某银行做校园卡缴费的测试过程中，发现成功缴费时间很长，大约需要75秒左右，原因分析：在做校园卡缴费的时候，首先是从数据库中查询到需要缴费的费项，然后再对该费项进行缴费，缴费成功后修改相应的状态，交易完成后，查看日志，发现下面的查询语句执行时间很长，在数据库中执行时间大约33.782 secs.秒
低效SQL（耗时：33.782 secs.）：
| 代码如下 |
复制代码 |
|
SELECT DISTINCT hmc.bjbh,
hmc.pxjgid,
hmc.bjmc,
csb.tjxbt,
hmc.pxlx,
hmc.bjrs,
hmc.bjsfjs,
csb.xzqhdm,
hmc.shyj,
hmc.kbrq
FROM px_bjhmc hmc, ldlsc_xtcsb csb, px_xyhmc c
WHERE hmc.pxjgid = csb.dwbh
AND hmc.pxjgid = c.pxjgid
AND hmc.shyj = '1' ---www.111com.net
AND hmc.bjsfjs = '1'
AND c.pxsfjs = '1'
AND hmc.jyrq >= '20130505000000'
AND hmc.jyrq <= '20140102235959'
AND hmc.pxjgid = '00000000000020'
|
优化方案一（耗时：6.485 secs.）：
| 代码如下 |
复制代码 |
SELECT DISTINCT hmc.bjbh,
hmc.pxjgid,
hmc.bjmc,
csb.tjxbt,
hmc.pxlx,
hmc.bjrs,
hmc.bjsfjs,
csb.xzqhdm,
hmc.shyj,
hmc.kbrq
FROM px_bjhmc hmc, ldlsc_xtcsb csb, px_xyhmc c
WHERE hmc.pxjgid = csb.dwbh
AND hmc.pxjgid = c.pxjgid
AND csb.dwbh = c.pxjgid ----在这里
AND hmc.shyj = '1'
AND hmc.bjsfjs = '1'
AND c.pxsfjs = '1'
AND hmc.jyrq >= '20130505000000'
AND hmc.jyrq <= '20140102235959'
AND hmc.pxjgid = '00000000000020' |
低效SQL（耗时：33.782 secs.）：
| 代码如下 |
复制代码 |
|
SELECT DISTINCT hmc.bjbh,
hmc.pxjgid,
hmc.bjmc,
csb.tjxbt,
hmc.pxlx,
hmc.bjrs,
hmc.bjsfjs,
csb.xzqhdm,
hmc.shyj,
hmc.kbrq
FROM px_bjhmc hmc, ldlsc_xtcsb csb, px_xyhmc c
WHERE hmc.pxjgid = csb.dwbh
AND hmc.pxjgid = c.pxjgid
AND hmc.shyj = '1'
AND hmc.bjsfjs = '1'
AND c.pxsfjs = '1'
AND hmc.jyrq >= '20130505000000'
AND hmc.jyrq <= '20140102235959'
AND hmc.pxjgid = '00000000000020'
|
优化方案二（耗时：0.031 secs.）：
| 代码如下 |
复制代码 |
|
SELECT DISTINCT hmc.bjbh,
hmc.pxjgid,
hmc.bjmc,
csb.tjxbt,
hmc.pxlx,
hmc.bjrs,
hmc.bjsfjs,
csb.xzqhdm,
hmc.shyj,
hmc.kbrq
FROM px_bjhmc hmc, ldlsc_xtcsb csb
WHERE hmc.pxjgid = csb.dwbh
AND EXISTS (SELECT 1
FROM px_xyhmc c
WHERE hmc.pxjgid = c.pxjgid
AND c.pxsfjs = '1')
AND hmc.shyj = '1'
AND hmc.bjsfjs = '1'
AND hmc.jyrq >= '20130505000000'
AND hmc.jyrq <= '20140102235959'
AND hmc.pxjgid = '00000000000020'
|
•多表同时关联：a.id = b.id， a.id = c.id， b.id = c.id；
•多表联接查询时，联少不联多，避免笛卡尔积操作；
•当一个表仅作为检索条件时，避免联接查询，多数情况下用Exists语句进行优化；
