Question

这是我的桌子：

在我的桌子上

Clustering_key（主键和自动增量键）
ID（索引列）
Data（“文本数据类型”列）
Position（索引列）保持Data的顺序

我的表有90,000行，其中相同的ID等于5。我想在前3行中的ID等于5，并且这样查询

Select * from mytable where ID=5 Limit 3;

ID列是索引列，所以我认为mysql仅扫描前3行，但mysql扫描约42000行。

在这里解释查询：

任何避免所有行扫描的可能性。

请给我一些解决方法

预先感谢

Answer 1

我模拟了这种情况。

使用


   CREATE TABLE mytable (
        Clustering_key INT NOT NULL AUTO_INCREMENT,
        ID INT NOT NULL,
        Data text NOT NULL,
        Position INT NOT NULL,
        PRIMARY KEY (Clustering_key),
        KEY(ID),
        KEY(Position)
    )

使用


    INSERT INTO mytable (ID,Data,Position) VALUES (5,CONCAT("Data-",5), 7);
    INSERT INTO mytable (ID,Data,Position) VALUES (5,CONCAT("Data-",5), 26);
    INSERT INTO mytable (ID,Data,Position) VALUES (5,CONCAT("Data-",51), 27);
    INSERT INTO mytable (ID,Data,Position) VALUES (5,CONCAT("Data-",56), 28);
    INSERT INTO mytable (ID,Data,Position) VALUES (5,CONCAT("Data-",57), 31);

解释


    mysql> explain Select * from mytable where ID=5 Limit 3
    +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
    | id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra |
    +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
    |  1 | SIMPLE      | mytable | NULL       | ref  | ID            | ID   | 4       | const |    5 |   100.00 | NULL  |
    +----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
    1 row in set, 1 warning (0.00 sec)

是的，explain显示检查的行是5，但不是3。但这似乎只是一个误导性信息。可以通过按照以下步骤为所有查询启用慢日志（设置long_query_time = 0）来验证运行时rows_examined的确切数量。

注意：您只能在自己的测试数据库中设置long_query_time = 0。测试之后，您必须将参数重置为先前的值。


     - set GLOBAL slow_query_log=1;
     - set global long_query_time=0;
     - set session long_query_time=0;
     mysql> show variables like '%slow%';
    +---------------------------+-------------------------------------------------+
    | Variable_name             | Value                                           |
    +---------------------------+-------------------------------------------------+
    | log_slow_admin_statements | OFF                                             |
    | log_slow_slave_statements | OFF                                             |
    | slow_launch_time          | 2                                               |
    | slow_query_log            | ON                                              |
    | slow_query_log_file       | /usr/local/mysql/data/slow.log                  |
    +---------------------------+-------------------------------------------------+
    5 rows in set (0.10 sec)
    mysql> select @@long_query_time;
    +-------------------+
    | @@long_query_time |
    +-------------------+
    |          0.000000 |
    +-------------------+

And then in the terminal, executing the query
<pre>
mysql> Select * from mytable where ID=5 Limit 3;
+----------------+----+---------+----------+
| Clustering_key | ID | Data    | Position |
+----------------+----+---------+----------+
|              5 |  5 | Data-5  |        7 |
|          26293 |  5 | Data-5  |       26 |
|          26294 |  5 | Data-51 |       27 |
+----------------+----+---------+----------+
3 rows in set (0.00 sec)

mysql> Select * from mytable where ID=5 Limit 1;

通过检查slow_query_log_file上方印刷的/usr/local/mysql/data/slow.log来检查慢速日志

您可以找到以下信息。


    # Time: 2019-04-26T01:48:19.890846Z
    # User@Host: root[root] @ localhost []  Id:  5124
    # Query_time: 0.000575  Lock_time: 0.000146 Rows_sent: 3  Rows_examined: 3 
    SET timestamp=1556243299;
    Select * from mytable where ID=5 Limit 3;
    # Time: 2019-04-26T01:48:34.672888Z
    # User@Host: root[root] @ localhost []  Id:  5124
    # Query_time: 0.000182  Lock_time: 0.000074 Rows_sent: 1  Rows_examined: 1 
    SET timestamp=1556243314;
    Select * from mytable where ID=5 Limit 1;

运行时Rows_exmained的值等于limit参数的值。该测试是在MySQL 5.7.18上完成的。

----------------------------------另一种验证方式---------- -------------------------



    mysql> show status like '%Innodb_rows_read%';
    +------------------+-------+
    | Variable_name    | Value |
    +------------------+-------+
    | Innodb_rows_read | 13    |
    +------------------+-------+
    1 row in set (0.00 sec)

    mysql> Select * from mytable where ID=5 Limit 1;
    +----------------+----+--------+----------+
    | Clustering_key | ID | Data   | Position |
    +----------------+----+--------+----------+
    |              5 |  5 | Data-5 |        7 |
    +----------------+----+--------+----------+
    1 row in set (0.00 sec)

    mysql> show status like '%Innodb_rows_read%';
    +------------------+-------+
    | Variable_name    | Value |
    +------------------+-------+
    | Innodb_rows_read | 14    |
    +------------------+-------+
    1 row in set (0.00 sec)

您可以看到Innodb_rows_read仅增加了1，即为限制1。如果您执行全表扫描查询，则可以看到该值将随着表的计数而增加。


    mysql> select count(*) from mytable;
    +----------+
    | count(*) |
    +----------+
    |   126296 |
    +----------+
    1 row in set (0.05 sec)

    mysql> show status like '%Innodb_rows_read%';
    +------------------+--------+
    | Variable_name    | Value  |
    +------------------+--------+
    | Innodb_rows_read | 505204 |
    +------------------+--------+
    1 row in set (0.00 sec)

    mysql> Select * from mytable where Data="Data-5";
    +----------------+----+--------+----------+
    | Clustering_key | ID | Data   | Position |
    +----------------+----+--------+----------+
    |              5 |  5 | Data-5 |        7 |
    |          26293 |  5 | Data-5 |       26 |
    |          26301 |  5 | Data-5 |        7 |
    +----------------+----+--------+----------+
    3 rows in set (0.09 sec)

    mysql> show status like '%Innodb_rows_read%';
    +------------------+--------+
    | Variable_name    | Value  |
    +------------------+--------+
    | Innodb_rows_read | 631500 |
    +------------------+--------+
    1 row in set (0.00 sec)

两种方式都证实了explain的限制似乎提供了有关所检查行的误导信息。

如何在不扫描所有行的情况下在索引列上的查询中使用限制？

1 个答案: