一条SQL语句执行得很慢的原因盘点

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"前言","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"说实话，这个问题可以涉及到 MySQL 的很多核心知识，可以扯出一大堆，就像要考你计算机网络的知识时，问你“输入URL回车之后，究竟发生了什么”一样，看看你能说出多少了。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以一篇文章肯定是讲不完的，我这篇文章，更多的是在分析“为什么”，例如可能你知道系统会选错索引，但是可能不并不清楚系统选择索引的原理。如果想搞清楚后者，可以点击领取我整理的","attrs":{}},{"type":"link","attrs":{"href":"https://mp.weixin.qq.com/s?__biz=MzkzNjE1NTcyNQ==&mid=2247484482&idx=1&sn=00b0d901e54138676e93efba888cc037&chksm=c2a24b60f5d5c276b8d81c354293fd13413ef50e5425f1dda832f96a1dba5e3a2f19804a1d7e&token=1831800176&lang=zh_CN#rd","title":"","type":null},"content":[{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"MySQL学习笔记","attrs":{}}]},{"type":"text","text":"，我在里面有详细的论述。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"好了，说回正题！","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"之前腾讯面试的实话，也问到这个问题了，不过答的很不好，之前没去想过相关原因，导致一时之间扯不出来。所以今天，我带大家来详细扯一下有哪些原因，相信你看完之后一定会有所收获，不然你打我。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"开始装逼：分类讨论","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一条 SQL 语句执行的很慢，那是每次执行都很慢呢？还是大多数情况下是正常的，偶尔出现很慢呢？所以我觉得，我们还得分以下两种情况来讨论。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"1、大多数情况是正常的，只是偶尔会出现很慢的情况。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"2、在数据量不变的情况下，这条SQL语句一直以来都执行的很慢。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"针对这两种情况，我们来分析下可能是哪些原因导致的。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"针对偶尔很慢的情况","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一条 SQL 大多数情况正常，偶尔才能出现很慢的情况，针对这种情况，我觉得这条SQL语句的书写本身是没什么问题的，而是其他原因导致的，那会是什么原因呢？","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"数据库在刷新脏页我也无奈啊","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当我们要往数据库插入一条数据、或者要更新一条数据的时候，我们知道数据库会在","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"内存","attrs":{}},{"type":"text","text":"中把对应字段的数据更新了，但是更新之后，这些更新的字段并不会马上同步持久化到","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"磁盘","attrs":{}},{"type":"text","text":"中去，而是把这些更新的记录写入到 redo log 日记中去，等到空闲的时候，在通过 redo log 里的日记把最新的数据同步到","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"磁盘","attrs":{}},{"type":"text","text":"中去。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不过，redo log 里的容量是有限的，如果数据库一直很忙，更新又很频繁，这个时候 redo log 很快就会被写满了，这个时候就没办法等到空闲的时候再把数据同步到磁盘的，只能暂停其他操作，全身心来把数据同步到磁盘中去的，而这个时候，","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"就会导致我们平时正常的SQL语句突然执行的很慢","attrs":{}},{"type":"text","text":"，所以说，数据库在在同步数据到磁盘的时候，就有可能导致我们的SQL语句执行的很慢了。","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"拿不到锁我能怎么办","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这个就比较容易想到了，我们要执行的这条语句，刚好这条语句涉及到的","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"表","attrs":{}},{"type":"text","text":"，别人在用，并且加锁了，我们拿不到锁，只能慢慢等待别人释放锁了。或者，表没有加锁，但要使用到的某个一行被加锁了，这个时候，我也没办法啊。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果要判断是否真的在等待锁，我们可以用 ","attrs":{}},{"type":"text","marks":[{"type":"strong","attrs":{}}],"text":"show processlist","attrs":{}},{"type":"text","text":"这个命令来查看当前的状态哦，这里我要提醒一下，有些命令最好记录一下，反正，我被问了好几个命令，都不知道怎么写，呵呵。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下来我们来访分析下第二种情况，我觉得第二种情况的分析才是最重要的","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"针对一直都这么慢的情况","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果在数据量一样大的情况下，这条 SQL 语句每次都执行的这么慢，那就就要好好考虑下你的 SQL 书写了，下面我们来分析下哪些原因会导致我们的 SQL 语句执行的很不理想。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我们先来假设我们有一个表，表里有下面两个字段,分别是主键 id，和两个普通字段 c 和 d。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"mysql> CREATE TABLE `t` (\n `id` int(11) NOT NULL,\n `c` int(11) DEFAULT NULL,\n `d` int(11) DEFAULT NULL,\n PRIMARY KEY (`id`)\n) ENGINE=InnoDB;\n","attrs":{}}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"扎心了，没用到索引","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"没有用上索引，我觉得这个原因是很多人都能想到的，例如你要查询这条语句","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":"text"},"content":[{"type":"text","text":"select * from t where 100