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)。
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
- <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>
- <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)。
- <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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+</span>
- <span style="color: #000000">
- mysql</span><span style="color: #808080">></span><span style="color: #000000"> show warnings;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+--------------------------------------------------------------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+--------------------------------------------------------------------------------+</span>
- <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>
- <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。
接下来我们将展示 explain 中每个列的信息。
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
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
- <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>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
2)from子句中的子查询
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+---------------+----------+---------+------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+---------------+----------+---------+------+------+-------------+</span>
- <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>
- <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>
- <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
- 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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
- <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>
- <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>
- <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>
- <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。
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
- 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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+-------+------+-------+</span>
- <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>
- <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
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+</span>
- <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>
- <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>
- <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>
- <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:
- 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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
- <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>
- <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>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+--------------+------------+------+---------------+------+---------+------+------+-----------------+</span>
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。
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
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+------------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+------------------------------+</span>
- <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>
- <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。
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。
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+</span>
- <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>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+</span>
- <span style="color: #000000">
- mysql</span><span style="color: #808080">></span><span style="color: #000000"> show warnings;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+---------------------------------------------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+---------------------------------------------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">-----+------+---------------------------------------------------------------+</span>
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+</span>
- <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>
- <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>
- <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。
- <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)
- 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";
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+</span>
- <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。
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+</span>
- <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>
- <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>
- <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。
- 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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+</span>
- <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>
- <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
- 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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+</span>
- <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>
- <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。
- 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">;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+---------+---------+------+------+-------------+</span>
- <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>
- <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。
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+-------+---------------+----------+---------+------+------+-------------+</span>
- <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>
- <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
- 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;
- </span><span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+---------+------+------+-------+</span>
- <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>
- <span style="color: #808080">+</span><span style="color: #008080">--</span><span style="color: #008080">--+-------------+-------+------+---------------+------+------- </span>