Oracle update性能优化

当执行批量刷新数据时,以前我写过最好是写成merge into,当然还有一种方式,如下:

SQL> create table test1 as select * from dba_objects where rownum <100;
SQL> create table test2 as select * from dba_objects where rownum <1000;
 SQL> create unique index ind_t1_object_id on test1(object_id);
 SQL> create unique index ind_t2_object_id on test2(object_id);
 SQL> exec dbms_stats.gather_table_stats(user,'test1');
 SQL> exec dbms_stats.gather_table_stats(user,'test2');
 SQL> set autotrace traceonly
 SQL> update test1 t1
        set t1.object_name = (select t2.object_name
                                from test2 t2
                              where t1.object_id = t2.object_id)
      where exists (select 1 from test2 t3 where t3.object_id = t1.object_id);--千万不能丢掉exists,否则很多匹配不上的记录object_name就为null。
 已更新99行。
 执行计划
----------------------------------------------------------
Plan hash value: 1549919212
 -------------------------------------------------------------------------------------------------
 | Id  | Operation                    | Name            | Rows  | Bytes | Cost (%CPU)| Time    |
 -------------------------------------------------------------------------------------------------
 |  0 | UPDATE STATEMENT            |                  |    99 |  1584 |  304  (33)| 00:00:04 |
 |  1 |  UPDATE                      | TEST1            |      |      |            |          |
 |*  2 |  HASH JOIN SEMI            |                  |    99 |  1584 |    7  (15)| 00:00:01 |
 |  3 |    TABLE ACCESS FULL        | TEST1            |    99 |  1188 |    3  (0)| 00:00:01 |
 |  4 |    INDEX FAST FULL SCAN      | IND_T2_OBJECT_ID |  999 |  3996 |    3  (0)| 00:00:01 |
 |  5 |  TABLE ACCESS BY INDEX ROWID| TEST2            |    1 |    20 |    2  (0)| 00:00:01 |
 |*  6 |    INDEX RANGE SCAN          | IND_T2_OBJECT_ID |    1 |      |    1  (0)| 00:00:01 |
 -------------------------------------------------------------------------------------------------
 Predicate Information (identified by operation id):
 ---------------------------------------------------
    2 - access("T3"."OBJECT_ID"="T1"."OBJECT_ID")
    6 - access("T2"."OBJECT_ID"=:B1)
统计信息
----------------------------------------------------------
          3  recursive calls
        103  db block gets
      217  consistent gets
          0  physical reads
      23656  redo size
        559  bytes sent via SQL*Net to client
        922  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          99  rows processed


 SQL> commit;
 SQL> update (select t1.object_name, t2.object_name new_object_name
              from test1 t1, test2 t2
              where t1.object_id = t2.object_id)
    set object_name = new_object_name;
已更新99行。
 执行计划
----------------------------------------------------------
Plan hash value: 1124869545
 -----------------------------------------------------------------------------
 | Id  | Operation          | Name  | Rows  | Bytes | Cost (%CPU)| Time    |
 -----------------------------------------------------------------------------
 |  0 | UPDATE STATEMENT    |      |    99 |  3168 |    10  (10)| 00:00:01 |
 |  1 |  UPDATE            | TEST1 |      |      |            |          |
 |*  2 |  HASH JOIN        |      |    99 |  3168 |    10  (10)| 00:00:01 |
 |  3 |    TABLE ACCESS FULL| TEST1 |    99 |  1188 |    3  (0)| 00:00:01 |
 |  4 |    TABLE ACCESS FULL| TEST2 |  999 | 19980 |    6  (0)| 00:00:01 |
 -----------------------------------------------------------------------------
 Predicate Information (identified by operation id):
 ---------------------------------------------------
    2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
统计信息
----------------------------------------------------------
          3  recursive calls
        103  db block gets
      25  consistent gets
          0  physical reads
      23736  redo size
        561  bytes sent via SQL*Net to client
        858  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          99  rows processed
 
SQL> commit;

总结:可以看到,第二种写法比第一种写法少扫描一张表,逻辑读也少了很多。

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