count(*)和sum(1) 的效率

count(*)和sum(*)的結果有時候是一樣的，所以有時候開放在寫存儲過程的時候會用到sum當count使用

這樣有不有問題呢？我們來討論下這2個的效率。

SQL> oradebug event 10046 trace name context forever,level 12;
Statement processed.
SQL>  select count(*) from business.PRPLCERTIFYIMGTEMPBAK;

  COUNT(*)
----------
 103287157

Elapsed: 00:00:10.01
SQL> select sum(1) from business.PRPLCERTIFYIMGTEMPBAK;

    SUM(1)
----------
 103287157

Elapsed: 00:00:13.27
SQL> oradebug event 10046 trace name context off;
Statement processed.
SQL> oradebug tracefile_name

從10046的信息，我們很容易就發現。sum的效率不足體現在cpu上，I/O完全一樣。

select count(*) 
from
 business.PRPLCERTIFYIMGTEMPBAK


call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.02          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        2      9.98       9.75     203828     203874          0           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4      9.98       9.77     203828     203874          0           1

Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: SYS

Rows     Row Source Operation
-------  ---------------------------------------------------
      1  SORT AGGREGATE (cr=203874 pr=203828 pw=0 time=9750091 us)
103287157   INDEX FAST FULL SCAN IDX_SERIALNO (cr=203874 pr=203828 pw=0 time=309861672 us)(object id 155232)


Elapsed times include waiting on following events:
  Event waited on                             Times   Max. Wait  Total Waited
  ----------------------------------------   Waited  ----------  ------------
  SQL*Net message to client                       2        0.00          0.00
  db file scattered read                      12791        0.00          0.65
  SQL*Net message from client                     2        3.47          3.47
********************************************************************************

select sum(1) 
from
 business.PRPLCERTIFYIMGTEMPBAK


call     count       cpu    elapsed       disk      query    current        rows
------- ------  -------- ---------- ---------- ---------- ----------  ----------
Parse        1      0.00       0.00          0          0          0           0
Execute      1      0.00       0.00          0          0          0           0
Fetch        2     13.26      12.95     203828     203874          0           1
------- ------  -------- ---------- ---------- ---------- ----------  ----------
total        4     13.26      12.95     203828     203874          0           1

Misses in library cache during parse: 1
Optimizer mode: ALL_ROWS
Parsing user id: SYS

Rows     Row Source Operation
-------  ---------------------------------------------------
      1  SORT AGGREGATE (cr=203874 pr=203828 pw=0 time=12957830 us)
103287157   INDEX FAST FULL SCAN IDX_SERIALNO (cr=203874 pr=203828 pw=0 time=206574428 us)(object id 155232)


Elapsed times include waiting on following events:
  Event waited on                             Times   Max. Wait  Total Waited
  ----------------------------------------   Waited  ----------  ------------
  SQL*Net message to client                       2        0.00          0.00
  db file scattered read                      12791        0.00          0.63
  SQL*Net message from client                     2       10.56         10.56

爲什麼sum的效率不如count呢。那就要了解count和sum的算法了
舉例說明他的算法：


在排序統計的時候
sum的算法爲1+1+1+1+1+1=6，sum是未知的，需要額外的CPU每一步都需要計算
count是1+2+3=6，就是說count的計算公式是定的，發現一行，加1，那麼就是1，下一次是從2開始加
顯然sum的效率不如count。

所以開發童鞋們。在遇到類似的時候儘量使用count，而不用sum。