要将表的限制条件写到与该表同级别的where中

测试目的：将朱查询的限制条件放到子查询的where中，查看性能影响。

测试数据：
create table t1 as select object_id,object_name from dba_objects;
create table t2 as select object_id,object_name from user_objects;
create table t3 as select rownum object_id,table_name object_name from user_tables;

analyze table t1 compute statistics for table for all columns;
analyze table t2 compute statistics for table for all columns;
analyze table t3 compute statistics for table for all columns;

SQL> select count(*) from t1;

  COUNT(*)
----------
     26341

SQL> select count(*) from t2;

  COUNT(*)
----------
     13135

SQL> select count(*) from t3;

  COUNT(*)
----------
      5891

开始测试：
1.主查询条件出现在子查询的where中（确实有开发是这样写的SQL，或许是动态生成SQL或者因为不太了解SQL编写规范手动写的）：
SQL> select *
  2    from t1
  3   where t1.object_id in
  4         (select t2.object_id
  5            from t2
  6           where t1.object_name in (select t3.object_name from t3));

已选择5890行。

执行计划
----------------------------------------------------------
Plan hash value: 1276346997

----------------------------------------------------------------------------------------
| Id  | Operation            | Name    | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |         | 25583 |  2723K|       |   244K  (1)| 00:48:50 |
|*  1 |  HASH JOIN SEMI      |         | 25583 |  2723K|  2280K|   244K  (1)| 00:48:50 |
|   2 |   TABLE ACCESS FULL  | T1      | 25585 |  1973K|       |    32   (0)| 00:00:01 |
|   3 |   VIEW               | VW_SQ_1 |    77M|  2213M|       | 89634   (1)| 00:17:56 |
|   4 |    NESTED LOOPS      |         |    77M|  2213M|       | 89634   (1)| 00:17:56 |
|   5 |     TABLE ACCESS FULL| T2      | 13135 |   166K|       |    17   (0)| 00:00:01 |
|   6 |     TABLE ACCESS FULL| T3      |  5891 |    97K|       |     7   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

   1 - access("T1"."OBJECT_ID"="T2.OBJECT_ID" AND "T1"."OBJECT_NAME"="ITEM_1")

Note
-----
   - dynamic sampling used for this statement (level=4)

统计信息
----------------------------------------------------------
         37  recursive calls
          0  db block gets
     316742  consistent gets
          0  physical reads
          0  redo size
     184781  bytes sent via SQL*Net to client
       4728  bytes received via SQL*Net from client
        394  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
       5890  rows processed

从执行计划可以看出，其实t2和t3走了笛卡尔积（SQL中这两个确实没有关联条件，从两个表nested loops后的返回基数77M可以看出，这两个表是走了笛卡尔积的）。笛卡尔积结果跟t1做hash join，t1将object_name传递给nested loop是的结果。返回5890条数据，消耗316742逻辑读。

这种性能方面的问题，我们是完全可以通过注意SQL的编码规范来规避的。

2.改写SQL，将主查询的限制条件放到与该其同级别的WHERE中。
SQL> select *
  2    from t1
  3   where t1.object_id in (select t2.object_id from t2)
  4     and t1.object_name in (select object_name from t3);

已选择5890行。

执行计划
----------------------------------------------------------
Plan hash value: 3620957986

------------------------------------------------------------------------------
| Id  | Operation             | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |      | 13135 |  1398K|    60   (4)| 00:00:01 |
|*  1 |  HASH JOIN RIGHT SEMI |      | 13135 |  1398K|    60   (4)| 00:00:01 |
|   2 |   TABLE ACCESS FULL   | T3   |  5891 |    97K|     9   (0)| 00:00:01 |
|*  3 |   HASH JOIN RIGHT SEMI|      | 13135 |  1180K|    50   (2)| 00:00:01 |
|   4 |    TABLE ACCESS FULL  | T2   | 13135 |   166K|    17   (0)| 00:00:01 |
|   5 |    TABLE ACCESS FULL  | T1   | 25585 |  1973K|    32   (0)| 00:00:01 |
------------------------------------------------------------------------------

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

   1 - access("T1"."OBJECT_NAME"="T3"."OBJECT_NAME")
   3 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")

Note
-----
   - dynamic sampling used for this statement (level=4)

统计信息
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        575  consistent gets
          0  physical reads
          0  redo size
     184781  bytes sent via SQL*Net to client
       4728  bytes received via SQL*Net from client
        394  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
       5890  rows processed
从执行计划来看，Oracle选择的表连接顺序都是最优的，逻辑读也从原来的316742下降到575，速度也提升很多。

切记：写SQL时一定要将对表的限制条件写到与该表同级别的where条件里。