SQL执行计划解读

声明

• 5.6中desc看不到show warnings,也看不到filtered列
• 5.7的desc等于5.6的desc extended,这样可以看show warnings,5.6中filtered列非常不准,5.7好一些

先看一个执行计划

(root@localhost) [test]> desc select * from l;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

Ⅰ、展开分析每个字段

id列——表示sql执行的顺序

• id相等,一般是简单关联,从上往下看即可
• id不相等,一般为出现了子查询,先看大的再看小的
• 同时存在相等与不相等,一般相等的为一组,先看大的再看小的

有个潜规则叫:id相等从上往下看,id不等从下往上看

select_type——select类型,用于区分子查询,关联查询等

• SIMPLE：简单查询,不包含子查询与union
• PRIMARY：查询中包含子查询,最外层查询则被标记为PRIMARY
• UNION：使用union连接select时,从第二个select开始都是UNION
• SUBQUERY：select或者where后面的子查询(非from之后)都可能是SUBQUERY
• DERIVED：在from中包含的子查询被标记为DERIVED(派生表)
• DEPENDENT SUBQUERY：依赖外部查询的SUBQUERY
• UNION RESULT：UNION的结果,对应ID为NULL

table——输出记录的表

• 查询中使用别名,则此处显示别名
• 不涉及表的操作,则显示为NULL
• < derivedN > / < subqueryN > 由ID为N的查询产生的结果
• < unionM,N > 由ID为M,N查询union产生的结果集

type——访问类型

• system：const的特例,表中只有一行记录
• const：使用唯一索引或主键,只取一行数据
• eq_ref：多表join时,驱动表只返回一行数据,且这行数据是被驱动表的主键或唯一索引,且必须not null
• ref：非唯一索引的扫描,通常为非唯一索引的等值查询
• range：检索给定范围的行,使用一个索引来选择行,通常为where条件中出现between、<、>、in、like等
• index：full index scan,根据索引读全表
• ALL：full table scan,扫整个数据文件
• fulltext：全文索引
• ref_or_null：使用普通索引进行查询，但要查询null值
• index_merge or：查询会使用到的类型,可能一条sql使用了两个索引,然后merge
• unique_subquery和index_subquery：很少出现 前一个是子查询的列是唯一索引,第二个是子查询的列是普通索引
• MATERIALIZED：物化子查询

主要优化对象是index和ALL,有两种情况可以考虑保留index
只查询索引列,不回表或者使用索引进行排序或者聚合

possible_keys

优化器可能使用到的索引

key

优化器实际选择的索引

key_len

使用索引的字节长度

ref

• 等值查询会显示const
• 连接查询的话被驱动表此处显示驱动表的join列

rows

优化器预估的记录数量

filtered

根据条件过滤得到的记录的百分比

extra

• Using index：优化器只需使用索引就能得到结果 索引覆盖
• Using index condition：优化器试用index condition pushdown优化，二级索引
• Using index for group by：优化器只需使用索引就能处理group by 或者distinct语句
  上面这3个基本上忽略吧,没什么参数好调的
• Using temporary：使用临时表,常见于order by,group by
• Using filesort：使用额外的排序 调整sort_buffer_size
• Using join buffer：优化器需要使用join buffer 调整join_buffer_size
• Using MRR：优化器使用MRR优化 调整read_cache_size
• Using temporary：优化器需要使用临时表 调整tmp_table_size
• Using where：优化器使用where过滤

Ⅱ、分析两个执行计划看看

案例1

(root@localhost) [dbt3]> DESC SELECT
    ->     *
    -> FROM
    ->     part
    -> WHERE
    ->     p_partkey IN (SELECT
    ->             l_partkey
    ->         FROM
    ->             lineitem
    ->         WHERE
    ->             l_shipdate BETWEEN '1997-01-01' AND '1997-02-01')
    -> ORDER BY p_retailprice DESC
    -> LIMIT 10;
+----+--------------+-------------+------------+--------+----------------------------------------------+--------------+---------+---------------------+--------+----------+----------------------------------+
| id | select_type  | table       | partitions | type   | possible_keys                                | key          | key_len | ref                 | rows   | filtered | Extra                            |
+----+--------------+-------------+------------+--------+----------------------------------------------+--------------+---------+---------------------+--------+----------+----------------------------------+
|  1 | SIMPLE       | part        | NULL       | ALL    | PRIMARY                                      | NULL         | NULL    | NULL                | 197706 |   100.00 | Using where; Using filesort      |
|  1 | SIMPLE       | <subquery2> | NULL       | eq_ref | <auto_key>                                   | <auto_key>   | 5       | dbt3.part.p_partkey |      1 |   100.00 | NULL                             |
|  2 | MATERIALIZED | lineitem    | NULL       | range  | i_l_shipdate,i_l_suppkey_partkey,i_l_partkey | i_l_shipdate | 4       | NULL                | 138672 |   100.00 | Using index condition; Using MRR |
+----+--------------+-------------+------------+--------+----------------------------------------------+--------------+---------+---------------------+--------+----------+----------------------------------+
3 rows in set, 1 warning (0.01 sec)

id 顺序
1 ② part表(外表)和subquery2(id=2产生的14w记录的表)进行关联,对于part表中所有记录都要关联,一共是19w行,再和l_partkey进行关联,最后排序用到using filesort
1 ③ 内表要加索引,所以mysql优化器自动把第一步取出来的数据添加了一个唯一索引,in里面是去重的(这其实是做了一个物化),所以是唯一索引,eq_ref表示通过唯一索引进行关联,和外表中的p_partkey关联
2 ① 先查lineitem表,是一个range范围查询,使用了i_l_shipdate索引,l_shipdate是date类型,占用四个字节,预估14万行记录,过滤出百分之百,materiallized表示产生了一张实际的表,并且去添加了索引,l_partkey,唯一索引(in里面是去重的)

注意一个细节

(root@localhost) [dbt3]> DESC SELECT
    ->     *
    -> FROM
    ->     part
    -> WHERE
    ->     p_partkey IN (SELECT
    ->             l_partkey
    ->         FROM
    ->             lineitem
    ->         WHERE
    ->             l_shipdate BETWEEN '1997-01-01' AND '1997-01-07')
    -> ORDER BY p_retailprice DESC
    -> LIMIT 10;
+----+--------------+-------------+------------+--------+----------------------------------------------+--------------+---------+-----------------------+-------+----------+----------------------------------------------+
| id | select_type  | table       | partitions | type   | possible_keys                                | key          | key_len | ref                   | rows  | filtered | Extra                                        |
+----+--------------+-------------+------------+--------+----------------------------------------------+--------------+---------+-----------------------+-------+----------+----------------------------------------------+
|  1 | SIMPLE       | <subquery2> | NULL       | ALL    | NULL                                         | NULL         | NULL    | NULL                  |  NULL |   100.00 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE       | part        | NULL       | eq_ref | PRIMARY                                      | PRIMARY      | 4       | <subquery2>.l_partkey |     1 |   100.00 | NULL                                         |
|  2 | MATERIALIZED | lineitem    | NULL       | range  | i_l_shipdate,i_l_suppkey_partkey,i_l_partkey | i_l_shipdate | 4       | NULL                  | 29148 |   100.00 | Using index condition; Using MRR             |
+----+--------------+-------------+------------+--------+----------------------------------------------+--------------+---------+-----------------------+-------+----------+----------------------------------------------+
3 rows in set, 1 warning (0.00 sec)

驱动表就变成了subquerry2,这时候优化器又把子查询作为了外表,说明优化器很聪明

in的子查询,优化器会帮你重写成join,并且帮你选择子查询到底是内表还是外表

(root@localhost) [dbt3]> DESC select
    ->     a.*
    -> from
    ->     part a,
    ->     (select distinct
    ->         l_partkey
    ->     from
    ->         lineitem
    ->     where l_shipdate between '1997-01-01' and '1997-02-01') b
    -> where
    ->     a.p_partkey=b.l_partkey
    -> order by a.p_retailprice desc
    -> limit 10;
+----+-------------+------------+------------+--------+----------------------------------------------+--------------+---------+-------------+--------+----------+---------------------------------------------------+
| id | select_type | table      | partitions | type   | possible_keys                                | key          | key_len | ref         | rows   | filtered | Extra                                             |
+----+-------------+------------+------------+--------+----------------------------------------------+--------------+---------+-------------+--------+----------+---------------------------------------------------+
|  1 | PRIMARY     | <derived2> | NULL       | ALL    | NULL                                         | NULL         | NULL    | NULL        | 138672 |   100.00 | Using where; Using temporary; Using filesort      |
|  1 | PRIMARY     | a          | NULL       | eq_ref | PRIMARY                                      | PRIMARY      | 4       | b.l_partkey |      1 |   100.00 | NULL                                              |
|  2 | DERIVED     | lineitem   | NULL       | range  | i_l_shipdate,i_l_suppkey_partkey,i_l_partkey | i_l_shipdate | 4       | NULL        | 138672 |   100.00 | Using index condition; Using MRR; Using temporary |
+----+-------------+------------+------------+--------+----------------------------------------------+--------------+---------+-------------+--------+----------+---------------------------------------------------+
3 rows in set, 1 warning (0.00 sec)

这么改写,b表永远是外表,子查询只是产生一个派生表,但是没办法给它建索引,如果子查询出来的结果集很大,这时候性能就不如in了,in的话优化器会把它作为内表

案例2

(root@localhost) [dbt3]> DESC select max(l_extendedprice)
    ->     from orders,lineitem
    -> where o_orderdate between '1995-01-01' and '1995-01-31'
    -> and l_orderkey=o_orderkey;
+----+-------------+----------+------------+-------+--------------------------------------------+---------------+---------+------------------------+-------+----------+--------------------------+
| id | select_type | table    | partitions | type  | possible_keys                              | key           | key_len | ref                    | rows  | filtered | Extra                    |
+----+-------------+----------+------------+-------+--------------------------------------------+---------------+---------+------------------------+-------+----------+--------------------------+
|  1 | SIMPLE      | orders   | NULL       | range | PRIMARY,i_o_orderdate                      | i_o_orderdate | 4       | NULL                   | 40696 |   100.00 | Using where; Using index |
|  1 | SIMPLE      | lineitem | NULL       | ref   | PRIMARY,i_l_orderkey,i_l_orderkey_quantity | PRIMARY       | 4       | dbt3.orders.o_orderkey |     3 |   100.00 | NULL                     |
+----+-------------+----------+------------+-------+--------------------------------------------+---------------+---------+------------------------+-------+----------+--------------------------+
2 rows in set, 1 warning (0.00 sec)

orderkey上有索引,但是没用,用的是pk,orders表示外表,根据过滤条件把数据过滤出来做外表,然后跟lineitem表关联,用的是pk,关联的列是orders.o_orderkey

如果强行走orderkey索引,成本很高,需要回表,通过主键不用回表

案例3

(root@localhost) [dbt3]> DESC select *
    -> from
    ->     lineitem
    -> where
    ->     l_shipdate <= '1995-12-32'
    -> union
    -> select
    ->     *
    -> from
    ->     lineitem
    -> where
    ->     l_shipdate >= '1997-01-01';
+----+--------------+------------+------------+------+---------------+------+---------+------+---------+----------+-----------------+
| id | select_type  | table      | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra           |
+----+--------------+------------+------------+------+---------------+------+---------+------+---------+----------+-----------------+
|  1 | PRIMARY      | lineitem   | NULL       | ALL  | i_l_shipdate  | NULL | NULL    | NULL | 5409799 |    33.33 | Using where     |
|  2 | UNION        | lineitem   | NULL       | ALL  | i_l_shipdate  | NULL | NULL    | NULL | 5409799 |    50.00 | Using where     |
|NULL| UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+---------+----------+-----------------+
3 rows in set, 3 warnings (0.10 sec)

union result合并两张表 会using temporary,使用临时表,union会去重,所以又去建了临时表,在上面加了唯一索引,这里就用了两个索引,所以一个sql只能用一条索引是不对的

案例4

(root@localhost) [employees]> DESC SELECT
    ->     emp_no,
    ->     dept_no,
    ->     (SELECT
    ->             COUNT(1)
    ->         FROM
    ->             dept_emp t2
    ->         WHERE
    ->             t1.emp_no <= t2.emp_no) AS row_num
    -> FROM
    ->     dept_emp t1;
+----+--------------------+-------+------------+-------+----------------+--------+---------+------+--------+----------+------------------------------------------------+
| id | select_type        | table | partitions | type  | possible_keys  | key    | key_len | ref  | rows   | filtered | Extra                                          |
+----+--------------------+-------+------------+-------+----------------+--------+---------+------+--------+----------+------------------------------------------------+
|  1 | PRIMARY            | t1    | NULL       | index | NULL           | emp_no | 4       | NULL | 331570 |   100.00 | Using index                                    |
|  2 | DEPENDENT SUBQUERY | t2    | NULL       | ALL   | PRIMARY,emp_no | NULL   | NULL    | NULL | 331570 |    33.33 | Range checked for each record (index map: 0x3) |
+----+--------------------+-------+------------+-------+----------------+--------+---------+------+--------+----------+------------------------------------------------+
2 rows in set, 2 warnings (0.00 sec)

对于这个sql,先执行了1再执行了2,2是dependent subquery,要依赖子查询,所以先执行了1,所以t1是外表,t2是内表,每次得关联33w * 33%次数,一共关联33w次,一共是33w * 10w次

行号问题,性能非常差

Ⅲ、补充

3.1 什么是物化？

通常来说查询的结果是不需要物化的,子查询产生的一张表,去重,加一个唯一键,上面还有个索引

A和B关联,B是子查询查出来的,本来这些数据都存放在内存中直接和A表关联,B肯定是外表,因为他没有索引,in的话就会去重,加唯一键,这时候就既可以是外表也可以是内表,这就是物化

explain format=json

这个东西还是蛮好用的,可以用来看看sql的执行成本

mysql优化器会选择一个成本最小的方式作为执行计划