用merge into进行性能优化

有时候开发组有这么一个需求，一个表和它的备份表，把备份表中的某些字段替换到原表中，当数据量非常大的时候就很很慢，这个时候如果我们用merge into往往会提高几倍的性能，下面我们来做个实验：

SQL> drop table test1 purge;

表已删除。

SQL> drop table test2 purge;

表已删除。

SQL> create table test1 as select * from dba_objects;

表已创建。

SQL> alter table test1 nologging;

表已更改。

SQL> begin
  2    for i in 1 .. 5 loop
  3      insert /*+append*/
  4      into test1
  5        select * from dba_objects;
  6      commit;
  7    end loop;
  8  end;
  9  /

PL/SQL 过程已成功完成。
SQL> update test1 set object_id = rownum;

已更新303258行。

SQL> commit;

提交完成。

SQL> create table test2 as select * from test1;

表已创建。

SQL> select count(*) from test1;

COUNT(*)
----------
    303258

SQL> select count(*) from test2;

COUNT(*)
----------
    303258

SQL> create index ind_object_id1 on test1(object_id) nologging;

索引已创建。

SQL> create index ind_object_id2 on test2(object_id) nologging;

索引已创建。

SQL> exec dbms_stats.gather_table_stats(user,'test1');

PL/SQL 过程已成功完成。

SQL> exec dbms_stats.gather_table_stats(user,'test2');

PL/SQL 过程已成功完成。

SQL> set timing on
SQL> set autotrace traceonly
SQL> update test1 t1
  2     set t1.object_type = (select object_type
  3                             from test2 t2
  4                            where t1.object_id = t2.object_id);

已更新303258行。

已用时间:  00: 00: 13.07

执行计划
----------------------------------------------------------
Plan hash value: 2560893763

-----------------------------------------------------------------------------------------------
| Id  | Operation                    | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | UPDATE STATEMENT             |                |   303K|  4146K|   949   (2)| 00:00:12 |
|   1 |  UPDATE                      | TEST1          |       |       |            |          |
|   2 |   TABLE ACCESS FULL          | TEST1          |   303K|  4146K|   949   (2)| 00:00:12 |
|   3 |   TABLE ACCESS BY INDEX ROWID| TEST2          |     1 |    14 |     4   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | IND_OBJECT_ID2 |     1 |       |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

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

4 - access("T2"."OBJECT_ID"=:B1)

统计信息
----------------------------------------------------------
        330  recursive calls
     338515  db block gets
    1250542  consistent gets
          1  physical reads
  107333692  redo size
        673  bytes sent via SQL*Net to client
        701  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
     303258  rows processed

SQL> commit;

提交完成。

已用时间:  00: 00: 00.00
SQL> merge into test1 t1
  2  using test2 t2
  3  on  (t1.object_id = t2.object_id)
  4  when matched then
  5    update set t1.object_type = t2.object_type;

303258 行已合并。

已用时间:  00: 00: 03.87

执行计划
----------------------------------------------------------
Plan hash value: 520388833

--------------------------------------------------------------------------------------
| Id  | Operation            | Name  | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | MERGE STATEMENT      |       |   303K|  5923K|       |  4947   (2)| 00:01:00 |
|   1 |  MERGE               | TEST1 |       |       |       |            |          |
|   2 |   VIEW               |       |       |       |       |            |          |
|*  3 |    HASH JOIN         |       |   303K|    53M|    30M|  4947   (2)| 00:01:00 |
|   4 |     TABLE ACCESS FULL| TEST2 |   303K|    26M|       |   957   (3)| 00:00:12 |
|   5 |     TABLE ACCESS FULL| TEST1 |   303K|    26M|       |   957   (3)| 00:00:12 |
--------------------------------------------------------------------------------------

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

3 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")

统计信息
----------------------------------------------------------
        378  recursive calls
     310584  db block gets
       8547  consistent gets
       3751  physical reads
   76712320  redo size
        678  bytes sent via SQL*Net to client
        671  bytes received via SQL*Net from client
          4  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
     303258  rows processed

SQL> commit;

提交完成。