SQL查询优化对比实例

时间：2022-06-29 07:59:01 编辑：袖梨来源：一聚教程网

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

   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；

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