union 優化方法

lovehewenyu發表於2013-10-25
優化
union 優化方法

union優化思路：
union=>remove duplicate=>sort=>index（don't remove sort）=>index fast full scan(read many index blocks once)
union總結：
返回值僅是索引列值的情況下，給返回值列加索引是可以優化union!

union and union all 簡介
union:合併結果集並去除重複（remove duplicate）記錄
union all:合併結果集並不去除重複記錄


實驗如下：
SQL> create table t_union as select * from dba_objects;

Table created.

SQL> alter table t_union modify object_id not null;

Table altered.

SQL> create table t_union_1 as select * from dba_objects;

Table created.

SQL> alter table t_union_1 modify object_id not null;

Table altered.

SQL> select object_id from t_union
  2  union
  3  select object_id from t_union_1;

81512 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 299238292

-----------------------------------------------------------------------------------------
| Id  | Operation           | Name      | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |           |   169K|  2153K|       |  1460  (52)| 00:00:18 |
|   1 |  SORT UNIQUE        |           |   169K|  2153K|  3352K|  1460  (52)| 00:00:18 |
|   2 |   UNION-ALL         |           |       |       |       |            |          |
|   3 |    TABLE ACCESS FULL| T_UNION   | 82154 |  1042K|       |   325   (1)| 00:00:04 |
|   4 |    TABLE ACCESS FULL| T_UNION_1 | 87476 |  1110K|       |   325   (1)| 00:00:04 |
-----------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       2332  consistent gets
          0  physical reads
          0  redo size
    1185377  bytes sent via SQL*Net to client
      60189  bytes received via SQL*Net from client
       5436  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      81512  rows processed

=>TempSpc和1  sorts (memory)證明了union是需要排序的;UNION-ALL代表了我們執行合併操作<=

SQL> select object_id from t_union
  2  union all
  3  select object_id from t_union_1;

163023 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3974255051

--------------------------------------------------------------------------------
| Id  | Operation          | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |           |   169K|  2153K|   650  (51)| 00:00:08 |
|   1 |  UNION-ALL         |           |       |       |            |          |
|   2 |   TABLE ACCESS FULL| T_UNION   | 82154 |  1042K|   325   (1)| 00:00:04 |
|   3 |   TABLE ACCESS FULL| T_UNION_1 | 87476 |  1110K|   325   (1)| 00:00:04 |
--------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      13034  consistent gets
          0  physical reads
          0  redo size
    2370239  bytes sent via SQL*Net to client
     119963  bytes received via SQL*Net from client
      10870  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
     163023  rows processed

=>sorts值為0，也證明了union all不需要排序的;UNION-ALL代表了我們執行合併操作<=


給返回值列，新增索引看union and union all是否得到優化
SQL> create index t1 on t_union(object_id);

Index created.

SQL> create index t2 on t_union_1(object_id);

Index created.

SQL> select object_id from t_union
  2   union
  3   select object_id from t_union_1;

81512 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 3228413298

---------------------------------------------------------------------------------------
| Id  | Operation              | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |      |   169K|  2153K|       |   919  (52)| 00:00:12 |
|   1 |  SORT UNIQUE           |      |   169K|  2153K|  3352K|   919  (52)| 00:00:12 |
|   2 |   UNION-ALL            |      |       |       |       |            |          |
|   3 |    INDEX FAST FULL SCAN| T1   | 82154 |  1042K|       |    54   (0)| 00:00:01 |
|   4 |    INDEX FAST FULL SCAN| T2   | 87476 |  1110K|       |    54   (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        378  consistent gets
          0  physical reads
          0  redo size
    1185377  bytes sent via SQL*Net to client
      60189  bytes received via SQL*Net from client
       5436  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      81512  rows processed

1、索引優化了union ==>INDEX FAST FULL SCAN
2、索引不能消除union的排序 ==>1  sorts (memory) 、TempSpc

SQL> select object_id from t_union
  2   union all
  3   select object_id from t_union_1;

163023 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 937658497

------------------------------------------------------------------------------
| Id  | Operation             | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      |   169K|  2153K|   109  (51)| 00:00:02 |
|   1 |  UNION-ALL            |      |       |       |            |          |
|   2 |   INDEX FAST FULL SCAN| T1   | 82154 |  1042K|    54   (0)| 00:00:01 |
|   3 |   INDEX FAST FULL SCAN| T2   | 87476 |  1110K|    54   (0)| 00:00:01 |
------------------------------------------------------------------------------

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
      11221  consistent gets
          0  physical reads
          0  redo size
    2370239  bytes sent via SQL*Net to client
     119963  bytes received via SQL*Net from client
      10870  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
     163023  rows processed

1、索引不能很好的優化union all


參考：《收穫，不止ORACLE》


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