Like SQL語(yǔ)句的優(yōu)化的一個(gè)小測(cè)試

1.盡量不要使用 like '%..%'

2.對(duì)于 like '..%..' (不以 % 開頭)，Oracle可以應(yīng)用 colunm上的index

3.對(duì)于 like '%...' 的 (不以 % 結(jié)尾)，可以利用reverse + function index 的形式，變化成 like '..%'

具體的測(cè)試流程： (測(cè)試工具：PLSQL Developer)

[1] '建測(cè)試表和Index，注意，重點(diǎn)在于帶reverse的function index。同時(shí)，一定要使用CBO才行。

已連接到 Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
已連接為 zxdbm_ismp

SQL> select reverse('123') from dual;

REVERSE('123')
--------------
321

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));  --建反向索引

Index created.

SQL> analyze table test_like compute statistics for table for all indexes;

Table analyzed.

SQL> set autotrace trace exp

[2] '常量開頭的like , 會(huì)利用index ，沒(méi)問(wèn)題…… '

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)

[3] '開頭和結(jié)尾都是 % ,對(duì)不起，很難優(yōu)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)

[4] '以常量結(jié)束，直接寫的時(shí)候是不能應(yīng)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)

[5] '以常量結(jié)束的，加個(gè)reverse 函數(shù)，又可以用上index了'(需要反向索引的支持)

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)