生产环境有一条sql语句执行比较频繁，占用了大量的cpu资源。原本执行需要花费11秒。在一次排查中引起了我的注意，决定看看cpu消耗到底在哪儿？
sql语句是比较简单的，通过查询SUBSCRIBER_FA_V是一个视图。在视图中关联了几个和业务核心表。
SELECT TO_CHAR(SUBSCRIBER_NO) SUBSCRIBER_ID,
SUB_STATUS,
SUB_STS_RSN_CD,
TO_CHAR(SUB_STATUS_DATE, 'yyyyMMdd') SUB_STATUS_DATE,
SUBSCRIBER_TYPE
FROM SUBSCRIBER_FA_V
WHERE BAN = :1
AND ROWNUM <= :2
AND SUB_STATUS NOT IN ('C', 'L', 'T')
ORDER BY INIT_ACT_DATE, SUBSCRIBER_NO
如果想做sql检查，对于sql中传入的变量，sql monitor提供了很方便的功能。
Binds
很清晰看到正在执行的sql语句对应的变量值。
还是来看看执行计划。
性能瓶颈都在几个全表扫描和一个分析函数相关的rank操作中。毕竟返回的记录数只有1条。如果过多的资源都消耗在一些不正确的连接访问中，是完全可以避免的。
视图的内容如下：
SELECT xxxxxxx
1 RANK
FROM subscriber, distribute ed, channel cpc
WHERE cpc.pym_channel_no = ed.target_pcn
AND ed.agreement_no = subscriber.subscriber_no
AND eg_dist_type = 'D'
AND ed.expiration_date IS NULL
AND SUBSCRIBER.SUB_STATUS NOT IN ('C', 'L', 'T')
UNION
SELECT xxxxxxx
FROM (SELECT cpc.ban,
subscriber.prim_resource_val,
subscriber.init_act_date,
SUBSCRIBER.CUSTOMER_ID,
subscriber.subscriber_no,
SUBSCRIBER.SUBSCRIBER_TYPE,
SUBSCRIBER.SUB_STATUS,
SUBSCRIBER.SUB_STS_RSN_CD,
SUBSCRIBER.SUB_STATUS_DATE,
SUBSCRIBER.EFFECTIVE_DATE,
ROW_NUMBER ()
OVER (
PARTITION BY subscriber.subscriber_no
ORDER BY
ed.expiration_date, subscriber.subscriber_no DESC)
AS RANK
FROM subscriber, distribute ed, channel cpc
WHERE cpc.pym_channel_no = ed.target_pcn
AND ed.agreement_no = subscriber.subscriber_no
AND eg_dist_type = 'D'
AND SUBSCRIBER.SUB_STATUS IN ('C', 'L', 'T'))
WHERE RANK = 1
根据执行计划，出问题的正式标黄的union子句。
根据传入的参数，是可以走索引的，但是在Union子句中，嵌入了子查询，导致在整个视图在数据的访问中，先全表扫描整个子查询，然后再匹配传入的参数。
视图的内容是不能随便改的，可能在这种场景中合适，其他的相关查询就有问题了。
所以尝试把视图的内容直接转换成直接的sql,标黄的部分是做的相应改动。
select xxxxxxx
from (SELECT xxxxxxx
1 RANK
FROM subscriber, event_distribute ed, csm_pay_channel cpc
WHERE cpc.pym_channel_no = ed.target_pcn
AND ed.agreement_no = subscriber.subscriber_no
AND eg_dist_type = 'D'
AND ed.expiration_date IS NULL
AND SUBSCRIBER.SUB_STATUS NOT IN ('C', 'L', 'T')
and cpc.ban = 10308170
UNION
SELECT "SUBSCRIBER_NO",
"INIT_ACT_DATE",
"SUB_STATUS",
"SUB_STS_RSN_CD",
"SUB_STATUS_DATE",
"SUBSCRIBER_TYPE",
"RANK"
FROM (SELECT subscriber.subscriber_no,
SUBSCRIBER.INIT_ACT_DATE,
SUBSCRIBER.SUBSCRIBER_TYPE,
SUBSCRIBER.SUB_STATUS,
SUBSCRIBER.SUB_STS_RSN_CD,
SUBSCRIBER.SUB_STATUS_DATE,
ROW_NUMBER() OVER(PARTITION BY subscriber.subscriber_no ORDER BY ed.expiration_date, subscriber.subscriber_no DESC) AS RANK
FROM subscriber, event_distribute ed, csm_pay_channel cpc
WHERE cpc.pym_channel_no = ed.target_pcn
AND ed.agreement_no = subscriber.subscriber_no
AND eg_dist_type = 'D'
AND SUBSCRIBER.SUB_STATUS IN ('C', 'L', 'T')
and cpc.ban = 10308170)
WHERE RANK = 1)
where SUB_STATUS NOT IN ('C','L','T') and rownum < 6
ORDER BY INIT_ACT_DATE, SUBSCRIBER_ID
看似语句挺长的，但是走了索引执行效率还是很高的。
Elapsed: 00:00:00.01
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
22 consistent gets
0 physical reads
0 redo size
857 bytes sent via SQL*Net to client
520 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
3 sorts (memory)
0 sorts (disk)
1 rows processed
本文转自ICT时空dbasdk的博客，原文链接： 生产环境sql语句调优实战第三篇 ，如需转载请自行联系原博主。
