oracle中like的优化

简单说两句,具体看例子

1。尽量不要使用 like '%..%'
2。对于 like '..%..' (不以 % 开头),Oracle可以应用 colunm上的index
3。对于 like '%...' 的 (不以 % 结尾),可以利用reverse + function index 的形式,变化成 like '..%'

代码:

-- '建测试表和Index,注意,重点在于带reverse的function index。同时,一定要使用CBO才行……
 
sys@mescp> select reverse('123') from dual;REVERSE('123')
--------------------------------
321
 
1 row selected.
sys@mescp> create table test_like as select object_id,object_name from dba_objects;
Table created.
sys@mescp> create index test_like__name on test_like(object_name);
Index created.
sys@mescp> create index test_like__name_reverse on test_like(reverse(object_name));
Index created.
sys@mescp> analyze table test_like compute statistics for table for all indexes;
Table analyzed.
sys@mescp> set autotrace trace exp
 
 

-- '常量开头的like , 会利用index ,没问题…… '
sys@mescp> select * from test_like where object_name like AS%';

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=655 Bytes=15720)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST_LIKE' (Cost=2 Card=655Bytes=15720)
2 1 INDEX (RANGE SCAN) OF 'TEST_LIKE__NAME' (NON-UNIQUE) (Cost=2 Card=118)

--'开头和结尾都是 % ,对不起,很难优化'
sys@mescp> select * from test_like where object_name like '%%';

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=655 Bytes=15720)
1 0 TABLE ACCESS (FULL) OF 'TEST_LIKE' (Cost=6 Card=655 ytes=15720)

-- '以常量结束,直接写的时候是不能应用index的'
sys@mescp> select * from test_like where object_name like '%S';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=6 Card=655 Bytes=15720)
1 0 TABLE ACCESS (FULL) OF 'TEST_LIKE' (Cost=6 Card=655 Bytes=15720)
--'以常量结束的,加个reverse 函数,又可以用上index了'
sys@mescp> select * from test_like where reverse(object_name)like reverse('%AS');
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=2 Card=655 Bytes=15720)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST_LIKE' (Cost=2 Card=655 Bytes=15720)
2 1 INDEX (RANGE SCAN) OF 'TEST_LIKE__NAME_REVERSE' (NON-UNIQUE) (Cost=2 Card=118)









本文转自 牛海彬 51CTO博客,原文链接:http://blog.51cto.com/newhappy/76994,如需转载请自行联系原作者
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