我正在读取具有最大 id 的记录的 tick 值。以下导致执行速度变慢的查询之间有什么区别？

慢查询:

SELECT tick
FROM   eventlog
WHERE  id IN (SELECT max(id) FROM eventlog)

快速查询:

SELECT max(id) INTO @id
FROM   eventlog;

SELECT tick
FROM   eventlog
WHERE  id = @id;

架构

CREATE TABLE eventlog (
    id INT (11) NOT NULL AUTO_INCREMENT,
    tick INT NOT NULL,
    eventType_id INT NOT NULL,
    compType INT (10) UNSIGNED NOT NULL,
    compID INT (10) UNSIGNED NOT NULL,
    value_double DOUBLE NOT NULL,
    value_int INT (10),
    hierarchy_id VARCHAR (255) NOT NULL,
    PRIMARY KEY (id),
    INDEX htet (
        hierarchy_id,
        tick,
        eventType_id
    )
)

最佳答案

因为 in 查询没有使用索引，mysql 将扫描所有记录以找到该行。

来自 http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html

B-Tree Index Characteristics

A B-tree index can be used for column comparisons in expressions that use the =, >, >=, <, <=, or BETWEEN operators.

没有IN

和

Hash Index Characteristics

Hash indexes have somewhat different characteristics from those just discussed:

They are used only for equality comparisons that use the = or <=> operators (but are very fast). They are not used for comparison operators such as < that find a range of values.

IN 也没有。

正如@tombom 提到的，foo IN ('bar', 'bla') 是 foo = 'bar' OR foo = 'bla' 的缩写，但是，我相信他们是不同的。所以我在一个有足够数据记录的表上进行测试，发现如下:

mysql> show columns from t_key;   
+-------+---------+------+-----+---------+----------------+
| Field | Type    | Null | Key | Default | Extra          |
+-------+---------+------+-----+---------+----------------+
| a     | int(11) | NO   | PRI | NULL    | auto_increment |
| b     | int(11) | YES  | MUL | NULL    |                |
+-------+---------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

mysql> select count(a) from t_key;
+----------+
| count(a) |
+----------+
|   989901 |
+----------+
1 row in set (0.00 sec)

mysql> explain select a from t_key where a in (select max(a) from t_key);  
+----+--------------------+-------+-------+---------------+---------+---------+------+--------+------------------------------+
| id | select_type        | table | type  | possible_keys | key     | key_len | ref  | rows   | Extra                        |
+----+--------------------+-------+-------+---------------+---------+---------+------+--------+------------------------------+
|  1 | PRIMARY            | t_key | index | NULL          | PRIMARY | 4       | NULL | 989901 | Using where; Using index     |
|  2 | DEPENDENT SUBQUERY | NULL  | NULL  | NULL          | NULL    | NULL    | NULL |   NULL | Select tables optimized away |
+----+--------------------+-------+-------+---------------+---------+---------+------+--------+------------------------------+
2 rows in set (0.00 sec)

mysql> explain select a from t_key where a =(select max(a) from t_key);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra                        |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
|  1 | PRIMARY     | t_key | const | PRIMARY       | PRIMARY | 4       | const |    1 | Using index                  |
|  2 | SUBQUERY    | NULL  | NULL  | NULL          | NULL    | NULL    | NULL  | NULL | Select tables optimized away |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
2 rows in set (0.00 sec)

然后我尝试使用静态序列的 IN 查询，它的工作原理与 @tombom 提到的一样:

mysql> explain select a from t_key where a in (100,200);
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t_key | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

mysql> explain select a from t_key where a=100 or a=200;
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t_key | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where; Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

我不知道 mysql 是否会在可能的情况下将 IN 查询转换为 OR 查询(例如，序列在查询之前已知)，我没有' 找到相关文档，但是explain 表明它确实在这种情况下扫描了表。

关于mysql - 查询性能慢的原因是什么，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/12471851/