<h1 color:#000000;font-size:16px;margin:0px;padding:0px;white-space:normal;background-color:#ffffff;"="" style="word-wrap: break-word; margin: 0px; padding: 0px;">Oracle查看执行计划的几种方法:http://blog.itpub.net/26736162/viewspace-2136865/
在Oracle数据库中判断得到的执行计划是否准确,就是看目标sql是否被真正执行过,真正执行过的sql所对应的执行计划就是准确的,反之则有可能不准,因此,通过10046事件及如下的几种方式得到的执行计划是最准确的,而从其它方式获取到的执行计划都有可能不准确。
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,NULL,'ADVANCED ALLSTATS'));
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR('sql_id/hash_value',CHILD_CURSOR_NUMBER,'ADVANCED ALLSTATS'));
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_AWR('sql_id'));
这里需要注意的是,虽然sql*Plus的AUTOTRACE功能有部分是真实执行了sql语句的(例如所有DML语句),但是,由于该命令所显示的执行计划来源于调用EXPLAIN PLAN命令,所以,其得到的执行计划依然可能不准确(特别是在使用了绑定变量的情况下)。那么,为什么EXPLAIN PLAN命令里显示的预估执行计划与该sql真实的执行计划不一样呢?原因有多个方面,常见的情况包括以下几个方面:
① 绑定变量窥视(Bind Peeking):EXPLAIN PLAN里不会进行绑定变量窥视,但是Runtime Plan里会进行绑定变量窥视,所以,如果发生这种情况,那么会使这两个执行计划产生差异。
② 隐式转换:Explain Plan里不会考虑绑定变量的类型,但是Runtime Plan里会考虑类型,从而有可能会根据绑定变量的类型出现隐式转换,所以谓词(Predicate)会发生变化,使得执行计划也会产生差异。
③ 优化器参数:执行Explain Plan的Session与Runtime Plan的Session不是同一个。如果各个Session之间存在优化器参数差异,那么执行计划也会产生差异。
④ 统计信息收集参数:Explain Plan始终是用最新的统计信息产生执行计划,但是,Runtime Plan不一定会用最新的统计信息。因此也会产生执行计划差异。在收集统计信息时,一个与缓存的游标是否失效的很重要的参数为NO_INVALIDATE。在重新收集统计信息时,可以指定NO_INVALIDATE选项。该选项有TRUE、FALSE和DBMS_STATS.AUTO_INVALIDATE这3个值。如果取值为TRUE,那么表示收集统计信息后不进行游标失效动作,原有的Shared Cursor保持原有状态。如果取值为FALSE,那么表示将统计信息对象相关的所有Cursor全部失效,目标sql语句在下次执行时就会使用硬解析。如果设置为AUTO_INVALIDATE,那么Oracle自己决定Shared Cursor失效动作,当sql再次执行时间距离上次收集统计信息的时间超过5小时(隐含参数“_OPTIMIZER_INVALIDATION_PERIOD”决定)则对sql重新做硬解析。AUTO_INVALIDATE为默认选项。有些DBA在收集统计信息时,没有使用NO_INVALIDATE=>FALSE选项,所以,即使收集了统计信息,执行计划也不会立即改变。可以在表级别设置让所有依赖于该表的游标不失效,设置方法为:
EXEC DBMS_STATS.SET_TABLE_PREFS('SH','SALES','NO_INVALIDATE','TRUE');--在收集SH.SALES表上的统计信息时,让所有依赖于该表的游标不失效
实验一:
CREATE TABLE TEST_EXPLAIN_LHR AS SELECT * FROM DBA_OBJECTS;
INSERT INTO TEST_EXPLAIN_LHR SELECT * FROM TEST_EXPLAIN_LHR;
COMMIT;
SELECT COUNT(*) FROM TEST_EXPLAIN_LHR;
CREATE INDEX IDX_OBJ_LHR ON TEST_EXPLAIN_LHR(OBJECT_ID);
EXEC DBMS_STATS.GATHER_TABLE_STATS(USER,'test_explain_lhr',ESTIMATE_PERCENT => 100,CASCADE => TRUE);
VAR X NUMBER;
VAR Y NUMBER;
EXEC :X := 0;
EXEC :Y := 100000;
EXPLAIN PLAN FOR SELECT COUNT(*) FROM TEST_EXPLAIN_LHR T WHERE T.OBJECT_ID BETWEEN :X AND :Y ;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
SET AUTOT ON
SELECT COUNT(*) FROM TEST_EXPLAIN_LHR T WHERE T.OBJECT_ID BETWEEN :X AND :Y ;
SET AUTOT OFF
SELECT COUNT(*) FROM TEST_EXPLAIN_LHR T WHERE T.OBJECT_ID BETWEEN :X AND :Y ;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(NULL,'advanced'));
下面实验验证了使用EXPLAIN PLAN FOR和SET AUTOT ON方式获取到的执行计划都是不准确的:
SYS@PROD1> clear scr
SYS@PROD1> CREATE TABLE test_explain_lhr AS SELECT * FROM Dba_Objects;
Table created.
SYS@PROD1> INSERT INTO test_explain_lhr SELECT * FROM test_explain_lhr;
72503 rows created.
SYS@PROD1> COMMIT;
Commit complete.
SYS@PROD1> SELECT COUNT(*) FROM test_explain_lhr;
COUNT(*)
----------
145006
SYS@PROD1> CREATE INDEX idx_obj_lhr ON test_explain_lhr(object_id);
Index created.
SYS@PROD1> EXEC dbms_stats.gather_table_stats(USER,estimate_percent => 100,cascade => TRUE);
PL/sql procedure successfully completed.
SYS@PROD1> VAR x NUMBER;
SYS@PROD1> VAR y NUMBER;
SYS@PROD1> EXEC :x := 0;
PL/sql procedure successfully completed.
SYS@PROD1> EXEC :y := 100000;
PL/sql procedure successfully completed.
SYS@PROD1> EXPLAIN PLAN FOR SELECT COUNT(*) FROM test_explain_lhr t WHERE t.object_id BETWEEN :x AND :y ;
Explained.
SYS@PROD1> set line 9999
SYS@PROD1> SELECT * FROM TABLE(dbms_xplan.display);
PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
Plan hash value: 3299589416
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
|* 2 | FILTER | | | | | |
|* 3 | INDEX RANGE SCAN| IDX_OBJ_LHR | 363 | 1815 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TO_NUMBER(:X)<=TO_NUMBER(:Y))
3 - access("T"."OBJECT_ID">=TO_NUMBER(:X) AND
"T"."OBJECT_ID"<=TO_NUMBER(:Y))
17 rows selected.
SYS@PROD1> set autot on
SYS@PROD1> SELECT COUNT(*) FROM test_explain_lhr t WHERE t.object_id BETWEEN :x AND :y ;
COUNT(*)
----------
145006
Execution Plan
----------------------------------------------------------
Plan hash value: 3299589416
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 5 | 3 (0)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
|* 2 | FILTER | | | | | |
|* 3 | INDEX RANGE SCAN| IDX_OBJ_LHR | 363 | 1815 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(TO_NUMBER(:X)<=TO_NUMBER(:Y))
3 - access("T"."OBJECT_ID">=TO_NUMBER(:X) AND
"T"."OBJECT_ID"<=TO_NUMBER(:Y))
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
329 consistent gets
0 physical reads
0 redo size
424 bytes sent via sql*Net to client
419 bytes received via sql*Net from client
2 sql*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
SYS@PROD1> SET AUTOT OFF
SYS@PROD1> SELECT COUNT(*) FROM test_explain_lhr t WHERE t.object_id BETWEEN :x AND :y ;
COUNT(*)
----------
145006
SYS@PROD1> SELECT * FROM TABLE(dbms_xplan.display_cursor(NULL,'advanced'));
PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------
sql_ID 1r87sg98rdkuf,child number 0
-------------------------------------
SELECT COUNT(*) FROM test_explain_lhr t WHERE t.object_id BETWEEN :x
AND :y
Plan hash value: 2428225634
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 90 (100)| |
| 1 | SORT AGGREGATE | | 1 | 5 | | |
|* 2 | FILTER | | | | | |
|* 3 | INDEX FAST FULL SCAN| IDX_OBJ_LHR | 145K| 708K| 90 (2)| 00:00:02 |
--------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1
3 - SEL$1 / T@SEL$1
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")
INDEX_FFS(@"SEL$1" "T"@"SEL$1" ("TEST_EXPLAIN_LHR"."OBJECT_ID"))
END_OUTLINE_DATA
*/
Peeked Binds (identified by position):
--------------------------------------
1 - :X (NUMBER): 0
2 - :Y (NUMBER): 100000
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(:X<=:Y)
3 - filter(("T"."OBJECT_ID">=:X AND "T"."OBJECT_ID"<=:Y))
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - (#keys=0) COUNT(*)[22]
53 rows selected.
1、“EXPLAIN PLAN FOR sql”不实际执行sql语句,生成的计划未必是真实执行的计划。但是,必须要有PLAN_TABLE表,可以执行脚本“@?/rdbms/admin/utlxplan.sql”来创建。
2、sql*Plus的AUTOTRACE功能,命令:SET AUTOTRACE TRACEONLY EXPLAIN。除SET AUTOTRACE TRACEONLY EXPLAIN外其它的AUTOTRACE方式均实际执行sql。但是,如果该命令后执行的是DML语句,那么该DML语句是确实被Oracle实际执行过的。
三、 如何获取历史执行计划?历史执行计划只能从AWR中获取,如果AWR没有记录的话,那么就无法获取历史执行计划了,获取历史执行计划的命令如下所示:
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_AWR('&sqlid'));
SELECT * FROM TABLE(DBMS_WORKLOAD_REPOSITORY.AWR_sql_REPORT_HTML(L_DBID =>,L_INST_NUM =>,L_BID =>,L_EID =>,L_sqlID => )) ;
其中,L_DBID代表数据库的DBID,L_INST_NUM代表数据库的实例号,单机环境为1,RAC环境填写具体的实例号,L_BID为开始的快照号,L_EID为结束的快照号,L_sqlID为要查看sql的sql_ID。
下面的例子可以直接从AWR中获取sql_ID为“bsa0wjtftg3uw”的执行计划,可以看到历史有2种执行计划,一个是全表扫描,一个是索引范围扫描:
SYS@RAC2LHR1> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_AWR(sql_ID => 'bsa0wjtftg3uw' )) ;
sql_ID bsa0wjtftg3uw
--------------------
SELECT file# FROM file$ WHERE ts#=:1
Plan hash value: 690176192
-----------------------------------------------
| Id | Operation | Name |
-----------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| FILE$ |
| 2 | INDEX RANGE SCAN | I_FILE2 |
-----------------------------------------------
Note
-----
- rule based optimizer used (consider using cbo)
sql_ID bsa0wjtftg3uw
--------------------
SELECT file# FROM file$ WHERE ts#=:1
Plan hash value: 3494626068
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%cpu)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 2 (100)| |
| 1 | TABLE ACCESS FULL| FILE$ | 1 | 6 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------
31 rows selected.
四、 给出一个执行计划的执行顺序阅读如下的执行计划,给出sql的执行顺序。
-----------------------------------------
| Id | Operation
-----------------------------------------
| 0 | SELECT STATEMENT
| 1 | SORT AGGREGATE
| 2 | VIEW
| 3 | UNION-ALL
|* 4 | FILTER
|* 5 | HASH JOIN
| 6 | TABLE ACCESS FULL
|* 7 | TABLE ACCESS FULL
|* 8 | TABLE ACCESS BY INDEX ROWID
|* 9 | INDEX UNIQUE SCAN
| 10 | NESTED LOOPS
| 11 | INDEX FULL SCAN
| 12 | TABLE ACCESS CLUSTER
|* 13 | INDEX UNIQUE SCAN
-------------------------------------------
分析:采用最右最上最先执行的原则看层次关系,在同一级如果某个动作没有子ID,那么就最先执行,首先,6、7、9、13最右,所以,6,7最先执行做HASH JOIN,为6,7,5。
第二,8有子节点,接下来是9,8。
第三,HASH的结果和8的结果做FILTER过滤。
第四,10这个节点根据原则是11,13,12,10。
第五,剩下依次是3,2,1,0。
所以,该图的执行顺序是6,7,5,9,8,4,11,13,12,10,3,2,1,0。