("T4"."C1">=:N1 AND "T4"."C1"<=:N2))
Note
-----
- dynamic sampling used for this statement (level=2)
OK, this plan changed a bit from when the SQL statement referenced tables T1 and T3. The execution plan is no longer using a nested loops join – in fact it is using the same plan as was used when the OPTIMIZER_MODE was set to ALL_ROWS (just like on Oracle 10.2.0.2).
Oracle 9i and earlier Oracle releases had a default OPTIMIZER_MODE of CHOOSE, so let’s see what happens when we use that optimizer mode with the same two tables:
ALTER SESSION SET OPTIMIZER_MODE='CHOOSE';
SELECT /*+ GATHER_PLAN_STATISTICS */
T4.C1,
SUBSTR(T2.C2,1,10) C2
FROM
T4,
T2
WHERE
T2.C1=T4.C1
AND T2.C1 BETWEEN :N1 AND :N2;
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ALLSTATS LAST'));
SQL_ID 94bv1jwkzcc38, child number 2
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T4.C1, SUBSTR(T2.C2,1,10) C2
FROM T4, T2 WHERE T2.C1=T4.C1 AND T2.C1 BETWEEN :N1 AND :N2
Plan hash value: 1136565161
-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | A-Rows | A-Time | Buffers | Reads |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 10 |00:00:00.03 | 386K| 39012 |
| 1 | NESTED LOOPS | | 1 | 10 |00:00:00.03 | 386K| 39012 |
| 2 | NESTED LOOPS | | 1 | 10 |00:00:00.03 | 386K| 39011 |
| 3 | TABLE ACCESS FULL | T4 | 1 | 1000K|00:00:00.38 | 37165 | 37133 |
|* 4 | INDEX UNIQUE SCAN | SYS_C0017308 | 1000K| 10 |00:00:02.30 | 348K| 1878 |
| 5 | TABLE ACCESS BY INDEX ROWID| T2 | 10 | 10 |00:00:00.01 | 10 | 1 |
-------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T2"."C1"="T4"."C1")
filter(("T2"."C1"<=:N2 AND "T2"."C1">=:N1))
Note
-----
- rule based optimizer used (consider using cbo)
Note that the Note section indicates that the rule based optimizer was used (just like with Oracle 10.2.0.2), even though the documentation for Oracle 10.2 states that as of Oracle 10.1 the RULE based optimizer is no longer supported. Also note that the execution plan is now using a nested loops join, the FILTER operation no longer appears, and the full table scan is again first joined with the index for the other table, just as it was when the OPTIMIZER_MODE was set to FIRST_ROWS_1 and the query accessed tables T1 and T3.
Let’s say that we are bored, and did not read chapter 15 by Pete Finnigan in the “Expert Oracle Practices” book… assume that column C1 contains a credit card number. Now for an experiment, we will retrieve all child cursors for SQL_ID cvq22z77c8fww with the bind variables that were submitted on the initial hard parse – be careful with who has access to this feature in a production environment, as it could expose sensitive information:
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY_CURSOR('cvq22z77c8fww',NULL,'ALLSTATS LAST +PEEKED_BINDS'));
SQL_ID cvq22z77c8fww, child number 0
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2
Plan hash value: 744086632
--------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 500K|00:00:08.01 | 123K| 12985 | | | |
|* 1 | FILTER | | 1 | | 500K|00:00:08.01 | 123K| 12985 | | | |
|* 2 | HASH JOIN | | 1 | 9 | 500K|00:00:07.72 | 123K| 12985 | 15M| 3723K| 23M (0)|
|* 3 | TABLE ACCESS FULL | T3 | 1 | 10 | 500K|00:00:00.11 | 37164 | 0 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 10 | 500K|00:00:06.72 | 86064 | 12985 | | | |
|* 5 | INDEX RANGE SCAN | SYS_C0017307 | 1 | 10 | 500K|00:00:00.43 | 34211 | 634 | | | |
--------------------------------------------------------------------------------------------------------------------------------------------
Peeked Binds (identified by position):
--------------------------------------
1 - (NUMBER): 1
2 - (NUMBER): 10
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
2 - access("T1"."C1"="T3"."C1")
3 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1">=:N1 AND "T1"."C1"<=:N2)
-
SQL_ID cvq22z77c8fww, child number 1
-------------------------------------
SELECT /*+ GATHER_PLAN_STATISTICS */ T3.C1, SUBSTR(T1.C2,1,10) C2
FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2
Plan hash value: 1340904442
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 500K|00:00:03.77 | 782K| 1 |
|* 1 | FILTER | | 1 | | 500K|00:00:03.77 | 782K| 1 |
| 2 | NESTED LOOPS | | 1 | | 500K|00:00:03.52 | 782K| 1 |
| 3 | NESTED LOOPS | | 1 | 2 | 500K|00:00:02.00 | 282K| 1 |
|* 4 | TABLE ACCESS FULL | T3 | 1 | 7 | 500K|00:00:00.39 | 70498 | 0 |
|* 5 | INDEX UNIQUE SCAN | SYS_C0017307 | 500K| 1 | 500K|00:00:00.95 | 212K| 1 |
| 6 | TABLE ACCESS BY INDEX ROWID| T1 | 500K| 1 | 500K|00:00:00.86 | 500K| 0 |
-----------------------------------------------------------------------------------------------------------------
Peeked Binds (identified by position):
--------------------------------------
1 - (NUMBER): 1
2 - (NUMBER): 10
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(:N1<=:N2)
4 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1"="T3"."C1")
filter(("T1"."C1"<=:N2 AND "T1"."C1">=:N1))
To learn more about DBMS_XPLAN.DISPLAY_CURSOR, see the documentation.
So, what if we want to know why the child cursors were created? We could do something like this:
DESC V$SQL_SHARED_CURSOR SET LINESIZE 200 SET HEADING ON BREAK ON SQL_ID SKIP 1 SELECT * FROM V$SQL_SHARED_CURSOR WHERE SQL_ID='94bv1jwkzcc38'; SQL_ID ADDRESS CHILD_ADDRESS CHILD_NUMBER USOOSLFEBPISTABDLTBIIRLIOEMUTNFAITDLDBPCSCPTMBMROPMFLPLAFLRLHPB ------------- ---------------- ---------------- ------------ --------------------------------------------------------------- 94bv1jwkzcc38 0000000257BD5498 0000000257E2E5B0 0 NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNN 0000000257BD5498 0000000257DC8F18 1 NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNYNNNNNNNNNNNNNN 0000000257BD5498 0000000257C8C3F8 2 NNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNNYNNNNNNNNNNNNNN
The above shows that the child cursors were created because of an optimizer mode mismatch (yes, we changed the OPTIMIZER_MODE). We could also check the bind variable definitions (not needed in this case because we used the undocumented PEEKED_BINDS format parameter with DBMS_XPLAN to see most of this information):
SELECT S.CHILD_NUMBER CN, SBM.* FROM V$SQL_BIND_METADATA SBM, V$SQL S WHERE S.SQL_ID='94bv1jwkzcc38' AND S.CHILD_ADDRESS=SBM.ADDRESS ORDER BY S.CHILD_NUMBER, SBM.POSITION; CN ADDRESS POSITION DATATYPE MAX_LENGTH ARRAY_LEN BIND_NAME -- ---------------- ---------- ---------- ---------- ---------- --------- 0 0000000257E2E5B0 1 2 22 0 N1 0 0000000257E2E5B0 2 2 22 0 N2 1 0000000257DC8F18 1 2 22 0 N1 1 0000000257DC8F18 2 2 22 0 N2 2 0000000257C8C3F8 1 2 22 0 N1 2 0000000257C8C3F8 2 2 22 0 N2
OK, now that we have moved off on a tangent, let’s return again to the topic of viewing execution plans. The above examples show the actual execution plan that was used, which may be different from that produced by explain plan. So, for fun, let’s look at the EXPLAIN PLAN FOR syntax (DBMS_XPLAN.DISPLAY is valid on Oracle 9.2.0.1 and higher):
ALTER SYSTEM FLUSH SHARED_POOL;
ALTER SESSION SET OPTIMIZER_MODE='ALL_ROWS';
EXPLAIN PLAN FOR
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN 1 AND 10;
Explained.
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY());
Plan hash value: 985299768
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 9 | 2394 | 10219 (1)| 00:02:03 |
|* 1 | HASH JOIN | | 9 | 2394 | 10219 (1)| 00:02:03 |
|* 2 | TABLE ACCESS FULL | T3 | 10 | 50 | 10214 (1)| 00:02:03 |
| 3 | TABLE ACCESS BY INDEX ROWID| T1 | 10 | 2610 | 4 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | SYS_C0017307 | 10 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."C1"="T3"."C1")
2 - filter("T3"."C1"<=10 AND "T3"."C1">=1)
4 - access("T1"."C1">=1 AND "T1"."C1"<=10)
The above plan appears to be identical to that of the first of the actual plans. Now a test with bind variables:
VARIABLE N1 NUMBER
VARIABLE N2 NUMBER
EXEC :N1:=1
EXEC :N2:=10
EXPLAIN PLAN FOR
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
Explained.
SELECT
*
FROM
TABLE(DBMS_XPLAN.DISPLAY());
Plan hash value: 744086632
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2499 | 649K| 10397 (1)| 00:02:05 |
|* 1 | FILTER | | | | | |
|* 2 | HASH JOIN | | 2499 | 649K| 10397 (1)| 00:02:05 |
|* 3 | TABLE ACCESS FULL | T3 | 2500 | 12500 | 10219 (1)| 00:02:03 |
| 4 | TABLE ACCESS BY INDEX ROWID| T1 | 2500 | 637K| 178 (0)| 00:00:03 |
|* 5 | INDEX RANGE SCAN | SYS_C0017307 | 4500 | | 11 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER(:N1)<=TO_NUMBER(:N2))
2 - access("T1"."C1"="T3"."C1")
3 - filter("T3"."C1">=TO_NUMBER(:N1) AND "T3"."C1"<=TO_NUMBER(:N2))
5 - access("T1"."C1">=TO_NUMBER(:N1) AND "T1"."C1"<=TO_NUMBER(:N2))
Notice in the above that there are TO_NUMBER entries surrounding each of the bind variables in the Predicate Information section, even though those bind variables were declared as a NUMBER data type. The cost has also increased a bit, and the estimated number of rows jumped from 9 to 2499 (bind peeking did not take place).
Let’s use AUTOTRACE to see the execution plan (AUTOTRACE starting in Oracle 10.1.0.1 uses DBMS_XPLAN to output the formatted execution plan).
SET AUTOTRACE TRACEONLY EXPLAIN
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
Plan hash value: 744086632
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2499 | 649K| 10397 (1)| 00:02:05 |
|* 1 | FILTER | | | | | |
|* 2 | HASH JOIN | | 2499 | 649K| 10397 (1)| 00:02:05 |
|* 3 | TABLE ACCESS FULL | T3 | 2500 | 12500 | 10219 (1)| 00:02:03 |
| 4 | TABLE ACCESS BY INDEX ROWID| T1 | 2500 | 637K| 178 (0)| 00:00:03 |
|* 5 | INDEX RANGE SCAN | SYS_C0017307 | 4500 | | 11 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER(:N1)<=TO_NUMBER(:N2))
2 - access("T1"."C1"="T3"."C1")
3 - filter("T3"."C1">=TO_NUMBER(:N1) AND "T3"."C1"<=TO_NUMBER(:N2))
5 - access("T1"."C1">=TO_NUMBER(:N1) AND "T1"."C1"<=TO_NUMBER(:N2))
OK, the same as the previous example on Oracle 10.1.0.1 and above, complete with the incorrect Predicate Information section. Again, displaying the runtime statistics and explain plan:
SET AUTOTRACE TRACEONLY STATISTICS EXPLAIN
SELECT
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2;
10 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 744086632
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2499 | 649K| 10397 (1)| 00:02:05 |
|* 1 | FILTER | | | | | |
|* 2 | HASH JOIN | | 2499 | 649K| 10397 (1)| 00:02:05 |
|* 3 | TABLE ACCESS FULL | T3 | 2500 | 12500 | 10219 (1)| 00:02:03 |
| 4 | TABLE ACCESS BY INDEX ROWID| T1 | 2500 | 637K| 178 (0)| 00:00:03 |
|* 5 | INDEX RANGE SCAN | SYS_C0017307 | 4500 | | 11 (0)| 00:00:01 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(TO_NUMBER(:N1)<=TO_NUMBER(:N2))
2 - access("T1"."C1"="T3"."C1")
3 - filter("T3"."C1">=TO_NUMBER(:N1) AND "T3"."C1"<=TO_NUMBER(:N2))
5 - access("T1"."C1">=TO_NUMBER(:N1) AND "T1"."C1"<=TO_NUMBER(:N2))
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
37170 consistent gets
7760 physical reads
0 redo size
812 bytes sent via SQL*Net to client
523 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
10 rows processed
Note that the above plan is not necessarily the actual plan, even though we are looking at the actual runtime statistics. This could be confusing since we are seeing a rough guess for an execution plan with the actual execution statistics.
For more information about Using EXPLAIN PLAN, see the documentation.
Now, another test. This time we will instruct Oracle to write the actual execution plan to a trace file every time Oracle must perform a hard parse. We will force a hard parse by adding a comment to the SQL statement:
ALTER SESSION SET TRACEFILE_IDENTIFIER = '10132_HARD_PARSE'; ALTER SESSION SET EVENTS '10132 TRACE NAME CONTEXT FOREVER, LEVEL 1'; SELECT /* TEST */ T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2; ALTER SESSION SET EVENTS '10132 TRACE NAME CONTEXT OFF';
If we take a look at the output in the trace file, we might see something like this:
user_id=68 user_name=TESTUSER module=SQL*Plus action=
sql_id=3g3fc5qyju0j3 plan_hash_value=744086632 problem_type=3
----- Current SQL Statement for this session (sql_id=3g3fc5qyju0j3) -----
SELECT /* TEST */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2
sql_text_length=119
sql=SELECT /* TEST */
T3.C1,
SUBSTR(T1.C2,1,10) C2
FROM
T3,
T1
WHERE
T1.C1=T3.C1
AND T1.C1 BETWEEN :N1 AND :N2
----- Explain Plan Dump -----
----- Plan Table -----
============
Plan Table
============
-----------------------------------------------------+-----------------------------------+
| Id | Operation | Name | Rows | Bytes | Cost | Time |
-----------------------------------------------------+-----------------------------------+
| 0 | SELECT STATEMENT | | | | 10K | |
| 1 | FILTER | | | | | |
| 2 | HASH JOIN | | 9 | 2394 | 10K | 00:02:03 |
| 3 | TABLE ACCESS FULL | T3 | 10 | 50 | 10K | 00:02:03 |
| 4 | TABLE ACCESS BY INDEX ROWID | T1 | 10 | 2610 | 4 | 00:00:01 |
| 5 | INDEX RANGE SCAN | SYS_C0017307| 10 | | 3 | 00:00:01 |
-----------------------------------------------------+-----------------------------------+
Predicate Information:
----------------------
1 - filter(:N1<=:N2)
2 - access("T1"."C1"="T3"."C1")
3 - filter(("T3"."C1"<=:N2 AND "T3"."C1">=:N1))
5 - access("T1"."C1">=:N1 AND "T1"."C1"<=:N2)
Content of other_xml column
===========================
db_version : 11.2.0.1
parse_schema : TESTUSER
plan_hash : 744086632
plan_hash_2 : 3461135235
Peeked Binds
============
Bind variable information
position=1
datatype(code)=2
datatype(string)=NUMBER
precision=0
scale=0
max length=22
value=1
Bind variable information
position=2
datatype(code)=2
datatype(string)=NUMBER
precision=0
scale=0
max length=22
value=10
Outline Data:
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('11.2.0.1')
DB_VERSION('11.2.0.1')
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
FULL(@"SEL$1" "T3"@"SEL$1")
INDEX_RS_ASC(@"SEL$1" "T1"@"SEL$1" ("T1"."C1"))
LEADING(@"SEL$1" "T3"@"SEL$1" "T1"@"SEL$1")
USE_HASH(@"SEL$1" "T1"@"SEL$1")
END_OUTLINE_DATA
*/
Optimizer state dump:
Compilation Environment Dump
optimizer_mode_hinted = false
optimizer_features_hinted = 0.0.0
parallel_execution_enabled = true
parallel_query_forced_dop = 0
parallel_dml_forced_dop = 0
parallel_ddl_forced_degree = 0
parallel_ddl_forced_instances = 0
_query_rewrite_fudge = 90
optimizer_features_enable = 11.2.0.1
_optimizer_search_limit = 5
cpu_count = 8
active_instance_count = 1
hash_area_size = 131072
bitmap_merge_area_size = 1048576
sort_area_size = 65536
sort_area_retained_size = 0
_sort_elimination_cost_ratio = 0
_optimizer_block_size = 8192
_sort_multiblock_read_count = 2
_hash_multiblock_io_count = 0
_db_file_optimizer_read_count = 8
_optimizer_max_permutations = 2000
pga_aggregate_target = 1843200 KB
_pga_max_size = 368640 KB
_query_rewrite_maxdisjunct = 257
_smm_auto_min_io_size = 56 KB
_smm_auto_max_io_size = 248 KB
_smm_min_size = 1024 KB
_smm_max_size = 184320 KB
_smm_px_max_size = 921600 KB
_cpu_to_io = 0
_optimizer_undo_cost_change = 11.2.0.1
parallel_query_mode = enabled
parallel_dml_mode = disabled
parallel_ddl_mode = enabled
optimizer_mode = all_rows
sqlstat_enabled = false
_optimizer_percent_parallel = 101
_always_anti_join = choose
_always_semi_join = choose
_optimizer_mode_force = true
_partition_view_enabled = true
_always_star_transformation = false
_query_rewrite_or_error = false
_hash_join_enabled = true
cursor_sharing = exact
_b_tree_bitmap_plans = true
star_transformation_enabled = false
_optimizer_cost_model = choose
_new_sort_cost_estimate = true
_complex_view_merging = true
_unnest_subquery = true
_eliminate_common_subexpr = true
_pred_move_around = true
_convert_set_to_join = false
_push_join_predicate = true
_push_join_union_view = true
_fast_full_scan_enabled = true
_optim_enhance_nnull_detection = true
_parallel_broadcast_enabled = true
_px_broadcast_fudge_factor = 100
_ordered_nested_loop = true
_no_or_expansion = false
optimizer_index_cost_adj = 100
optimizer_index_caching = 0
_system_index_caching = 0
_disable_datalayer_sampling = false
query_rewrite_enabled = true
query_rewrite_integrity = enforced
_query_cost_rewrite = true
_query_rewrite_2 = true
_query_rewrite_1 = true
_query_rewrite_expression = true
_query_rewrite_jgmigrate = true
_query_rewrite_fpc = true
_query_rewrite_drj = true
_full_pwise_join_enabled = true
_partial_pwise_join_enabled = true
_left_nested_loops_random = true
_improved_row_length_enabled = true
_index_join_enabled = true
_enable_type_dep_selectivity = true
_improved_outerjoin_card = true
_optimizer_adjust_for_nulls = true
_optimizer_degree = 0
_use_column_stats_for_function = true
_subquery_pruning_enabled = true
_subquery_pruning_mv_enabled = false
_or_expand_nvl_predicate = true
_like_with_bind_as_equality = false
_table_scan_cost_plus_one = true
_cost_equality_semi_join = true
_default_non_equality_sel_check = true
_new_initial_join_orders = true
_oneside_colstat_for_equijoins = true
_optim_peek_user_binds = true
_minimal_stats_aggregation = true
_force_temptables_for_gsets = false
workarea_size_policy = auto
_smm_auto_cost_enabled = true
_gs_anti_semi_join_allowed = true
_optim_new_default_join_sel = true
optimizer_dynamic_sampling = 2
_pre_rewrite_push_pred = true
_optimizer_new_join_card_computation = true
_union_rewrite_for_gs = yes_gset_mvs
_generalized_pruning_enabled = true
_optim_adjust_for_part_skews = true
_force_datefold_trunc = false
statistics_level = typical
_optimizer_system_stats_usage = true
skip_unusable_indexes = true
_remove_aggr_subquery = true
_optimizer_push_down_distinct = 0
_dml_monitoring_enabled = true
_optimizer_undo_changes = false
_predicate_elimination_enabled = true
_nested_loop_fudge = 100
_project_view_columns = true
_local_communication_costing_enabled = true
_local_communication_ratio = 50
_query_rewrite_vop_cleanup = true
_slave_mapping_enabled = true
_optimizer_cost_based_transformation = linear
_optimizer_mjc_enabled = true
_right_outer_hash_enable = true
_spr_push_pred_refspr = true
_optimizer_cache_stats = false
_optimizer_cbqt_factor = 50
_optimizer_squ_bottomup = true
_fic_area_size = 131072
_optimizer_skip_scan_enabled = true
_optimizer_cost_filter_pred = false
_optimizer_sortmerge_join_enabled = true
_optimizer_join_sel_sanity_check = true
_mmv_query_rewrite_enabled = true
_bt_mmv_query_rewrite_enabled = true
_add_stale_mv_to_dependency_list = true
_distinct_view_unnesting = false
_optimizer_dim_subq_join_sel = true
_optimizer_disable_strans_sanity_checks = 0
_optimizer_compute_index_stats = true
_push_join_union_view2 = true
_optimizer_ignore_hints = false
_optimizer_random_plan = 0
_query_rewrite_setopgrw_enable = true
_optimizer_correct_sq_selectivity = true
_disable_function_based_index = false
_optimizer_join_order_control = 3
_optimizer_cartesian_enabled = true
_optimizer_starplan_enabled = true
_extended_pruning_enabled = true
_optimizer_push_pred_cost_based = true
_optimizer_null_aware_antijoin = true
_optimizer_extend_jppd_view_types = true
_sql_model_unfold_forloops = run_time
_enable_dml_lock_escalation = false
_bloom_filter_enabled = true
_update_bji_ipdml_enabled = 0
_optimizer_extended_cursor_sharing = udo
_dm_max_shared_pool_pct = 1
_optimizer_cost_hjsmj_multimatch = true
_optimizer_transitivity_retain = true
_px_pwg_enabled = true
optimizer_secure_view_merging = true
_optimizer_join_elimination_enabled = true
flashback_table_rpi = non_fbt
_optimizer_cbqt_no_size_restriction = true
_optimizer_enhanced_filter_push = true
_optimizer_filter_pred_pullup = true
_rowsrc_trace_level = 0
_simple_view_merging = true
_optimizer_rownum_pred_based_fkr = true
_optimizer_better_inlist_costing = all
_optimizer_self_induced_cache_cost = false
_optimizer_min_cache_blocks = 10
_optimizer_or_expansion = depth
_optimizer_order_by_elimination_enabled = true
_optimizer_outer_to_anti_enabled = true
_selfjoin_mv_duplicates = true
_dimension_skip_null = true
_force_rewrite_enable = false
_optimizer_star_tran_in_with_clause = true
_optimizer_complex_pred_selectivity = true
_optimizer_connect_by_cost_based = true
_gby_hash_aggregation_enabled = true
_globalindex_pnum_filter_enabled = true
_px_minus_intersect = true
_fix_control_key = 0
_force_slave_mapping_intra_part_loads = false
_force_tmp_segment_loads = false
_query_mmvrewrite_maxpreds = 10
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_query_mmvrewrite_maxcmaps = 20
_query_mmvrewrite_maxregperm = 512
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_query_mmvrewrite_maxqryinlistvals = 500
_disable_parallel_conventional_load = false
_trace_virtual_columns = false
_replace_virtual_columns = true
_virtual_column_overload_allowed = true
_kdt_buffering = true
_first_k_rows_dynamic_proration = true
_optimizer_sortmerge_join_inequality = true
_optimizer_aw_stats_enabled = true
_bloom_pruning_enabled = true
result_cache_mode = MANUAL
_px_ual_serial_input = true
_optimizer_skip_scan_guess = false
_enable_row_shipping = true
_row_shipping_threshold = 80
_row_shipping_explain = false
transaction_isolation_level = read_commited
_optimizer_distinct_elimination = true
_optimizer_multi_level_push_pred = true
_optimizer_group_by_placement = true
_optimizer_rownum_bind_default = 10
_enable_query_rewrite_on_remote_objs = true
_optimizer_extended_cursor_sharing_rel = simple
_optimizer_adaptive_cursor_sharing = true
_direct_path_insert_features = 0
_optimizer_improve_selectivity = true
optimizer_use_pending_statistics = false
_optimizer_enable_density_improvements = true
_optimizer_aw_join_push_enabled = true
_optimizer_connect_by_combine_sw = true
_enable_pmo_ctas = 0
_optimizer_native_full_outer_join = force
_bloom_predicate_enabled = true
_optimizer_enable_extended_stats = true
_is_lock_table_for_ddl_wait_lock = 0
_pivot_implementation_method = choose
optimizer_capture_sql_plan_baselines = false
optimizer_use_sql_plan_baselines = true
_optimizer_star_trans_min_cost = 0
_optimizer_star_trans_min_ratio = 0
_with_subquery = OPTIMIZER
_optimizer_fkr_index_cost_bias = 10
_optimizer_use_subheap = true
parallel_degree_policy = manual
parallel_degree = 0
parallel_min_time_threshold = 10
_parallel_time_unit = 10
_optimizer_or_expansion_subheap = true
_optimizer_free_transformation_heap = true
_optimizer_reuse_cost_annotations = true
_result_cache_auto_size_threshold = 100
_result_cache_auto_time_threshold = 1000
_optimizer_nested_rollup_for_gset = 100
_nlj_batching_enabled = 1
parallel_query_default_dop = 0
is_recur_flags = 0
optimizer_use_invisible_indexes = false
flashback_data_archive_internal_cursor = 0
_optimizer_extended_stats_usage_control = 224
_parallel_syspls_obey_force = true
cell_offload_processing = true
_rdbms_internal_fplib_enabled = false
db_file_multiblock_read_count = 128
_bloom_folding_enabled = true
_mv_generalized_oj_refresh_opt = true
cell_offload_compaction = ADAPTIVE
parallel_degree_limit = 65535
parallel_force_local = false
parallel_max_degree = 16
total_cpu_count = 8
cell_offload_plan_display = AUTO
_optimizer_coalesce_subqueries = true
_optimizer_fast_pred_transitivity = true
_optimizer_fast_access_pred_analysis = true
_optimizer_unnest_disjunctive_subq = true
_optimizer_unnest_corr_set_subq = true
_optimizer_distinct_agg_transform = true
_aggregation_optimization_settings = 0
_optimizer_connect_by_elim_dups = true
_optimizer_eliminate_filtering_join = true
_connect_by_use_union_all = true
dst_upgrade_insert_conv = true
advanced_queuing_internal_cursor = 0
_optimizer_unnest_all_subqueries = true
_bloom_predicate_pushdown_to_storage = true
_bloom_vector_elements = 0
_bloom_pushing_max = 524288
parallel_autodop = 0
parallel_ddldml = 0
_parallel_cluster_cache_policy = adaptive
_parallel_scalability = 50
iot_internal_cursor = 0
_optimizer_instance_count = 0
_optimizer_connect_by_cb_whr_only = false
_suppress_scn_chk_for_cqn = nosuppress_1466
_optimizer_join_factorization = true
_optimizer_use_cbqt_star_transformation = true
_optimizer_table_expansion = true
_and_pruning_enabled = true
_deferred_constant_folding_mode = DEFAULT
_optimizer_distinct_placement = true
partition_pruning_internal_cursor = 0
parallel_hinted = none
_sql_compatibility = 0
_optimizer_use_feedback = true
_optimizer_try_st_before_jppd = true
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Query Block Registry:
SEL$1 0x6e975eb0 (PARSER) [FINAL]
:
call(in-use=25024, alloc=82024), compile(in-use=102048, alloc=164272), execution(in-use=12872, alloc=16200)
End of Optimizer State Dump
Dumping Hints
=============
Note in the above that we are able to see the actual execution plan, the peeked bind variables, the set of hints that will reproduce the execution plan, and a large number of normal and hidden optimizer parameters. Comparing the output with that of Oracle Database 10.2.0.2, you will notice that server additional parameters are listed in the output.
An example with a 10046 trace at level 4 (by default, on Oracle 11.1.0.6 and higher we no longer need to execute another SQL statement after the SQL statement under investigation so that the STAT lines for our SQL statement are written to the trace file):
ALTER SESSION SET TRACEFILE_IDENTIFIER = '10046_EXECUTION_PLAN'; ALTER SESSION SET EVENTS '10046 TRACE NAME CONTEXT FOREVER, LEVEL 4'; SELECT /* TEST */ T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2; SELECT SYSDATE FROM DUAL;
The resulting trace file looks something like this (you did read Chapter 15 of the book, didn’t you – so be sure to delete the trace files when no longer needed):
===================== PARSING IN CURSOR #6 len=118 dep=0 uid=68 oct=3 lid=68 tim=1264989049019258 hv=3172794915 ad='25b889600' sqlid='3g3fc5qyju0j3' SELECT /* TEST */ T3.C1, SUBSTR(T1.C2,1,10) C2 FROM T3, T1 WHERE T1.C1=T3.C1 AND T1.C1 BETWEEN :N1 AND :N2 END OF STMT PARSE #6:c=0,e=165,p=0,cr=0,cu=0,mis=1,r=0,dep=0,og=1,plh=0,tim=1264989049019257 BINDS #6: Bind#0 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=03 fl2=1000000 frm=00 csi=00 siz=48 off=0 kxsbbbfp=2b7f6eb34ea8 bln=22 avl=02 flg=05 value=1 Bind#1 oacdty=02 mxl=22(22) mxlc=00 mal=00 scl=00 pre=00 oacflg=03 fl2=1000000 frm=00 csi=00 siz=0 off=24 kxsbbbfp=2b7f6eb34ec0 bln=22 avl=02 flg=01 value=10 EXEC #6:c=999,e=1078,p=0,cr=0,cu=0,mis=1,r=0,dep=0,og=1,plh=744086632,tim=1264989049020428 FETCH #6:c=74988,e=75221,p=0,cr=37168,cu=0,mis=0,r=1,dep=0,og=1,plh=744086632,tim=1264989049095700 FETCH #6:c=0,e=50,p=0,cr=2,cu=0,mis=0,r=9,dep=0,og=1,plh=744086632,tim=1264989049095954 STAT #6 id=1 cnt=10 pid=0 pos=1 obj=0 op='FILTER (cr=37170 pr=0 pw=0 time=0 us)' STAT #6 id=2 cnt=10 pid=1 pos=1 obj=0 op='HASH JOIN (cr=37170 pr=0 pw=0 time=0 us cost=10219 size=2394 card=9)' STAT #6 id=3 cnt=10 pid=2 pos=1 obj=70288 op='TABLE ACCESS FULL T3 (cr=37164 pr=0 pw=0 time=0 us cost=10214 size=50 card=10)' STAT #6 id=4 cnt=10 pid=2 pos=2 obj=70284 op='TABLE ACCESS BY INDEX ROWID T1 (cr=6 pr=0 pw=0 time=0 us cost=4 size=2610 card=10)' STAT #6 id=5 cnt=10 pid=4 pos=1 obj=70285 op='INDEX RANGE SCAN SYS_C0017307 (cr=4 pr=0 pw=0 time=36 us cost=3 size=0 card=10)' CLOSE #6:c=0,e=10,dep=0,type=0,tim=1264989049096413 =====================
If you compare the above output with that of Oracle 10.2.0.2, you will notice that the SQL_ID for the SQL statement is written to the trace file, the STAT lines include cardinality estimates, and when a cursor is closed a line is written to the trace file. We could, of course, just read the actual plan direct from the 10046 trace file, as well as the bind variable values and data types. But, we will use TKPROF instead (we will not be able to see the bind variable values when using TKPROF).
> tkprof or112_ora_5062_10046_EXECUTION_PLAN.trc or112_ora_5062_10046_EXECUTION_PLAN.txt
The output might look like this:
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 2 0.07 0.07 0 37170 0 10 ------- ------ -------- ---------- ---------- ---------- ---------- ---------- total 4 0.07 0.07 0 37170 0 10 Misses in library cache during parse: 1 Misses in library cache during execute: 1 Optimizer mode: ALL_ROWS Parsing user id: 68 Rows Row Source Operation ------- --------------------------------------------------- 10 FILTER (cr=37170 pr=0 pw=0 time=0 us) 10 HASH JOIN (cr=37170 pr=0 pw=0 time=0 us cost=10219 size=2394 card=9) 10 TABLE ACCESS FULL T3 (cr=37164 pr=0 pw=0 time=0 us cost=10214 size=50 card=10) 10 TABLE ACCESS BY INDEX ROWID T1 (cr=6 pr=0 pw=0 time=0 us cost=4 size=2610 card=10) 10 INDEX RANGE SCAN SYS_C0017307 (cr=4 pr=0 pw=0 time=36 us cost=3 size=0 card=10)(object id 70285)
What else has changed? Nested – Nested Loops joins… interesting.
Charles Hooper's Oracle Notes 
[…] It? Charles Hooper-Execution Plans – What is the Plan, and Where Do I Find It 2? Charles Hooper-Execution Plans – What is the Plan, and Where Do I Find It 3? 20-Faulty quotes about %100 CPU utilization ? (comments) Charles Hooper-Faulty Quotes 6 – CPU […]