以通配符（%）开始的like字符串，走索引

在对oracle的SQL优化过程中经常会遇到【like'%abc'】破坏索引的问题，但是如果真有此类需求，该如何在不破坏索引的基础上进行查询呢。

[sql] view plain copy
sys@mescp> select reverse('') from dual;  

REVERSE('')
--------------------------------  

 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  

----------------------------------------------------------
 SELECT STATEMENT Optimizer=CHOOSE (Cost= Card= Bytes=)
  TABLE ACCESS (BY INDEX ROWID) OF 'TEST_LIKE' (Cost= Card=655Bytes=)
  INDEX (RANGE SCAN) OF 'TEST_LIKE__NAME' (NON-UNIQUE) (Cost= Card=)
--'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  

----------------------------------------------------------
 SELECT STATEMENT Optimizer=CHOOSE (Cost= Card= Bytes=)
  TABLE ACCESS (FULL) OF 'TEST_LIKE' (Cost= Card= ytes=)  

-- '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
----------------------------------------------------------
 SELECT STATEMENT Optimizer=CHOOSE (Cost= Card= Bytes=)  

  TABLE ACCESS (FULL) OF 'TEST_LIKE' (Cost= Card= Bytes=)
--'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  

----------------------------------------------------------  

 SELECT STATEMENT Optimizer=CHOOSE (Cost= Card= Bytes=)
  TABLE ACCESS (BY INDEX ROWID) OF 'TEST_LIKE' (Cost= Card= Bytes=)  

  INDEX (RANGE SCAN) OF 'TEST_LIKE__NAME_REVERSE' (NON-UNIQUE) (Cost= Card=)  

了解SQL的同学，都知道，like关键字可以走索引，只要字符串不是以通配符（%）开始。

如果类似 like "%xxx" 的sql，如何走索引呢？基于REVERSE()函数来创建一个函数索引。

1、准备数据：

CREATE TABLE jka AS SELECT ROWNUM id, dbms_random.string('x',10) v FROM dual CONNECT BY LEVEL <= 10000;

2、创建原始索引：

CREATE INDEX jka_normal ON jka (v);

BEGIN
  dbms_stats.gather_table_stats
    (USER
    ,'JKA'
    ,estimate_percent=>100
    ,cascade=>TRUE);
END;

3、以下SQL基于jka_normal索引走范围扫描：

SELECT * FROM jka WHERE v LIKE 'ABC%';

4、但是下面的SQL将走全表扫描（不会使用索引）：

SELECT * FROM jka WHERE v LIKE '%ABC';

5、现在，创建一个函数索引（不要与反向索引[REVERSE INDEX]混淆）：

CREATE INDEX jka_reverse ON jka(REVERSE(v));

BEGIN
  dbms_stats.gather_table_stats
    (USER
    ,'JKA'
    ,estimate_percent=>100
    ,cascade=>TRUE);
END;

6、下面的SQL将基于jka_reverse索引走范围扫描：

SELECT * FROM jka WHERE REVERSE(v) LIKE REVERSE('%ABC');

问题解决。