count(*)和sum(1) 的效率
count(*)和sum(*)的结果有时候是一样的，所以有时候开放在写存储过程的时候会用到sum当count使用

这样有不有问题呢？我们来讨论下这2个的效率。

select count(*) from business.PRPLCERTIFYIMGTEMPBAKcall     count       cpu    elapsed       disk      query    current        rows------- ------  -------- ---------- ---------- ---------- ----------  ----------Parse        1      0.00       0.02          0          0          0           0Execute      1      0.00       0.00          0          0          0           0Fetch        2      9.98       9.75     203828     203874          0           1------- ------  -------- ---------- ---------- ---------- ----------  ----------total        4      9.98       9.77     203828     203874          0           1Misses in library cache during parse: 1Optimizer mode: ALL_ROWSParsing user id: SYSRows     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.PRPLCERTIFYIMGTEMPBAKcall     count       cpu    elapsed       disk      query    current        rows------- ------  -------- ---------- ---------- ---------- ----------  ----------Parse        1      0.00       0.00          0          0          0           0Execute      1      0.00       0.00          0          0          0           0Fetch        2     13.26      12.95     203828     203874          0           1------- ------  -------- ---------- ---------- ---------- ----------  ----------total        4     13.26      12.95     203828     203874          0           1Misses in library cache during parse: 1Optimizer mode: ALL_ROWSParsing user id: SYSRows     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

为什么sum的效率不如count呢。那就要了解count和sum的算法了
举例说明他的算法：


在排序统计的时候
sum的算法为1+1+1+1+1+1=6，sum是未知的，需要额外的CPU每一步都需要计算
count是1+2+3=6，就是说count的计算公式是定的，发现一行，加1，那么就是1，下一次是从2开始加
显然sum的效率不如count。

所以开发童鞋们。在遇到类似的时候尽量使用count，而不用sum。