Oracle一条SQL语句时快时慢

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Oracle一条SQL语句时快时慢

来源: 作者: 时间:2016-01-29 09:07 【

今天碰到一个非常奇怪的问题问题,一条SQL语句在PL SQL developer中很慢,需要9s,问题SQL: SELECT * FROM GG_function_location f WHERE f parent_id = 03000000000001 ; 表GG_function

今天碰到一个非常奇怪的问题问题,一条SQL语句在PL/SQL developer中很慢,需要9s,问题SQL:

SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'; 表GG_function_location有5千万的数据,parent_id上是有索引的。

诊断第一步:就在PL/SQL developer中按F5,看到的执行计划是走索引的,应该不会慢啊。

第二步:在sqlplus中用autotrace看,非常快,0.06s。

第三部:我想要重现这种慢,于是在PL/SQL developer中开一个窗口,天啊!单独执行SQL非常慢,但使用下面的语句就非常快,真是太神奇了。

alter session set tracefile_identifier = 'gg_test';
alter session set events '10046 trace name context forever ,level 12' ;
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' ;
alter session set events '10046 trace name context off' ;

第四部:我想到v$sql中找到这条SQL的执行计划,终于有了发现。

SQL> select banner from v$version;
BANNER
--------------------------------------------------------------------------------
Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production
PL/SQL Release 12.1.0.1.0 - Production
CORE 12.1.0.1.0 Production
TNS for : Version 12.1.0.1.0 - Production
NLSRTL Version 12.1.0.1.0 - Production

SQL> select s.SQL_TEXT,s.SQL_ID

from v$sql s
where s.SQL_TEXT like
'SELECT * FROM GG_function_location f WHERE f.parent_id =%'
and s.SQL_TEXT not like '%AND%';
SQL_TEXT SQL_ID
------------------------------------------------------------------------- ------------
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' dk02nb8mkchna
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' 2zav8x5kwxb32
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001' bc0k800k6u0x3

先找到SQL_ID,再找到对应的执行计划

select hash_value, child_number, sql_text from v$sql s
where s.SQL_ID = 'bc0k800k6u0x3';
select * from table(dbms_xplan.display_cursor(611124131, 0, 'advanced'));

执行计划一:
HASH_VALUE 656818826, child number 0
-------------------------------------
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'
Plan hash value: 1550360901
-----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
-----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 7 (100)| | | |
| 1 | TABLE ACCESS BY GLOBAL INDEX ROWID BATCHED| GG_FUNCTION_LOCATION | 3 | 999 | 7 (0)| 00:00:01 | ROWID | ROWID |
|* 2 | INDEX RANGE SCAN | IDX_GG_FL_PARENT_ID | 3 | | 4 (0)| 00:00:01 | | |
-----------------------------------------------------------------------------------------------------------------------------------

执行计划二:
HASH_VALUE 611124131, child number 0
-------------------------------------
SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001'
Plan hash value: 3374024865
------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | Pstart| Pstop |
------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 68 (100)| | | |
| 1 | PARTITION LIST ALL | | 1 | 247 | 68 (0)| 00:00:01 | 1 | 2 |
| 2 | PARTITION LIST ALL| | 1 | 247 | 68 (0)| 00:00:01 | 1 | 20 |
|* 3 | TABLE ACCESS FULL| GG_FUNCTION_LOCATION | 1 | 247 | 68 (0)| 00:00:01 | 1 | 40 |
------------------------------------------------------------------------------------------------------------

分析:我判断是解析这条SQL语句走错了执行计划,SELECT * FROM GG_function_location f WHERE f.parent_id ='03000000000001',于是我把改为

SELECT /*+gg*/* FROM GG_function_location f WHERE f.parent_id ='03000000000001',非常快。接近就简单了,把索引删除后,重建,会让此SQL重新解析。

解决方案:
drop index IDX_GG_FL_PARENT_ID;
create index IDX_GG_FL_PARENT_ID on GG_FUNCTION_LOCATION (PARENT_ID) nologging;
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