使用变量时不使用索引

Question

背景：服务器应用程序 (C#) 使用以下查询（已简化）从数据库中获取订单数据。我记录了这需要的平均时间，现在执行时间突然增加（平均从 5 毫秒增加到 50 毫秒）。所以我开始检查这个查询。

查询是从服务器应用程序执行的，如下所示：

String ordernr = "123456789";
String sql = "SELECT * FROM MYDB.`MYTABLE` WHERE id = @id";
using (MySqlCommand cmd = new MySqlCommand(sql, conn))
{
    cmd.Parameters.Add("@id", MySqlDbType.VarChar, 16).Value = ordernr;
    using (MySqlDataReader reader = cmd.ExecuteReader())
    {
        //reading data
    }
}

当我检查 mysql 中的查询时，我得到以下结果：

MariaDB [MYDB]> SET @id  = '123456789';
Query OK, 0 rows affected (0.00 sec)

MariaDB [MYDB]> explain SELECT SQL_NO_CACHE * FROM MYDB.`MYTABLE` WHERE id  = @id ;
+------+-------------+-----------+------+---------------+------+---------+------+---------+-------------+
| id   | select_type | table     | type | possible_keys | key  | key_len | ref  | rows    | Extra       |
+------+-------------+-----------+------+---------------+------+---------+------+---------+-------------+
|    1 | SIMPLE      | MYTABLE | ALL  | NULL          | NULL | NULL    | NULL | 1448219 | Using where |
+------+-------------+-----------+------+---------------+------+---------+------+---------+-------------+
1 row in set (0.00 sec)

MariaDB [MYDB]> explain SELECT SQL_NO_CACHE * FROM MYDB.`MYTABLE` WHERE id  = '123456789';
+------+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
| id   | select_type | table     | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+------+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
|    1 | SIMPLE      | MYTABLE | const | PRIMARY       | PRIMARY | 18      | const |    1 |       |
+------+-------------+-----------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [MYDB]>

现在我的问题是，为什么在使用变量时忽略了 PRIMARY 索引？

我不知道这是否是性能下降的原因（因为在本例中两个查询都在 0.00 秒内 return），但这让我皱眉，为什么会有差异。我也在服务器应用程序中使用带有变量的准备好的查询。所以我想检查一下这是否相关。

谁能解释一下？

Answer 1

索引长度表明 id 实际上是一个 char/varchar 列（VARCHAR(16) 是我的猜测）。

然后，问题是 table 或列的字符集与其中一个连接不匹配。考虑以下因素：

MariaDB [test]> create table t1 (id varchar(16) primary key, i int) charset latin1;
Query OK, 0 rows affected (0.72 sec)

MariaDB [test]> insert into t1 values ('123456789',1),('987654321',2);
Query OK, 2 rows affected (0.08 sec)
Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> set names latin1;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> set @a='123456789';
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> explain select * from t1 where id = @a;
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id   | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|    1 | SIMPLE      | t1    | const | PRIMARY       | PRIMARY | 18      | const |    1 |       |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

MariaDB [test]> set names utf8;
Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> set @a='123456789';
Query OK, 0 rows affected (0.01 sec)

MariaDB [test]> explain select * from t1 where id = @a;
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

您需要更改 table，或在会话中设置字符集，或使用显式转换：

MariaDB [test]> explain select * from t1 where id = @a;
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
|    1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where |
+------+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

MariaDB [test]> explain select * from t1 where id = convert(@a using latin1);
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id   | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|    1 | SIMPLE      | t1    | const | PRIMARY       | PRIMARY | 18      | const |    1 |       |
+------+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

使用变量时不使用索引

Index is not used when using a variable

c#

mysql

mariadb