[ORALCE]SQL 优化案例之 组合索引的前缀和单列索引一致

• 组合索引的前缀和单列索引一致,走INDEX RANGE SCAN

drop table TX1 purge;
create table TX1 as select * from dba_objects;
SQL> select count(*) from (select distinct object_id from TX1);

  COUNT(*)
----------
     73396
SQL> select count(*) from (select distinct object_type from TX1);

  COUNT(*)
----------
    47
create index idx_object_id on TX1(object_id,object_type);
set autotrace on
set linesize 150

select * from TX1 where object_id=19;
Execution Plan
----------------------------------------------------------
Plan hash value: 1750502627

-----------------------------------------------------------------------------------------------------
| Id  | Operation                | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |            |      1 |    481 |      3   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TX1        |      1 |    481 |      3   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN            | IDX_OBJECT_ID |      1 |        |      2   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"=19)

Note
-----
   - dynamic statistics used: dynamic sampling (level=2)


Statistics
----------------------------------------------------------
     34  recursive calls
      0  db block gets
    164  consistent gets
      1  physical reads
      0  redo size
       2686  bytes sent via SQL*Net to client
    398  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed

• 组合索引的前缀和单列索引不一致,走FULL TABLE SCAN

drop index idx_object_id;
create index idx_object_id on TX1(object_type,object_id);
select * from TX1 where object_id=19;
Execution Plan
----------------------------------------------------------
Plan hash value: 2923622636

--------------------------------------------------------------------------
| Id  | Operation      | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |     |    16 |  7696 |   459   (1)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| TX1  |    16 |  7696 |   459   (1)| 00:00:01 |
--------------------------------------------------------------------------

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

   1 - filter("OBJECT_ID"=19)

Note
-----
   - dynamic statistics used: dynamic sampling (level=2)


Statistics
----------------------------------------------------------
      5  recursive calls
      0  db block gets
       1514  consistent gets
      0  physical reads
      0  redo size
       2686  bytes sent via SQL*Net to client
    398  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed