Oracle技术之Index Merge与and_equal 的变迁
and_equal是Oracle支持的一种特定操作，可以将多个单列索引进行合并（Index Merge）输出查询结果。
同时and_equal可以通过Hints来进行强制，最少指定两个索引，最多指定5个。
以下的执行计划是常见的and_equal执行方式：
SQL> select /*+ and_equal(t1 iu ii) */ username,password from t1 where username='EYGLE' and user_id=58;
USERNAME PASSWORD
------------------------------ ------------------------------
EYGLE B726E09FE21F8E83
执行计划
----------------------------------------------------------
Plan hash value: 1425017857
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 47 | 2 (0)| 00:00:01 |
|* 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 47 | 2 (0)| 00:00:01 |
| 2 | AND-EQUAL | | | | | |
|* 3 | INDEX RANGE SCAN | II | 1 | | 1 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IU | 1 | | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("USERNAME"='EYGLE' AND "USER_ID"=58)
3 - access("USER_ID"=58)
4 - access("USERNAME"='EYGLE')
但是从Oracle 10g开始，and_equal操作被废弃（depricated）掉，Oracle不再支持。这里废弃的含义并不是被彻底移除，而是说不再进行改进，通过hints仍然可以强制实现Index Merge的and_equal操作。
以上的输出就是来自Oracle 10.2.0.4，在使用rule提示时，Oracle会主动选择and_equal，所以不再支持是因为，这种方法限制过多，而且远不如复合索引来的灵活：
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Prod
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for 32-bit Windows: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production
SQL> select /*+ rule */ username,password from t1 where username='EYGLE' and user_id=58;
USERNAME PASSWORD
------------------------------ ------------------------------
EYGLE B726E09FE21F8E83
执行计划
----------------------------------------------------------
Plan hash value: 3072843751
--------------------------------------------
| Id | Operation | Name |
--------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | TABLE ACCESS BY INDEX ROWID| T1 |
| 2 | AND-EQUAL | |
|* 3 | INDEX RANGE SCAN | IU |
|* 4 | INDEX RANGE SCAN | II |
--------------------------------------------
新的可选替代方案是NDEX_COMBINE，从9i开始，初始化参数 _b_tree_bitmap_plans设置为true，允许为B*Tree索引使用,bitmap convert:
SQL> select /*+ index_combine(t1 iu ii) */ username,password from t1 where username='EYGLE' and user_id=58;
USERNAME PASSWORD
------------------------------ ------------------------------
EYGLE B726E09FE21F8E83
执行计划
----------------------------------------------------------
Plan hash value: 1808973554
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 47 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID | T1 | 1 | 47 | 2 (0)| 00:00:01 |
| 2 | BITMAP CONVERSION TO ROWIDS | | | | | |
| 3 | BITMAP AND | | | | | |
| 4 | BITMAP CONVERSION FROM ROWIDS| | | | | |
|* 5 | INDEX RANGE SCAN | II | | | 1 (0)| 00:00:01 |
| 6 | BITMAP CONVERSION FROM ROWIDS| | | | | |
|* 7 | INDEX RANGE SCAN | IU | | | 1 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("USER_ID"=58)
7 - access("USERNAME"='EYGLE')
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