Adaptive Cursor Sharing in Oracle Database 11g Release 1
DBAs are always encouraging developers to use bind variables, but when bind variables are used against columns containing skewed data they sometimes lead to less than optimum execution plans. This is because the optimizer peeks at the bind variable value during the hard parse of the statement, so the value of a bind variable when the statement is first presented to the server can affect every execution of the statement, regardless of the bind variable values.
Oracle 11g uses Adaptive Cursor Sharing to solve this problem by allowing the server to compare the effectiveness of execution plans between executions with different bind variable values. If it notices suboptimal plans, it allows certain bind variable values, or ranges of values, to use alternate execution plans for the same statement. This functionality requires no additional configuration. The following code provides an example of adaptive cursor sharing.
First we create and populate a test table.
DROP TABLE acs_test_tab;
CREATE TABLE acs_test_tab (
id NUMBER,
record_type NUMBER,
description VARCHAR2(50),
CONSTRAINT acs_test_tab_pk PRIMARY KEY (id)
);
CREATE INDEX acs_test_tab_record_type_i ON acs_test_tab(record_type);
DECLARE
TYPE t_acs_test_tab IS TABLE OF acs_test_tab%ROWTYPE;
l_tab t_acs_test_tab := t_acs_test_tab();
BEGIN
FOR i IN 1 .. 100000 LOOP
l_tab.extend;
IF MOD(i,2)=0 THEN
l_tab(l_tab.last).record_type := 2;
ELSE
l_tab(l_tab.last).record_type := i;
END IF;
l_tab(l_tab.last).id := i;
l_tab(l_tab.last).description := 'Description for ' || i;
END LOOP;
FORALL i IN l_tab.first .. l_tab.last
INSERT INTO acs_test_tab VALUES l_tab(i);
COMMIT;
END;
/
EXEC DBMS_STATS.gather_table_stats(USER, 'acs_test_tab', method_opt=>'for all indexed columns size skewonly', cascade=>TRUE);
The data in the RECORD_TYPE column is skewed, as shown by the presence of a histogram against the column.
SELECT column_name, histogram FROM user_tab_cols WHERE table_name = 'ACS_TEST_TAB'; COLUMN_NAME HISTOGRAM ------------------------------ --------------- ID NONE RECORD_TYPE HEIGHT BALANCED DESCRIPTION NONE 3 rows selected. SQL>
Next, we query the table and limit the rows returned based on the RECORD_TYPE column with a literal value of "1".
SET LINESIZE 200
SELECT MAX(id) FROM acs_test_tab WHERE record_type = 1;
SELECT * FROM TABLE(DBMS_XPLAN.display_cursor);
MAX(ID)
----------
1
1 row selected.
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
SQL_ID cgt92vnmcytb0, child number 0
-------------------------------------
SELECT MAX(id) FROM acs_test_tab WHERE record_type = 1
Plan hash value: 3987223107
-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB | 1 | 9 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | ACS_TEST_TAB_RECORD_TYPE_I | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------
This query has used the index as we would expect. Now we repeat the query, but this time use a bind variable.
VARIABLE l_record_type NUMBER;
EXEC :l_record_type := 1;
SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type;
SELECT * FROM TABLE(DBMS_XPLAN.display_cursor);
MAX(ID)
----------
1
1 row selected.
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
SQL_ID 9bmm6cmwa8saf, child number 0
-------------------------------------
SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type
Plan hash value: 3987223107
-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB | 1 | 9 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | ACS_TEST_TAB_RECORD_TYPE_I | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------
So we ran what amounted to the same query, and got the same result and execution plan. The optimizer picked an execution plan that it thinks is optimium for query by peeking at the value of the bind variable. The only problem is, this would be totally the wrong thing to do for other bind values.
VARIABLE l_record_type NUMBER;
EXEC :l_record_type := 2;
SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type;
SELECT * FROM TABLE(DBMS_XPLAN.display_cursor);
MAX(ID)
----------
100000
1 row selected.
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
SQL_ID 9bmm6cmwa8saf, child number 0
-------------------------------------
SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type
Plan hash value: 3987223107
-----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB | 1 | 9 | 2 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | ACS_TEST_TAB_RECORD_TYPE_I | 1 | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------
If we look at the V$SQL view entry for this query, we can see the IS_BIND_SENSITIVE column is marked as 'Y', so Oracle is aware this query may require differing execution plans depending on the bind variable values, but currently the IS_BIND_AWARE column is marked as 'N', so Oracle has not acted on this yet.
SELECT sql_id, child_number, is_bind_sensitive, is_bind_aware FROM v$sql WHERE sql_text = 'SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type'; SQL_ID CHILD_NUMBER I I ------------- ------------ - - 9bmm6cmwa8saf 0 Y N 1 row selected. SQL>
If we run the statement using the second bind variable again, we can see that Oracle has decided to use an alternate, more efficient plan for this statement.
VARIABLE l_record_type NUMBER;
EXEC :l_record_type := 2;
SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type;
SELECT * FROM TABLE(DBMS_XPLAN.display_cursor);
MAX(ID)
----------
100000
1 row selected.
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
SQL_ID 9bmm6cmwa8saf, child number 1
-------------------------------------
SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type
Plan hash value: 509473618
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 138 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
|* 2 | TABLE ACCESS FULL| ACS_TEST_TAB | 48031 | 422K| 138 (2)| 00:00:02 |
-----------------------------------------------------------------------------------
This change in behavior is also reflected in the V$SQL view, which now has the IS_BIND_AWARE column maked as "Y".
SELECT sql_id, child_number, is_bind_sensitive, is_bind_aware FROM v$sql WHERE sql_text = 'SELECT MAX(id) FROM acs_test_tab WHERE record_type = :l_record_type'; SQL_ID CHILD_NUMBER I I ------------- ------------ - - 9bmm6cmwa8saf 0 Y N 9bmm6cmwa8saf 1 Y Y 2 rows selected. SQL>
Information about the cursor sharing histograms, statistics and selectivity is displayed using the V$SQL_CS_HISTOGRAM, V$SQL_CS_STATISTICS and V$SQL_CS_SELECTIVITY views respectively.
SQL> SELECT * FROM v$sql_cs_histogram WHERE sql_id = '9bmm6cmwa8saf'; ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER BUCKET_ID COUNT -------- ---------- ------------- ------------ ---------- ---------- 319A4A1C 4171522382 9bmm6cmwa8saf 1 0 0 319A4A1C 4171522382 9bmm6cmwa8saf 1 1 1 319A4A1C 4171522382 9bmm6cmwa8saf 1 2 0 319A4A1C 4171522382 9bmm6cmwa8saf 0 0 1 319A4A1C 4171522382 9bmm6cmwa8saf 0 1 1 319A4A1C 4171522382 9bmm6cmwa8saf 0 2 0 6 rows selected. SQL> SELECT * FROM v$sql_cs_statistics WHERE sql_id = '9bmm6cmwa8saf'; ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER BIND_SET_HASH_VALUE P EXECUTIONS ROWS_PROCESSED BUFFER_GETS CPU_TIME -------- ---------- ------------- ------------ ------------------- - ---------- -------------- ----------- ---------- 319A4A1C 4171522382 9bmm6cmwa8saf 1 2064090006 Y 1 50001 499 0 319A4A1C 4171522382 9bmm6cmwa8saf 0 2342552567 Y 1 3 3 0 2 rows selected. SQL> SELECT * FROM v$sql_cs_selectivity WHERE sql_id = '9bmm6cmwa8saf'; ADDRESS HASH_VALUE SQL_ID CHILD_NUMBER PREDICATE RANGE_ID LOW HIGH -------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ---------- 319A4A1C 4171522382 9bmm6cmwa8saf 1 =L_RECORD_T 0 0.432283 0.528346 1 row selected. SQL>