use_hash的使用实例

SELECT count(*)
  FROM FEEDBACK f, company r, feedback_detail d, category c
 WHERE f.feedback_status = 2
   and f.feedback_target <> 5
   and f.feedback_target <> 6
   and f.receiver_company_id = r.id
   and r.is_validate = 'enabled'
   and f.id = d.feedback_id(+)
   and f.FEEDBACK_CATEGORY = c.id(+)
   and f.gmt_create >
       to_date('2008-03-05 14:30:00', 'yyyy-mm-dd hh24:mi:ss')
   and f.gmt_create <
       to_date('2008-09-05 14:30:00', 'yyyy-mm-dd hh24:mi:ss')

执行计划如下：
Execution Plan
----------------------------------------------------------
   0    SELECT STATEMENT ptimizer=FIRST_ROWS (Cost=5278 Card=1 Bytes=57)
   1  0   SORT (AGGREGATE)
   2  1     HASH JOIN (Cost=5278 Card=2843 Bytes=162051)
   3  2 NESTED LOOPS (OUTER) (Cost=5220 Card=2977 Bytes=128011)
   4  3   HASH JOIN (OUTER) (Cost=5220 Card=2977 Bytes=116103)
   5  4     TABLE ACCESS (FULL) OF 'FEEDBACK' (Cost=5217 Card=2977 Bytes=77402)
   6  4     TABLE ACCESS (FULL) OF 'FEEDBACK_DETAIL' (Cost=2 Card=82 Bytes=1066)
   7  3   INDEX (RANGE SCAN) OF 'CATEGORY_PK' (NON-UNIQUE)
   8  2 INDEX (FAST FULL SCAN) OF 'COMPANY_VLD_CTY_ID_IND' (NON-UNIQUE) (Cost=55 Card=25367 Bytes=355138)

 

 

Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
      79616  consistent gets
      0  physical reads
      0  redo size
    520  bytes sent via SQL*Net to client
    655  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed
     
     
走的是nested loop  

增加hint走 hash join 

SELECT /*+use_hash(f,r,d,c) */
count(*)
FROM FEEDBACK f,company r,feedback_detail d,category c
WHERE   f.feedback_status = 2 and f.feedback_target <> 5 and f.feedback_target <> 6
and f.receiver_company_id = r.id and r.is_validate = 'enabled' and f.id=d.feedback_id(+)
and f.FEEDBACK_CATEGORY = c.id(+)
and f.gmt_create > to_date('2008-03-05 14:30:00','yyyy-mm-dd hh24:mi:ss')
and f.gmt_create < to_date('2008-09-05 14:30:00','yyyy-mm-dd hh24:mi:ss')

执行计划如下：

Execution Plan
----------------------------------------------------------
   0    SELECT STATEMENT ptimizer=FIRST_ROWS (Cost=5281 Card=1 Byte
      s=57)

   1  0   SORT (AGGREGATE)
   2  1     HASH JOIN (OUTER) (Cost=5281 Card=2843 Bytes=162051)
   3  2 HASH JOIN (Cost=5277 Card=2843 Bytes=125092)
   4  3   HASH JOIN (OUTER) (Cost=5220 Card=2977 Bytes=89310)
   5  4     TABLE ACCESS (FULL) OF 'FEEDBACK' (Cost=5217 Card=
      2977 Bytes=77402)

   6  4     INDEX (FAST FULL SCAN) OF 'CATEGORY_PK' (NON-UNIQU
      E) (Cost=2 Card=836 Bytes=3344)

   7  3   INDEX (FAST FULL SCAN) OF 'COMPANY_VLD_CTY_ID_IND' (
      NON-UNIQUE) (Cost=55 Card=25367 Bytes=355138)

   8  2 TABLE ACCESS (FULL) OF 'FEEDBACK_DETAIL' (Cost=2 Card=
      82 Bytes=1066)

 

 

Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
      64638  consistent gets
      0  physical reads
      0  redo size
    520  bytes sent via SQL*Net to client
    655  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed  
     
     
一致性读减少了 ，，由于数据量比较小，若数据量很大的话，一致性读的减少量更大。如

japan@JAPAN>SELECT
  2  count(*)
  3  FROM FEEDBACK f,company r,feedback_detail d,category c
  4  WHERE   f.feedback_status = 2 and f.feedback_target <> 5 and f.feedback_target <> 6
  5  and f.receiver_company_id = r.id and r.is_validate = 'enabled' and f.id=d.feedback_id(+)
  6  and f.FEEDBACK_CATEGORY = c.id(+)
  7  /

  COUNT(*)
----------
    267142

Elapsed: 00:00:00.99

Execution Plan
----------------------------------------------------------
   0    SELECT STATEMENT ptimizer=FIRST_ROWS (Cost=5305 Card=1 Byte
      s=50)

   1  0   SORT (AGGREGATE)
   2  1     NESTED LOOPS (OUTER) (Cost=5305 Card=52351 Bytes=2617550
      )

   3  2 HASH JOIN (Cost=5305 Card=52351 Bytes=2408146)
   4  3   INDEX (FAST FULL SCAN) OF 'COMPANY_VLD_CTY_ID_IND' (
      NON-UNIQUE) (Cost=55 Card=25367 Bytes=355138)

   5  3   HASH JOIN (OUTER) (Cost=5233 Card=54835 Bytes=175472
      0)

   6  5     TABLE ACCESS (FULL) OF 'FEEDBACK' (Cost=5217 Card=
      54835 Bytes=1041865)

   7  5     TABLE ACCESS (FULL) OF 'FEEDBACK_DETAIL' (Cost=2 C
      ard=82 Bytes=1066)

   8  2 INDEX (RANGE SCAN) OF 'CATEGORY_PK' (NON-UNIQUE)

 

Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
     254410  consistent gets
      0  physical reads
      0  redo size
    520  bytes sent via SQL*Net to client
    655  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed
     

nested loop 的一致性读为 254410    

japan@JAPAN>SELECT /*+use_hash(f,r,d,c) */
  2  count(*)
  3  FROM FEEDBACK f,company r,feedback_detail d,category c
  4  WHERE   f.feedback_status = 2 and f.feedback_target <> 5 and f.feedback_target <> 6
  5  and f.receiver_company_id = r.id and r.is_validate = 'enabled' and f.id=d.feedback_id(+)
  6  and f.FEEDBACK_CATEGORY = c.id(+)
  7  /

  COUNT(*)
----------
    267251

Elapsed: 00:00:00.67

Execution Plan
----------------------------------------------------------
   0    SELECT STATEMENT ptimizer=FIRST_ROWS (Cost=5325 Card=1 Byte
      s=50)

   1  0   SORT (AGGREGATE)
   2  1     HASH JOIN (Cost=5325 Card=52351 Bytes=2617550)
   3  2 INDEX (FAST FULL SCAN) OF 'COMPANY_VLD_CTY_ID_IND' (NO
      N-UNIQUE) (Cost=55 Card=25367 Bytes=355138)

   4  2 HASH JOIN (OUTER) (Cost=5251 Card=54835 Bytes=1974060)
   5  4   HASH JOIN (OUTER) (Cost=5233 Card=54835 Bytes=126120
      5)

   6  5     TABLE ACCESS (FULL) OF 'FEEDBACK' (Cost=5217 Card=
      54835 Bytes=1041865)

   7  5     INDEX (FAST FULL SCAN) OF 'CATEGORY_PK' (NON-UNIQU
      E) (Cost=2 Card=836 Bytes=3344)

   8  4   TABLE ACCESS (FULL) OF 'FEEDBACK_DETAIL' (Cost=2 Car
      d=82 Bytes=1066)

 

 

Statistics
----------------------------------------------------------
      0  recursive calls
      0  db block gets
      64638  consistent gets
      0  physical reads
      0  redo size
    520  bytes sent via SQL*Net to client
    655  bytes received via SQL*Net from client
      2  SQL*Net roundtrips to/from client
      0  sorts (memory)
      0  sorts (disk)
      1  rows processed   

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