SQL 语句中对于like 的调优

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SQL> create table test as select * from dba_objects;

表已创建。

SQL> select owner,object_name from test where owner like 'ROBINSON%';

OWNER                                                        OBJECT_NAME
------------------------------------------------------------ ------------------------------
ROBINSON                                                     TEST
ROBINSON                                                     T2
ROBINSON                                                     T1
ROBINSON                                                     T3
ROBINSON                                                     T4
ROBINSON                                                     HEX_TO_BIN

已选择6行。

SQL> create index i_owner on test(owner);

索引已创建。

SQL> update test set owner='SYSMAN' where owner!='ROBINSON';

已更新50353行。

SQL> update test set owner='SYSTEM' where object_id>1000 and object_id<1020;

已更新19行。

SQL> commit;

提交完成。

SQL> update test set owner='SCOTT' where object_id>2000 and object_id<2005;

已更新4行。

SQL> select distinct owner from test;

OWNER
------------------------------------------------------------
ROBINSON
SYSTEM
SCOTT
SYS

SQL> BEGIN
2     DBMS_STATS.GATHER_TABLE_STATS(ownname => 'ROBINSON',
3                                   tabname => 'TEST',
4                                   estimate_percent => DBMS_STATS.AUTO_SAMPLE_SIZE,
5                                   method_opt => 'for all columns size repeat',
6                                   degree => DBMS_STATS.AUTO_DEGREE,
7                                   cascade=>TRUE
8                                   );
9 END;
10 /

PL/SQL 过程已成功完成。

SQL> set autot traceonly
SQL> select owner,object_name from test where owner not like 'SYS%';

已选择10行。

执行计划
----------------------------------------------------------
Plan hash value: 1357081020

--------------------------------------------------------------------------
| Id | Operation         | Name | Rows | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT |      | 25180 |   737K|   142   (3)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| TEST | 25180 |   737K|   142   (3)| 00:00:02 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("OWNER" NOT LIKE 'SYS%')

统计信息
----------------------------------------------------------
          0 recursive calls
          0 db block gets
          0 consistent gets
          0 physical reads
          0 redo size
          0 bytes sent via SQL*Net to client
          0 bytes received via SQL*Net from client
          0 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
         10 rows processed

从此处可以看到选择了10行，但是却走了全表扫描，现在对其加HINT提示

SQL> select /*+ INDEX (test I_OWNER) */ owner,object_name from test where owner not like 'SYS%';

已选择10行。

执行计划
----------------------------------------------------------
Plan hash value: 469605390

---------------------------------------------------------------------------------------
| Id | Operation                   | Name    | Rows | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         | 25180 |   737K|   490   (1)| 00:00:06 |
|   1 | TABLE ACCESS BY INDEX ROWID| TEST    | 25180 |   737K|   490   (1)| 00:00:06 |
|* 2 |   INDEX FULL SCAN           | I_OWNER | 25180 |       |   140   (3)| 00:00:02 |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("OWNER" NOT LIKE 'SYS%')

我们发现，强制加HINT提示所花的成本比全表扫描成本更高，这里要想优化这条SQL 必须改like

SQL> drop index i_owner;

索引已删除。

SQL> create index f_owner on test(instr(owner,'SYS',1,1)) ;

索引已创建。

SQL> select owner,object_name from test where instr(owner,'SYS',1,1)=0;

已选择10行。

执行计划
----------------------------------------------------------
Plan hash value: 535182827

---------------------------------------------------------------------------------------
| Id | Operation                   | Name    | Rows | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |         |   504 | 15120 |    52   (0)| 00:00:01 |
|   1 | TABLE ACCESS BY INDEX ROWID| TEST    |   504 | 15120 |    52   (0)| 00:00:01 |
|* 2 |   INDEX RANGE SCAN          | F_OWNER |   201 |       |    49   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access(INSTR("OWNER",'SYS',1,1)=0)

当使用函数索引后，COST由原来的142下降到现在的52

总结：对于LIKE语句，我们可以使用instr函数来进行SQL调优