复合索引的前导列如何选择？

     在表上建复合索引是常见的事情，那索引中键值的顺序是什么呢？通过下面的实验可以得出三个结论：

--制造实验数据，并收集统计信息

SQL> create table test as select * from dba_objects;
SQL> create index ind_id_owner on test(object_id,owner);
SQL> create index ind_owner_id on test(owner,object_id);
SQL> exec dbms_stats.gather_table_stats(user,'test',cascade=>true);

SQL> select count(distinct owner) owner_count,  count(distinct object_id) id_count, count(*)  from test;

OWNER_COUNT   ID_COUNT   COUNT(*)
        -----------            ----------        ----------
         25                     50583      50584
SQL> set autotrace traceonly

1. 如果是等值查询，那字段无先后之分，结合着索引的存储结构，索引存储的是键值和rowid，复合索引就是存储两个键值，如果是等值查询，可以看作是一个值，跟单个字段索引查询一样。

SQL> select /*+index(test,ind_id_owner)*/* from test    where owner ='TEST' and object_id =52623;
执行计划
----------------------------------------------------------
Plan hash value: 1096520809
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |     1 |    93 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |     1 |    93 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IND_ID_OWNER |     1 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID"=52623 AND "OWNER"='TEST')
统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1199  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> select /*+index(test,ind_owner_id)*/* from test  where owner ='TEST' and object_id =52623;
执行计划
----------------------------------------------------------
Plan hash value: 724495818
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |     1 |    93 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |     1 |    93 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IND_OWNER_ID |     1 |       |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OWNER"='TEST' AND "OBJECT_ID"=52623)
统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          4  consistent gets
          0  physical reads
          0  redo size
       1199  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

2. 如果是一个是等值，一个是范围查询，等值的字段作为前导列好一些。同样结合索引的存储特性，如果是object_id作为前导列，那oracle 会扫描30000,test到50584,***的数据，丢弃owner !=‘TEST’的列。

    如果是owner作为前导列,那oracle 会扫描test,30000到test,50584的数据,没有丢弃。
SQL> select /*+index(test,ind_id_owner)*/* from test  where owner ='TEST' and object_id >=30000;
已选择148行。
执行计划
----------------------------------------------------------
Plan hash value: 1096520809
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |  1076 |    97K|    90   (2)| 00:00:02 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |  1076 |    97K|    90   (2)| 00:00:02 |
|*  2 |   INDEX RANGE SCAN          | IND_ID_OWNER |  1076 |       |    73   (2)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID">=30000 AND "OWNER"='TEST' AND "OBJECT_ID" IS NOT NULL)
       filter("OWNER"='TEST')
统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        114  consistent gets
          0  physical reads
          0  redo size
      17139  bytes sent via SQL*Net to client
        484  bytes received via SQL*Net from client
         11  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        148  rows processed
SQL> select /*+index(test,ind_owner_id)*/* from test  where owner ='TEST' and object_id >=30000;
已选择148行。
执行计划
----------------------------------------------------------
Plan hash value: 724495818
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |  1076 |    97K|    36   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |  1076 |    97K|    36   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | IND_OWNER_ID |  1076 |       |     5   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OWNER"='TEST' AND "OBJECT_ID">=30000 AND "OBJECT_ID" IS NOT NULL)
统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         50  consistent gets
          0  physical reads
          0  redo size
      17139  bytes sent via SQL*Net to client
        484  bytes received via SQL*Net from client
         11  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        148  rows processed

3. 如果两个条件都是范围查询，则前导列为，单个字段过滤后的数据量接近两个字段过滤后的数据量。以下面的列子来说，最终结果是830条数据，用 owner >='TEST'为1076条，用object_id >= 30000是21029条，这样会减少filter的操作，所以选择owner作为前导列。
SQL> select /*+index(test,ind_id_owner)*/*from test  where owner >='TEST' and object_id >= 30000;
已选择830行。
执行计划
----------------------------------------------------------
Plan hash value: 1096520809
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |  5377 |   488K|   156   (1)| 00:00:02 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |  5377 |   488K|   156   (1)| 00:00:02 |
|*  2 |   INDEX RANGE SCAN          | IND_ID_OWNER |  5377 |       |    73   (2)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("OBJECT_ID">=30000 AND "OWNER">='TEST' AND "OBJECT_ID" IS NOT NULL)
       filter("OWNER">='TEST')
统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        228  consistent gets
          0  physical reads
          0  redo size
      86578  bytes sent via SQL*Net to client
        990  bytes received via SQL*Net from client
         57  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        830  rows processed
SQL> select /*+index(test,ind_owner_id)*/*from test  where owner >='TEST' and object_id >= 30000;
已选择830行。
执行计划
----------------------------------------------------------
Plan hash value: 724495818
--------------------------------------------------------------------------------------------
| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |              |  5377 |   488K|   188   (1)| 00:00:03 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST         |  5377 |   488K|   188   (1)| 00:00:03 |
|*  2 |   INDEX RANGE SCAN          | IND_OWNER_ID |  5377 |       |    36   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
  2 - access("OWNER">='TEST' AND "OBJECT_ID">=30000 AND "OWNER" IS NOT NULL)
       filter("OBJECT_ID">=30000)
统计信息
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        164  consistent gets
          0  physical reads
          0  redo size
      86578  bytes sent via SQL*Net to client
        990  bytes received via SQL*Net from client
         57  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
        830  rows processed
SQL> set autotrace off
SQL> select count(*) from test where owner >='TEST';
  COUNT(*)
----------
      1076
SQL> select count(*) from test where object_id >= 30000;
  COUNT(*)
----------
     21029