oracle-组合索引字段位置与查询效率之间的关系

Oracle索引组合字段的位置不同，当查询条件不能覆盖索引时，影响查询效率。查询条件是不是索引字段的第一列影响执行计划，实验验证

实验1：查询条件为组合索引的第一列
--创建测试表
create table test_C
as
select * from user_objects where 1=2;

--创建测试表
create table test_a
as
select * from user_objects where 1=1;
--创建组合索引
create index test_a_1 on test_a(object_id,object_type);
set linesize 1000;
set autotrace trace;
select * from test_a where object_id=161728;
****************************************************
执行计划
----------------------------------------------------------
Plan hash value: 3036018411

------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |          |     1 |   116 |     3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TEST_A   |     1 |   116 |     3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | TEST_A_1 |     1 |       |     2   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"=161728)


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
       1933  bytes sent via SQL*Net to client
        500  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

查询条件object_id为索引的前导列。执行计划：INDEX RANGE SCAN，cost是3，逻辑读是3.

drop index test_a_1;
drop index test_a_2;
--创建索引2，查询条件在索引中的位置是第二列
create index test_a_2 on test_a(object_name,object_ID);
--再次查询
select * from test_a where object_id=161728;
****************************************************
执行计划
----------------------------------------------------------
Plan hash value: 219338936

------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |          |     1 |   116 |     7   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TEST_A   |     1 |   116 |     7   (0)| 00:00:01 |
|*  2 |   INDEX SKIP SCAN                   | TEST_A_2 |     1 |       |     6   (0)| 00:00:01 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("OBJECT_ID"=161728)
       filter("OBJECT_ID"=161728)


统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
          8  consistent gets
          0  physical reads
          0  redo size
       1933  bytes sent via SQL*Net to client
        500  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

查询条件object_id为索引的第二列。执行计划：INDEX SKIP SCAN，cost是7，逻辑读是8.

同样object_id作为查询条件，当object_id作为索引的第一列或者作为索引的第二列，它们在cost与逻辑读上是有区别的，也就是索引跳跃扫描(INDEX SKIP SCAN)没有索引范围扫描(INDEX RANGE SCAN)的效率高。

组合索引字段的位置影响查询效率，所以在建索引的时候需注意.

建议:把等值条件的字段放在组合索引的前导列,范围字段放在第二列.