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mysql in语句子查询效率慢的解决办法


本篇文章介绍了mysql in语句子查询效率慢的解决办法,下面小编给大家介绍详细的解决步骤。

本篇文章介绍了mysql.html" target="_blank">mysql in语句子查询效率慢的解决办法,下面小编给大家介绍详细的解决步骤。

表结构如下,文章只有690篇。

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文章表article(id,title,content)
标签表tag(tid,tag_name)
标签文章中间表article_tag(id,tag_id,article_id)

其中有个标签的tid是135,查询标签tid是135的文章列表。

690篇文章,用以下的语句查询,奇慢:

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select id,title from article where id in(
select article_id from article_tag where tag_id=135
)

其中这条速度很快:

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select article_id from article_tag where tag_id=135

查询结果是五篇文章,id为428,429,430,431,432

用下面sql来查文章也很快:

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select id,title from article where id in(
428,429,430,431,432
)

解决方法:

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select id,title from article where id in(
select article_id from (select article_id from article_tag where tag_id=135) as tbt
)

其它解决方法:(举例)

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mysql> select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');

为了节省篇幅,省略了输出内容,下同。

67 rows in set (12.00 sec)

只有67行数据返回,却花了12秒,而系统中可能同时会有很多这样的查询,系统肯定扛不住。用desc看一下(注:explain也可)

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mysql> desc select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');
+----+--------------------+------------------+--------+-----------------+-------+---------+------------+---------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------------+--------+-----------------+-------+---------+------------+---------+--------------------------+
| 1 | PRIMARY | abc_number_prop | ALL | NULL | NULL | NULL | NULL | 2679838 | Using where |
| 2 | DEPENDENT SUBQUERY | abc_number_phone | eq_ref | phone,number_id | phone | 70 | const,func | 1 | Using where; Using index |
+----+--------------------+------------------+--------+-----------------+-------+---------+------------+---------+--------------------------+
2 rows in set (0.00 sec)

可以看出,在执行此查询时会扫描两百多万行,难道是没有创建索引吗,看一下

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mysql>show index from abc_number_phone;
+------------------+------------+-------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------------+------------+-------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| abc_number_phone | 0 | PRIMARY | 1 | number_phone_id | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 0 | phone | 1 | phone | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 0 | phone | 2 | number_id | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 1 | number_id | 1 | number_id | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 1 | created_by | 1 | created_by | A | 36879 | NULL | NULL | | BTREE | | |
| abc_number_phone | 1 | modified_by | 1 | modified_by | A | 36879 | NULL | NULL | YES | BTREE | | |
+------------------+------------+-------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (0.06 sec)
mysql>show index from abc_number_prop;
+-----------------+------------+-------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------------+------------+-------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| abc_number_prop | 0 | PRIMARY | 1 | number_prop_id | A | 311268 | NULL | NULL | | BTREE | | |
| abc_number_prop | 1 | number_id | 1 | number_id | A | 311268 | NULL | NULL | | BTREE | | |
| abc_number_prop | 1 | created_by | 1 | created_by | A | 311268 | NULL | NULL | | BTREE | | |
| abc_number_prop | 1 | modified_by | 1 | modified_by | A | 311268 | NULL | NULL | YES | BTREE | | |
+-----------------+------------+-------------+--------------+----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
4 rows in set (0.15 sec)

从上面的输出可以看出,这两张表在number_id字段上创建了索引的。
看看子查询本身有没有问题。

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mysql> desc select number_id from abc_number_phone where phone = '82306839';
+----+-------------+------------------+------+---------------+-------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------------+------+---------------+-------+---------+-------+------+--------------------------+
| 1 | SIMPLE | abc_number_phone | ref | phone | phone | 66 | const | 6 | Using where; Using index |
+----+-------------+------------------+------+---------------+-------+---------+-------+------+--------------------------+
1 row in set (0.00 sec)

没有问题,只需要扫描几行数据,索引起作用了。

查询出来看看:

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mysql> select number_id from abc_number_phone where phone = '82306839';
+-----------+
| number_id |
+-----------+
| 8585 |
| 10720 |
| 148644 |
| 151307 |
| 170691 |
| 221897 |
+-----------+
6 rows in set (0.00 sec)

直接把子查询得到的数据放到上面的查询中

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mysql> select * from abc_number_prop where number_id in (8585, 10720, 148644, 151307, 170691, 221897);
67 rows in set (0.03 sec)

速度也快,看来MySQL在处理子查询的时候是不够好。我在MySQL 5.1.42 和 MySQL 5.5.19 都进行了尝试,都有这个问题。

搜索了一下网络,发现很多人都遇到过这个问题:

参考资料1:MySQL优化之使用连接(join)代替子查询

参考资料2:MYSQL子查询和嵌套查询优化实例解析

根据网上这些资料的建议,改用join来试试。
修改前:

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select * from abc_number_prop where number_id in (select number_id from abc_number_phone where phone = '82306839');

修改后:

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select a.* from abc_number_prop a inner join abc_number_phone b on a.number_id = b.number_id where phone = '82306839';
mysql> select a.* from abc_number_prop a inner join abc_number_phone b on a.number_id = b.number_id where phone = '82306839';
67 rows in set (0.00 sec)

效果不错,查询所用时间几乎为0。看一下MySQL是怎么执行这个查询的

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mysql>desc select a.* from abc_number_prop a inner join abc_number_phone b on a.number_id = b.number_id where phone = '82306839';
+----+-------------+-------+------+-----------------+-----------+---------+-----------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+-----------------+-----------+---------+-----------------+------+--------------------------+
| 1 | SIMPLE | b | ref | phone,number_id | phone | 66 | const | 6 | Using where; Using index |
| 1 | SIMPLE | a | ref | number_id | number_id | 4 | eap.b.number_id | 3 | |
+----+-------------+-------+------+-----------------+-----------+---------+-----------------+------+--------------------------+
2 rows in set (0.00 sec)

小结:当子查询速度慢时,可用JOIN来改写一下该查询来进行优化。

网上也有文章说,使用JOIN语句的查询不一定总比使用子查询的语句快。

mysql手册也提到过,具体的原文在mysql文档的这个章节:
I.3. Restrictions on Subqueries
13.2.8. Subquery Syntax

摘抄:

1)关于使用IN的子查询:

Subquery optimization for IN is not as effective as for the = operator or for IN(value_list) constructs.

A typical case for poor IN subquery performance is when the subquery returns a small number of rows but the outer query returns a large number of rows to be compared to the subquery result.

The problem is that, for a statement that uses an IN subquery, the optimizer rewrites it as a correlated subquery. Consider the following statement that uses an uncorrelated subquery:

SELECT ... FROM t1 WHERE t1.a IN (SELECT b FROM t2);

The optimizer rewrites the statement to a correlated subquery:

SELECT ... FROM t1 WHERE EXISTS (SELECT 1 FROM t2 WHERE t2.b = t1.a);

If the inner and outer queries return M and N rows, respectively, the execution time becomes on the order of O(M×N), rather than O(M+N) as it would be for an uncorrelated subquery.

An implication is that an IN subquery can be much slower than a query written using an IN(value_list) construct that lists the same values that the subquery would return.

2)关于把子查询转换成join的:

The optimizer is more mature for joins than for subqueries, so in many cases a statement that uses a subquery can be executed more efficiently if you rewrite it as a join.

An exception occurs for the case where an IN subquery can be rewritten as a SELECT DISTINCT join. Example:

SELECT col FROM t1 WHERE id_col IN (SELECT id_col2 FROM t2 WHERE condition);

That statement can be rewritten as follows:

SELECT DISTINCT col FROM t1, t2 WHERE t1.id_col = t2.id_col AND condition;

But in this case, the join requires an extra DISTINCT operation and is not more efficient than the subquery

总结

以上所述就是mysql in语句子查询效率慢的解决办法的介绍,希望大家能够喜欢



发布时间:2018/11/2 9:43:59
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