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,注重,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')--------------------------------3211 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)