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优化拥有谓词or的子查询

原创 Linux操作系统 作者:myownstars 时间:2011-07-13 11:20:22 0 删除 编辑

于节点2上发现一个sql,其结构如下:
select distinct t.attr, t.item
 from a t
where attr = :1
  and (pro = :2 or exists
       (select 1
          from b hps
         where hps.pro = :3
           and t.pro = hps.sub_pro))
鉴于以往的经验,一开始就觉得子查询中的连接谓词or有问题。
注:
表a有50多万条记录,attr和pro上分别有索引;表b有10万条记录,pro上有单独索引,同时(pro,sub_pro)组成unique索引;
同时表a上的attr,pro字段的分布情况如下,对于表a,pro字段的选择性比attr高出了很多
SQL> select count(distinct pro),count(distinct attr) from a;
 
COUNT(DISTINCT pro)           COUNT(DISTINCT attr)
------------------------- ---------------------------
          164067                         716
          
 
先通过set autotrace traceonly查看一把其执行计划和consistent gets
 --------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                        |    63 |   945 |    83   (2)| 00:00:01 |
|   1 |  HASH UNIQUE                  |                        |    63 |   945 |    83   (2)| 00:00:01 |
|*  2 |   FILTER                      |                        |       |       |            |          |
|   3 |    TABLE ACCESS BY INDEX ROWID| a                      |  1268 | 19020 |    82   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | idx_a_attr             |  1268 |       |     8   (0)| 00:00:01 |
|*  5 |    INDEX UNIQUE SCAN          | idx_b_un_pro_sub       |     1 |    12 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------

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

   2 - filter("pro"=TO_NUMBER(:B) OR  EXISTS (SELECT 0 FROM "b" "HPS" WHERE
              "HPS"."SUB_pro"=:B1 AND "HPS"."pro"=TO_NUMBER(:C)))
   4 - access("attr"=TO_NUMBER(:A))
   5 - access("HPS"."pro"=TO_NUMBER(:C) AND "HPS"."SUB_pro"=:B1)


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
       4690  consistent gets
          0  physical reads
          0  redo size
        589  bytes sent via SQL*Net to client
        487  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
 
从执行计划看出,访问表a的时候选择了attr上的索引,实际效果不佳,逻辑读有4690;
通过等价改写后的sql如下,去除了谓词or,改用了union
select distinct t.attr, t.item
       from a t
      where attr = :a
        and pro = :b
union
select distinct t.attr, t.item
       from a t
      where attr = :a
            and exists
             (select 1
                from b hps
               where hps.pro = :c
                 and t.pro = hps.sub_pro)
改写后的sql的执行计划如下               
--------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                        |     6 |   150 |    16  (75)| 00:00:01 |
|   1 |  SORT UNIQUE                  |                        |     6 |   150 |    16  (75)| 00:00:01 |
|   2 |   UNION-ALL                   |                        |       |       |            |          |
|*  3 |    TABLE ACCESS BY INDEX ROWID| a                      |     1 |    15 |     4   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | idx_a_pro              |     3 |       |     3   (0)| 00:00:01 |
|*  5 |    TABLE ACCESS BY INDEX ROWID| a                      |     1 |    15 |     3   (0)| 00:00:01 |
|   6 |     NESTED LOOPS              |                        |     5 |   135 |    10  (10)| 00:00:01 |
|   7 |      SORT UNIQUE              |                        |     5 |    60 |     2   (0)| 00:00:01 |
|*  8 |       INDEX RANGE SCAN        | idx_b_un_pro_sub       |     5 |    60 |     2   (0)| 00:00:01 |
|*  9 |      INDEX RANGE SCAN         | idx_a_pro              |     3 |       |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------

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

   3 - filter("attr"=TO_NUMBER(:A))
   4 - access("pro"=TO_NUMBER(:B))
   5 - filter("attr"=TO_NUMBER(:A))
   8 - access("HPS"."pro"=TO_NUMBER(:C))
   9 - access("T"."pro"="HPS"."SUB_pro")


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         24  consistent gets
          0  physical reads
          0  redo size
        589  bytes sent via SQL*Net to client
        487  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          2  sorts (memory)
          0  sorts (disk)
          1  rows processed
访问表a时选择了pro上的索引,虽然增加了一次访问次数,但是逻辑读却下降到了24.

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