sql 语句中对like 的优化

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......

SQL> select reverse('123') from dual;
REVERSE('123')
--------------------------------
321
1 row selected.

SQL> create table test_like as select object_id,object_name from dba_objects;
Table created.

SQL> create index test_like__name on test_like(object_name);
Index created.

SQL> create index test_like__name_reverse on test_like(reverse(object_name));
Index created.

SQL> analyze table test_like compute statistics for table for all indexes;
Table analyzed.

SQL> set autot trace

--常量开头的like , 会利用index ，没问题......
SQL> 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

SQL> 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
SQL> 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'
SQL> 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)