LikeSQL语句的优化.doc

Like SQL语句的优化Like SQL语句的优化的一个小测试1.尽量不要使用 like %.% 2.对于 like .%. (不以 % 开头)，Oracle可以应用 colunm上的index3.对于 like %. 的 (不以 % 结尾)，可以利用reverse + function index 的形式，变化成 like .%具体的测试流程： (测试工具：PLSQL Developer)1 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。已连接到 Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 已连接为 zxdbm_ismpSQL select reverse(123) from dual;REVERSE(123)-321SQL 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 exp2 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)3 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)4 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)5 以常量结束的，加个reverse 函数，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