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mysql explain详解

时间：2021-07-01 10:21:17 帮助过：9人阅读

下面是使用 explain 的例子： 

在 select 语句之前增加 explain 关键字，MySQL 会在查询上设置一个标记，执行查询时，会返回执行计划的信息，而不是执行这条SQL（如果 from 中包含子查询，仍会执行该子查询，将结果放入临时表中）。

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span><span style="color: #000000"> actor;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>  <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> actor <span style="color: #808080">|</span> <span style="color: #808080">ALL</span>  <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>

在查询中的每个表会输出一行，如果有两个表通过 join 连接查询，那么会输出两行。表的意义相当广泛：可以是子查询、一个 union 结果等。

explain 有两个变种：

1）explain extended：会在 explain  的基础上额外提供一些查询优化的信息。紧随其后通过 show warnings 命令可以 得到优化后的查询语句，从而看出优化器优化了什么。额外还有 filtered 列，是一个半分比的值，rows * filtered/100 可以估算出将要和 explain 中前一个表进行连接的行数（前一个表指 explain 中的id值比当前表id值小的表）。

  1. <span style="font-size: 12px">mysql<span style="color: #808080">></span> explain extended <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #0000ff">where</span> id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">1</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>     <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> filtered <span style="color: #808080">|</span> Extra <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film  <span style="color: #808080">|</span> const <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span> <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span>   <span style="color: #800000; font-weight: bold">100.00</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+</span>
  7. <span style="color: #000000">
  8. mysql</span><span style="color: #808080">></span><span style="color: #000000"> show warnings;
  9. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+--------------------------------------------------------------------------------+</span>
  10. <span style="color: #808080">|</span> <span style="color: #0000ff">Level</span> <span style="color: #808080">|</span> Code <span style="color: #808080">|</span> Message                                                                        <span style="color: #808080">|</span>
  11. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+--------------------------------------------------------------------------------+</span>
  12. <span style="color: #808080">|</span> Note  <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">1003</span> <span style="color: #808080">|</span> <span style="color: #008080">/*</span><span style="color: #008080"> select#1 </span><span style="color: #008080">*/</span> <span style="color: #0000ff">select</span> <span style="color: #ff0000">‘</span><span style="color: #ff0000">1</span><span style="color: #ff0000">‘</span> <span style="color: #0000ff">AS</span> `id`,<span style="color: #ff0000">‘</span><span style="color: #ff0000">film1</span><span style="color: #ff0000">‘</span> <span style="color: #0000ff">AS</span> `name` <span style="color: #0000ff">from</span> `test`.`film` <span style="color: #0000ff">where</span> <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span>
  13. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+--------------------------------------------------------------------------------+<br></span></span>

2）explain partitions：相比 explain 多了个 partitions 字段，如果查询是基于分区表的话，会显示查询将访问的分区。

explain 中的列

接下来我们将展示 explain 中每个列的信息。

1. id列

id列的编号是 select 的序列号，有几个 select 就有几个id，并且id的顺序是按 select 出现的顺序增长的。MySQL将 select 查询分为简单查询和复杂查询。复杂查询分为三类：简单子查询、派生表（from语句中的子查询）、union 查询。

1）简单子查询

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> (<span style="color: #0000ff">select</span> <span style="color: #800000; font-weight: bold">1</span> <span style="color: #0000ff">from</span> actor limit <span style="color: #800000; font-weight: bold">1</span>) <span style="color: #0000ff">from</span><span style="color: #000000"> film;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>      <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>     <span style="color: #808080">|</span> film  <span style="color: #808080">|</span> <span style="color: #0000ff">index</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> idx_name <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">32</span>      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> SUBQUERY    <span style="color: #808080">|</span> actor <span style="color: #808080">|</span> <span style="color: #0000ff">index</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>  <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  7. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span> 

2）from子句中的子查询

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> id <span style="color: #0000ff">from</span> (<span style="color: #0000ff">select</span> id <span style="color: #0000ff">from</span> film) <span style="color: #0000ff">as</span><span style="color: #000000"> der;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+---------------+----------+---------+------+------+-------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>      <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+---------------+----------+---------+------+------+-------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>     <span style="color: #808080">|</span> <span style="color: #808080"><</span>derived2<span style="color: #808080">></span> <span style="color: #808080">|</span> <span style="color: #808080">ALL</span>   <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>     <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>        <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> DERIVED     <span style="color: #808080">|</span> film       <span style="color: #808080">|</span> <span style="color: #0000ff">index</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> idx_name <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">32</span>      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  7. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+---------------+----------+---------+------+------+-------------+</span>

这个查询执行时有个临时表别名为der，外部 select 查询引用了这个临时表

3）union查询

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #800000; font-weight: bold">1</span> <span style="color: #0000ff">union</span> <span style="color: #808080">all</span> <span style="color: #0000ff">select</span> <span style="color: #800000; font-weight: bold">1</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type  <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>  <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra           <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> No tables used  <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> <span style="color: #0000ff">UNION</span>        <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> No tables used  <span style="color: #808080">|</span>
  7. <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">UNION</span> RESULT <span style="color: #808080">|</span> <span style="color: #808080"><</span>union1,<span style="color: #800000; font-weight: bold">2</span><span style="color: #808080">></span> <span style="color: #808080">|</span> <span style="color: #808080">ALL</span>  <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">temporary</span> <span style="color: #808080">|</span>
  8. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>

union结果总是放在一个匿名临时表中，临时表不在SQL总出现，因此它的id是NULL。

2. select_type列

select_type 表示对应行是是简单还是复杂的查询，如果是复杂的查询，又是上述三种复杂查询中的哪一种。

1）simple：简单查询。查询不包含子查询和union

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #0000ff">where</span> id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">2</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+-------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>     <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+-------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film  <span style="color: #808080">|</span> const <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span> <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+-------+</span>

2）primary：复杂查询中最外层的 select

3）subquery：包含在 select 中的子查询（不在 from 子句中）

4）derived：包含在 from 子句中的子查询。MySQL会将结果存放在一个临时表中，也称为派生表（derived的英文含义）

用这个例子来了解 primary、subquery 和 derived 类型

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> (<span style="color: #0000ff">select</span> <span style="color: #800000; font-weight: bold">1</span> <span style="color: #0000ff">from</span> actor <span style="color: #0000ff">where</span> id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">1</span>) <span style="color: #0000ff">from</span> (<span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #0000ff">where</span> id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">1</span><span style="color: #000000">) der;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type   <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>     <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>     <span style="color: #808080">|</span> <span style="color: #808080"><</span>derived3<span style="color: #808080">></span> <span style="color: #808080">|</span> system <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>        <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">3</span> <span style="color: #808080">|</span> DERIVED     <span style="color: #808080">|</span> film       <span style="color: #808080">|</span> const  <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span> <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>        <span style="color: #808080">|</span>
  7. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> SUBQUERY    <span style="color: #808080">|</span> actor      <span style="color: #808080">|</span> const  <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span> <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  8. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+</span> 

5）union：在 union 中的第二个和随后的 select

6）union result：从 union 临时表检索结果的 select

用这个例子来了解 union 和 union result 类型：

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #800000; font-weight: bold">1</span> <span style="color: #0000ff">union</span> <span style="color: #808080">all</span> <span style="color: #0000ff">select</span> <span style="color: #800000; font-weight: bold">1</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type  <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>  <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra           <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> No tables used  <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> <span style="color: #0000ff">UNION</span>        <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> No tables used  <span style="color: #808080">|</span>
  7. <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">UNION</span> RESULT <span style="color: #808080">|</span> <span style="color: #808080"><</span>union1,<span style="color: #800000; font-weight: bold">2</span><span style="color: #808080">></span> <span style="color: #808080">|</span> <span style="color: #808080">ALL</span>  <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">temporary</span> <span style="color: #808080">|</span>
  8. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>

3. table列

这一列表示 explain 的一行正在访问哪个表。

当 from 子句中有子查询时，table列是 <derivenN> 格式，表示当前查询依赖 id=N 的查询，于是先执行 id=N 的查询。当有 union 时，UNION RESULT 的 table 列的值为 <union1,2>，1和2表示参与 union 的 select 行id。

4. type列

这一列表示关联类型或访问类型，即MySQL决定如何查找表中的行。

依次从最优到最差分别为：system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

NULL：mysql能够在优化阶段分解查询语句，在执行阶段用不着再访问表或索引。例如：在索引列中选取最小值，可以单独查找索引来完成，不需要在执行时访问表

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #ff00ff">min</span>(id) <span style="color: #0000ff">from</span><span style="color: #000000"> film;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+------------------------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>  <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra                        <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+------------------------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">Select</span> tables optimized away <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+------------------------------+</span>

const, system：mysql能对查询的某部分进行优化并将其转化成一个常量（可以看show warnings 的结果）。用于 primary key 或 unique key 的所有列与常数比较时，所以表最多有一个匹配行，读取1次，速度比较快。

  1. mysql<span style="color: #808080">></span> explain extended <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> (<span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #0000ff">where</span> id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">1</span><span style="color: #000000">) tmp;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type   <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>     <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> filtered <span style="color: #808080">|</span> Extra <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>     <span style="color: #808080">|</span> <span style="color: #808080"><</span>derived2<span style="color: #808080">></span> <span style="color: #808080">|</span> system <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span>   <span style="color: #800000; font-weight: bold">100.00</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> DERIVED     <span style="color: #808080">|</span> film       <span style="color: #808080">|</span> const  <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span> <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span>   <span style="color: #800000; font-weight: bold">100.00</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>
  7. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+</span>
  8. <span style="color: #000000">
  9. mysql</span><span style="color: #808080">></span><span style="color: #000000"> show warnings;
  10. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+---------------------------------------------------------------+</span>
  11. <span style="color: #808080">|</span> <span style="color: #0000ff">Level</span> <span style="color: #808080">|</span> Code <span style="color: #808080">|</span> Message                                                       <span style="color: #808080">|</span>
  12. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+---------------------------------------------------------------+</span>
  13. <span style="color: #808080">|</span> Note  <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">1003</span> <span style="color: #808080">|</span> <span style="color: #008080">/*</span><span style="color: #008080"> select#1 </span><span style="color: #008080">*/</span> <span style="color: #0000ff">select</span> <span style="color: #ff0000">‘</span><span style="color: #ff0000">1</span><span style="color: #ff0000">‘</span> <span style="color: #0000ff">AS</span> `id`,<span style="color: #ff0000">‘</span><span style="color: #ff0000">film1</span><span style="color: #ff0000">‘</span> <span style="color: #0000ff">AS</span> `name` <span style="color: #0000ff">from</span> dual <span style="color: #808080">|</span>
  14. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+---------------------------------------------------------------+</span>
eq_ref：primary key 或 unique key 索引的所有部分被连接使用 ，最多只会返回一条符合条件的记录。这可能是在 const 之外最好的联接类型了，简单的 select 查询不会出现这种 type。 
  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film_actor <span style="color: #808080">left</span> <span style="color: #808080">join</span> film <span style="color: #0000ff">on</span> film_actor.film_id <span style="color: #808080">=</span><span style="color: #000000"> film.id;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type   <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>               <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref                     <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film_actor <span style="color: #808080">|</span> <span style="color: #0000ff">index</span>  <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> idx_film_actor_id <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">8</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>                    <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">3</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film       <span style="color: #808080">|</span> eq_ref <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>           <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> test.film_actor.film_id <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>        <span style="color: #808080">|</span>
  7. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+</span>

ref：相比 eq_ref，不使用唯一索引，而是使用普通索引或者唯一性索引的部分前缀，索引要和某个值相比较，可能会找到多个符合条件的行。

  1. <span style="color: #800000; font-weight: bold">1</span>. 简单 <span style="color: #0000ff">select</span><span style="color: #000000"> 查询，name是普通索引（非唯一索引）
  2. mysql</span><span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #0000ff">where</span> name <span style="color: #808080">=</span><span style="color: #000000"> "film1";
  3. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+</span>
  4. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>      <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra                    <span style="color: #808080">|</span>
  5. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+</span>
  6. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film  <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> idx_name      <span style="color: #808080">|</span> idx_name <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">33</span>      <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">where</span>; Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  7. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+</span>
  8. <span style="color: #800000; font-weight: bold">2</span><span style="color: #000000">.关联表查询，idx_film_actor_id是film_id和actor_id的联合索引，这里使用到了film_actor的左边前缀film_id部分。
  9. mysql</span><span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #808080">left</span> <span style="color: #808080">join</span> film_actor <span style="color: #0000ff">on</span> film.id <span style="color: #808080">=</span><span style="color: #000000"> film_actor.film_id;
  10. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+</span>
  11. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span>      <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys     <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>               <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref          <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  12. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+</span>
  13. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film       <span style="color: #808080">|</span> <span style="color: #0000ff">index</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>              <span style="color: #808080">|</span> idx_name          <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">33</span>      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>         <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">3</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  14. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film_actor <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> idx_film_actor_id <span style="color: #808080">|</span> idx_film_actor_id <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> test.film.id <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  15. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+</span>

ref_or_null：类似ref，但是可以搜索值为NULL的行。

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> film <span style="color: #0000ff">where</span> name <span style="color: #808080">=</span> "film1" <span style="color: #808080">or</span> name <span style="color: #0000ff">is</span> <span style="color: #0000ff">null</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type        <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>      <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref   <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra                    <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film  <span style="color: #808080">|</span> ref_or_null <span style="color: #808080">|</span> idx_name      <span style="color: #808080">|</span> idx_name <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">33</span>      <span style="color: #808080">|</span> const <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">where</span>; Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+</span>

index_merge：表示使用了索引合并的优化方法。 例如下表：id是主键，tenant_id是普通索引。or 的时候没有用 primary key，而是使用了 primary key(id) 和 tenant_id 索引

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> role <span style="color: #0000ff">where</span> id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">11011</span> <span style="color: #808080">or</span> tenant_id <span style="color: #808080">=</span> <span style="color: #800000; font-weight: bold">8888</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type        <span style="color: #808080">|</span> possible_keys         <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>                   <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra                                           <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> role  <span style="color: #808080">|</span> index_merge <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>,idx_tenant_id <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>,idx_tenant_id <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>,<span style="color: #800000; font-weight: bold">4</span>     <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">134</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">union</span>(<span style="color: #0000ff">PRIMARY</span>,idx_tenant_id); Using <span style="color: #0000ff">where</span> <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+</span>

range：范围扫描通常出现在 in(), between ,> ,<, >= 等操作中。使用一个索引来检索给定范围的行。

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span> actor <span style="color: #0000ff">where</span> id <span style="color: #808080">></span> <span style="color: #800000; font-weight: bold">1</span><span style="color: #000000">;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+------+------+-------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>     <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+------+------+-------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> actor <span style="color: #808080">|</span> range <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">PRIMARY</span> <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">4</span>       <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">where</span> <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+------+------+-------------+</span>

index：和ALL一样，不同就是mysql只需扫描索引树，这通常比ALL快一些。

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #ff00ff">count</span>(<span style="color: #808080">*</span>) <span style="color: #0000ff">from</span><span style="color: #000000"> film;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type  <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>      <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra       <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> film  <span style="color: #808080">|</span> <span style="color: #0000ff">index</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> idx_name <span style="color: #808080">|</span> <span style="color: #800000; font-weight: bold">33</span>      <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">3</span> <span style="color: #808080">|</span> Using <span style="color: #0000ff">index</span> <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>

ALL：即全表扫描，意味着mysql需要从头到尾去查找所需要的行。通常情况下这需要增加索引来进行优化了

  1. mysql<span style="color: #808080">></span> explain <span style="color: #0000ff">select</span> <span style="color: #808080">*</span> <span style="color: #0000ff">from</span><span style="color: #000000"> actor;
  2. </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
  3. <span style="color: #808080">|</span> id <span style="color: #808080">|</span> select_type <span style="color: #808080">|</span> <span style="color: #0000ff">table</span> <span style="color: #808080">|</span> type <span style="color: #808080">|</span> possible_keys <span style="color: #808080">|</span> <span style="color: #0000ff">key</span>  <span style="color: #808080">|</span> key_len <span style="color: #808080">|</span> ref  <span style="color: #808080">|</span> rows <span style="color: #808080">|</span> Extra <span style="color: #808080">|</span>
  4. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
  5. <span style="color: #808080">|</span>  <span style="color: #800000; font-weight: bold">1</span> <span style="color: #808080">|</span> SIMPLE      <span style="color: #808080">|</span> actor <span style="color: #808080">|</span> <span style="color: #808080">ALL</span>  <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>          <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>    <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span> <span style="color: #808080">|</span>    <span style="color: #800000; font-weight: bold">2</span> <span style="color: #808080">|</span> <span style="color: #0000ff">NULL</span>  <span style="color: #808080">|</span>
  6. <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+-------                    </span>

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