简单说两句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