介绍
本篇主要通过汇总网上的大牛的知识,简单介绍一下如何使用mysql的执行计划,并根据执行计划判断如何优化和是否索引最优。
执行计划可显示估计查询语句执行计划,从中可以分析查询的执行情况是否最优,有助于对不使用索引的语句进行优化。EXPLAIN对每个查询返回一行信息,列出了有序的表格,MySQL处理语句的时候读取他们。MySQL解决所有的连接使用嵌套连接方法。这意味读取第一张一行,然后匹配第二张表的所有行,第三张表甚至更多表。当所有的表在处理时,MySQL会输出已经查询出来的列,并且回溯到表继续查找直到所有的行被找到,从该表读取下一行,直到程序继续处理下一张表。
使用关键词 EXTENDED ,EXPLAIN 会处理通过 SHOW WARNINGS 看到的一些额外信息。EXPLAIN EXTENDED 会显示这些滤出的列。
语法:
EXPLAIN <select statement>;
输出表格字段如下:
mysql> explain select * from mysql.user where user='root'; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
Column |
JSONName |
Meaning |
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查询标识。id越大优先执行;id相同自上而下执行; |
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None |
查询的类型 |
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查询的表 |
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Thematching partitions |
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连接类型 |
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可能选择的索引 |
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实际使用的索引 |
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使用的索引长度 |
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哪一列或常数在查询中与索引键列一起使用 |
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估计查询的行数 |
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被条件过滤掉的行数百分比 |
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None |
解决查询的一些额外信息 |
以下主要举例说明3个字段:select_type 、type、Extra
【select_type
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alue |
JSONName |
Meaning |
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None |
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None |
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None |
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None |
子查询中第一个SELECT |
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子查询中第一个SELECT,独立于外部查询 |
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None |
子查询在 FROM子句中 |
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物化子查询(不清楚是什么样的查询语句?) |
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结果集不能被缓存的子查询,必须重新评估外层查询的每一行 |
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创建测试表:
create table tabname ( id int auto_increment not null primary key, name varchar(10) null, indate datetime null, tid int null, key(tid), key(indate) )engine=innodb; create table tabname2 ( id int auto_increment not null primary key, name varchar(10) null, indate datetime null, tid int null, key(tid), key(indate) )engine=myisam; insert into tabname(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4); insert into tabname2(name,indate,tid) values('love',now(),2),('lucky',now(),3),('passion',now(),4);
#SIMPLE
#PRIMARY / DERIVED
mysql> explain select * from (select * from tabname) as a; +----+-------------+------------+------+---------------+------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+------+---------+------+------+-------+ | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 3 | | | 2 | DERIVED | tabname | ALL | NULL | NULL | NULL | NULL | 3 | | +----+-------------+------------+------+---------------+------+---------+------+------+-------+
#PRIMARY / UNION / UNION RESULT
mysql> explain select * from tabname where id=(select max(id) from tabname); +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+ | 1 | PRIMARY | tabname | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+---------+-------+---------------+---------+---------+-------+------+------------------------------+
#PRIMARY / SUBQUERY
#PRIMARY / DEPENDENT SUBQUERY
mysql> explain select * from tabname a where exists(select 1 from tabname b where a.id=b.id); mysql> explain select *,(select name from tabname b where a.id=b.id) from tabname a; mysql> explain select * from tabname where id not in(select id from tabname); +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+ | 1 | PRIMARY | tabname | ALL | NULL | NULL | NULL | NULL | 3 | Using where | | 2 | DEPENDENT SUBQUERY | tabname | unique_subquery | PRIMARY | PRIMARY | 4 | func | 1 | Using index | +----+--------------------+---------+-----------------+---------------+---------+---------+------+------+-------------+
#PRIMARY / DEPENDENT UNION / DEPENDENT SUBQUERY / UNION RESULT
mysql> explain select * from tabname where id in (select id from tabname union select id from tabname); +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+ | 1 | PRIMARY | tabname | ALL | NULL | NULL | NULL | NULL | 3 | Using where | | 2 | DEPENDENT SUBQUERY | tabname | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | Using index | | 3 | DEPENDENT UNION | tabname | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | Using index | | NULL| UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | | +----+--------------------+------------+--------+---------------+---------+---------+------+------+-------------+
【type】
type |
Meaning |
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表仅一行数据 (=system table).这是const连接类型的特例。 |
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对于前面表的结果集匹配查询的所有行,当连接使用索引key时,或者索引不是 |
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使用全文索引时出现。 |
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使用了索引合并优化。(未成功) |
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该类型将ref替换成以下子查询的格式: valueIN (SELECTprimary_key FROMsingle_table WHERE some_expr) |
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与 valueIN (SELECTkey_columnFROMsingle_table WHERE some_expr) |
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使用索引检索给定范围内的行。 |
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对于前面表的结果集中,进行了全表扫描。最差的一种类型,应考虑查询优化了! |
查询类型性能由优到差:
system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
#system
#const
mysql> explain select * from tabname as a,tabname as b where a.id=b.id and a.id=1; mysql> explain select * from tabname where id=1; +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+ | 1 | SIMPLE | tabname | const | PRIMARY | PRIMARY | 4 | const | 1 | | +----+-------------+---------+-------+---------------+---------+---------+-------+------+-------+
#eq_ref(engine=myisam)
mysql> explain select * from tabname2 as a,tabname2 as b where a.id=b.id; +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+ | 1 | SIMPLE | a | ALL | PRIMARY | NULL | NULL | NULL | 3 | | | 1 | SIMPLE | b | eq_ref | PRIMARY | PRIMARY | 4 | test.a.id | 1 | | +----+-------------+-------+--------+---------------+---------+---------+-----------+------+-------+
#ref
#ref_or_null
mysql> explain select id,tid from tabname where tid=2 or tid is null; +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+ | 1 | SIMPLE | tabname | ref_or_null | tid | tid | 5 | const | 2 | Using where; Using index | +----+-------------+---------+-------------+---------------+------+---------+-------+------+--------------------------+
#fulltext
#index_merge(未成功)
#unique_subquery
#index_subquery
mysql> explain select * from tabname where tid in(select tid from tabname); mysql> explain select * from tabname where id in(select tid from tabname); +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+ | 1 | PRIMARY | tabname | ALL | NULL | NULL | NULL | NULL | 3 | Using where | | 2 | DEPENDENT SUBQUERY | tabname | index_subquery | tid | tid | 5 | func | 1 | Using index; Using where | +----+--------------------+---------+----------------+---------------+------+---------+------+------+--------------------------+
#range
mysql> explain select * from tabname where tid between 1 and 2; mysql> explain select * from tabname where id>1; +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | tabname | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where | +----+-------------+---------+-------+---------------+---------+---------+------+------+-------------+
#index
mysql> explain select id,tid from tabname; mysql> explain select tid from tabname; +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tabname | index | NULL | tid | 5 | NULL | 3 | Using index | +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
#ALL
mysql> explain select * from tabname where tid<>2; mysql> explain select * from tabname; +----+-------------+---------+------+---------------+------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+------+---------------+------+---------+------+------+-------+ | 1 | SIMPLE | tabname | ALL | NULL | NULL | NULL | NULL | 3 | | +----+-------------+---------+------+---------------+------+---------+------+------+-------+
【Extra】
该列输出关MySQL如何解决查询的额外信息。(下面列出部分常见的)
Extra |
Meaning |
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使用过滤条件 |
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没有groupby情况下使用min(),max(),或者count(*) |
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在leftjoin中匹配一行之后将不再继续查询查询 |
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查找到第一个匹配的行之后,MySQL则会停止对当前行的搜索 |
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where子句总数失败的查询 |
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使用连接缓存 |
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#using where
mysql> explain select * from tabname where id>2; mysql> explain select * from tabname where tid=2; +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+ | 1 | SIMPLE | tabname | ref | tid | tid | 5 | const | 1 | Using where | +----+-------------+---------+------+---------------+------+---------+-------+------+-------------+
#using index
mysql> explain select tid from tabname; +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tabname | index | NULL | tid | 5 | NULL | 3 | Using index | +----+-------------+---------+-------+---------------+------+---------+------+------+-------------+
#using temporary
mysql> explain select distinct name from tabname; +----+-------------+---------+------+---------------+------+---------+------+------+-----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+------+---------------+------+---------+------+------+-----------------+ | 1 | SIMPLE | tabname | ALL | NULL | NULL | NULL | NULL | 3 | Using temporary | +----+-------------+---------+------+---------------+------+---------+------+------+-----------------+
#select tables optimized away
mysql> explain select max(tid) from tabname; +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
#using filesort
mysql> explain select id,name from tabname group by id,name; mysql> explain select * from tabname order by name; +----+-------------+---------+------+---------------+------+---------+------+------+----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+------+---------------+------+---------+------+------+----------------+ | 1 | SIMPLE | tabname | ALL | NULL | NULL | NULL | NULL | 3 | Using filesort | +----+-------------+---------+------+---------------+------+---------+------+------+----------------+
#not exists
#distinct
#impossible where
#impossible having
mysql> explain select id,count(*) from tabname group by id having 1=2; mysql> explain select count(*) from tabname having 1=2; +----+-------------+-------+------+---------------+------+---------+------+------+-------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible HAVING | +----+-------------+-------+------+---------------+------+---------+------+------+-------------------+
现在使用 EXTENDED 情况:
语法:
- EXPLAIN EXTENDED <select statement>;
不使用 extended 和使用extended 的分析情况:
可以看到,使用 extended 时,输出的最下面多了 1 条警告。 此时可以用 show warnings 来查看:
show warnings 显示了优化器中是怎么规范表和字段名的,在通过重写和优化规则之后的 select 语句是什么样子。
更多参考:
EXPLAIN EXTENDED Output Format
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