教你快速掌握Oracle数据库中的like优化-性能调优

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

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