Like SQL語句的優化的一個小測試
1.儘量不要使用 like '%..%'
2.對於 like '..%..' (不以 % 開頭)，Oracle可以應用 colunm上的index
3.對於 like '%...' 的 (不以 % 結尾)，可以利用reverse + function index 的形式，變化成 like '..%'
具體的測試流程： (測試工具：PLSQL Developer)
[1] '建測試表和Index，注意，重點在於帶reverse的function index。同時，一定要使用CBO才行。
已連接到 Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
已連接爲 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 , 會利用index ，沒問題…… '
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] '開頭和結尾都是 % ,對不起，很難優化'
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] '以常量結束，直接寫的時候是不能應用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] '以常量結束的，加個reverse 函數，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)