EXPLAIN the statement to check the execution plan that the CBO has created. This will often reveal that the query is not using the relevant indexes, or indexes to support the query are missing. Interpretation of the execution plan is beyond the scope of this article.Plan Table
The explain plan process stores data in thePLAN_TABLE. This table can be located in the current schema or a shared schema and is created using in SQL*Plus as follows:SQL> CONN sys/password AS SYSDBA Connected SQL> @$ORACLE_HOME/rdbms/admin/utlxplan.sql SQL> GRANT ALL ON sys.plan_table TO public; SQL> CREATE PUBLIC SYNONYM plan_table FOR sys.plan_table;
AUTOTRACE - The Easy Option?
Switching on theAUTOTRACE parameter in SQL*Plus causes an explain to be performed on every query.SQL> SET AUTOTRACE ON
SQL> SELECT *
2 FROM emp e, dept d
3 WHERE e.deptno = d.deptno
4 AND e.ename = 'SMITH';
EMPNO ENAME JOB MGR HIREDATE SAL COMM
---------- ---------- --------- ---------- --------- ---------- ----------
DEPTNO DEPTNO DNAME LOC
---------- ---------- -------------- -------------
7369 SMITH CLERK 7902 17-DEC-80 800
20 20 RESEARCH DALLAS
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 NESTED LOOPS
2 1 TABLE ACCESS (FULL) OF 'EMP'
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'DEPT'
4 3 INDEX (UNIQUE SCAN) OF 'PK_DEPT' (UNIQUE)
Statistics
----------------------------------------------------------
81 recursive calls
4 db block gets
27 consistent gets
0 physical reads
0 redo size
941 bytes sent via SQL*Net to client
425 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>TRACEONLY option of AUTOTRACE seems to remove this issue, but this option merely suppresses the output of the query data, it doesn't prevent the statement being run. As such, long running queries will still take a long time to complete, but they will not present their data. The following example show this in practice.CREATE OR REPLACE FUNCTION pause_for_secs(p_seconds IN NUMBER) RETURN NUMBER A
BEGIN
DBMS_LOCK.sleep(p_seconds);
RETURN p_seconds;
END;
/
Function created.
SQL> SET TIMING ON
SQL> SET AUTOTRACE ON
SQL> SELECT pause_for_secs(10) FROM DUAL;
PAUSE_FOR_SECS(10)
------------------
10
1 row selected.
Elapsed: 00:00:10.28
Execution Plan
----------------------------------------------------------
Plan hash value: 1550022268
-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:00:01 |
| 1 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------
Statistics
----------------------------------------------------------
189 recursive calls
0 db block gets
102 consistent gets
0 physical reads
0 redo size
331 bytes sent via SQL*Net to client
332 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
13 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> SET AUTOTRACE TRACEONLY
SQL> SELECT pause_for_secs(10) FROM DUAL;
1 row selected.
Elapsed: 00:00:10.26
Execution Plan
----------------------------------------------------------
Plan hash value: 1550022268
-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 2 (0)| 00:00:01 |
| 1 | FAST DUAL | | 1 | | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
331 bytes sent via SQL*Net to client
332 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SQL>TRACEONLY option is used or not. If the TRACEONLY option prevented the query running, you would expect it to return instantly, like an EXPLAIN PLAN.The solution to this is to use the
TRACEONLY EXPLAIN option, which only performs the EXPLAIN PLAN, rather than running the statement.EXPLAIN PLAN
TheEXPLAIN PLAN method doesn't require the query to be run, greatly reducing the time it takes to get an execution plan for long-running queries compared to AUTOTRACE. First the query must be explained:Then the execution plan displayed:SQL> EXPLAIN PLAN FOR 2 SELECT * 3 FROM emp e, dept d 4 WHERE e.deptno = d.deptno 5 AND e.ename = 'SMITH'; Explained. SQL>
For parallel queries use the utlxplp.sql script instead of utlxpls.sql.SQL> @$ORACLE_HOME/rdbms/admin/utlxpls.sql Plan Table -------------------------------------------------------------------------------- | Operation | Name | Rows | Bytes| Cost | Pstart| Pstop | -------------------------------------------------------------------------------- | SELECT STATEMENT | | | | | | | | NESTED LOOPS | | | | | | | | TABLE ACCESS FULL |EMP | | | | | | | TABLE ACCESS BY INDEX RO|DEPT | | | | | | | INDEX UNIQUE SCAN |PK_DEPT | | | | | | -------------------------------------------------------------------------------- 8 rows selected. SQL>
Statement ID
If multiple people are accessing the same plan table, or you would like to keep a history of the execution plans you should use theSTATEMENT_ID. This associates a user specified ID with each plan which can be used when retrieving the data.SQL> EXPLAIN PLAN SET STATEMENT_ID = 'TIM' FOR
2 SELECT *
3 FROM emp e, dept d
4 WHERE e.deptno = d.deptno
5 AND e.ename = 'SMITH';
Explained.
SQL> @explain.sql TIM
PLAN OBJECT_NAME OBJECT_TYPE BYTES COST PARTITION_START PARTITION_STOP
-------------------------------------- --------------- --------------- ----- ----- --------------- ---------------
Select Statement 57 4
1.1 Nested Loops 57 4
2.1 Table Access (Full) EMP TABLE 37 3
2.2 Table Access (By Index Rowid) DEPT TABLE 20 1
3.1 Index (Unique Scan) PK_DEPT INDEX (UNIQUE) 0
5 rows selected.
SQL>