1. 先说删除吧，因为刚搞了。
删除缓慢的原因：主要是约束的问题。（数据库在有约束的时候，进行操作，会根据约束对相关表进行验证，可想而知，20W的数据验证要耗费多久的时间）。其次就是sql的编写。（sql如果查询中包含子查询等的可以优化的where会影响匹配的速度<查询的话就不多逼逼了>）。索引的问题 请看下面的 补充部分
具体解决方法：
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ALTER TABLE tableName DISABLE TRIGGER ALL;
delete 目标语句
ALTER TABLE tableName ENABLE TRIGGER ALL;
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2. 更新，原因和删除大体一致，这里主要描述一下自己遇到的问题。
利用mybatis批量更新，第一张图是原本的批量更新的写法，发现在本地还可以的但是放在服务器会由于时间太久出现IO错误。
后来换成第二张图的方式来更新。当然也可以java代码for循环来单条更新来达到批量的目的。（对于这三种方式，第二种最快）
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<update id="batchUpdate2" parameterType="java.util.List">
UPDATE lt_dzheinfo
<trim prefix="set" suffixOverrides=",">
<trim prefix="dzzhao =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerBarcode}
</foreach>
</trim>
<trim prefix="xming =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerName}
</foreach>
</trim>
<trim prefix="ztai =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerState}
</foreach>
</trim>
<trim prefix="mima =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerPsw}
</foreach>
</trim>
<trim prefix="dzlxid =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerType}
</foreach>
</trim>
<trim prefix="zjlx =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.identityType}
</foreach>
</trim>
<trim prefix="zjhma =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.identityId}
</foreach>
</trim>
<trim prefix="qyrqi =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then TO_TIMESTAMP(#{reader.enableDate}, 'YYYY-MM-DD HH24:MI:SS')
</foreach>
</trim>
<trim prefix="jzrqi =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then TO_TIMESTAMP(#{reader.disableDate}, 'YYYY-MM-DD HH24:MI:SS')
</foreach>
</trim>
<trim prefix="libid =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerOfLibCode}
</foreach>
</trim>
<trim prefix="xingbie =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then
<choose>
<when test="reader.readerSex=='男'.toString()">
true
</when>
<otherwise>
false
</otherwise>
</choose>
</foreach>
</trim>
<trim prefix="userid =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerOperatorId}
</foreach>
</trim>
<trim prefix="beizhu =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then #{reader.readerRemark}
</foreach>
</trim>
<trim prefix="bzrqi =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then TO_TIMESTAMP(#{reader.enableDate}, 'YYYY-MM-DD HH24:MI:SS')
</foreach>
</trim>
<trim prefix="optime =case" suffix="end,">
<foreach collection="list" item="reader" index="index">
when dzid=#{reader.readerId} then TO_TIMESTAMP(#{reader.enableDate}, 'YYYY-MM-DD HH24:MI:SS')
</foreach>
</trim>
</trim>
where dzid in
<foreach collection="list" index="index" item="reader" separator="," open="(" close=")">
#{reader.readerId}
</foreach>
</update>
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<update id="batchUpdate" parameterType="java.util.List">
<foreach collection="list" index="index" item="reader" separator=";">
UPDATE lt_dzheinfo SET
dzzhao = #{reader.readerBarcode},
xming = #{reader.readerName},
ztai = #{reader.readerState},
mima = #{reader.readerPsw},
dzlxid = #{reader.readerType},
zjlx = #{reader.identityType},
zjhma = #{reader.identityId},
qyrqi = TO_TIMESTAMP(#{reader.enableDate}, 'YYYY-MM-DD HH24:MI:SS'),
jzrqi = TO_TIMESTAMP(#{reader.disableDate}, 'YYYY-MM-DD HH24:MI:SS'),
libid = #{reader.readerOfLibCode},
xingbie =
<choose>
<when test="reader.readerSex=='男'.toString()">
true
</when>
<otherwise>
false
</otherwise>
</choose>,
userid = #{reader.readerOperatorId},
beizhu = #{reader.readerRemark},
bzrqi = TO_TIMESTAMP(#{reader.enableDate}, 'YYYY-MM-DD HH24:MI:SS'),
optime = TO_TIMESTAMP(#{reader.enableDate}, 'YYYY-MM-DD HH24:MI:SS')
<where>
dzid = #{reader.readerId}
</where>
</foreach>
</update>
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补充：数据库突然变慢...索引作怪！
本人在最近发现，有些比容量较大的SQL数据库，在使用一段时间后，会突然变很慢很慢。特别体现在一些经常进行操作及查询的大表中。经过深入研究，这一般是索引的问题！如果把此表的索引重建一次，问题即可解决。
为更好地解决此问题，建意在SQL中新建一个数据库维护计划，选中“重新组织数据各索引页”选项，并设置每天运行一次。
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/*
刷新数据库视图.sql
*/
ALTER PROC P_RefreshView
as
DECLARE @vName sysname
DECLARE refresh_cursor CURSOR FOR
SELECT Name from sysobjects WHERE xtype = 'V' order by crdate
FOR READ ONLY
OPEN refresh_cursor
FETCH NEXT FROM refresh_cursor
INTO @vName
WHILE @@FETCH_STATUS = 0
BEGIN
--刷新指定视图的元数据。由于视图所依赖的基础对象的更改，视图的持久元数据会过期。
exec sp_refreshview @vName
PRINT '视图' + @vName + '已更新'
FETCH NEXT FROM refresh_cursor
INTO @vName
END
CLOSE refresh_cursor
DEALLOCATE refresh_cursor
GO
SET QUOTED_IDENTIFIER OFF
GO
SET ANSI_NULLS ON
GO
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以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。如有错误或未考虑完全的地方，望不吝赐教。
原文链接：https://blog.csdn.net/weixin_38289303/article/details/82185081