查询具有字符串数据类型的 int 列对 mysql 查询有任何性能影响吗？

Question

假设我有一个 table 作为：

create table any_table (any_column_1 int, any_column_2 varchar(255));
create index any_table_any_column_1_IDX USING BTREE ON any_table (any_column_1);

（注意：此处索引类型无关紧要）

我想知道使用 int 或 string 查询 any_column 是否对性能有任何影响，即

select * from any_table where any_column_1 = 12345;

与这个在性能方面有什么不同吗？

select * from any_table where any_column_1 = '12345';

我在网上看了一圈，确实没有遇到过这种特殊情况。

Answer 1

对于索引整数列，无论哪种方式都可以。当您将整数列与常量进行比较时，无论您将其格式化为整数还是字符串，常量值都会转换为整数。

您可以使用 EXPLAIN 确认这一点。在这两种情况下，EXPLAIN 表明它将使用索引（type: ref 表示索引查找），并且性能将相同。

mysql> explain select * from any_table where any_column_1 = 12345;
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys              | key                        | key_len | ref   | rows | filtered | Extra |
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | any_table | NULL       | ref  | any_table_any_column_1_IDX | any_table_any_column_1_IDX | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+

mysql> explain select * from any_table where any_column_1 = '12345';
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys              | key                        | key_len | ref   | rows | filtered | Extra |
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | any_table | NULL       | ref  | any_table_any_column_1_IDX | any_table_any_column_1_IDX | 5       | const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+

如果您在您的示例 any_column_2 中为字符串列编制了索引，那将会有所不同，因为字符串列的排序规则必须与您与之比较的值的排序规则相匹配。默认情况下，字符串文字将转换为兼容的排序规则，因此它使用索引：

create index any_table_any_column_2_IDX USING BTREE ON any_table (any_column_2);

mysql> explain select * from any_table where any_column_2 = '12345';
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+
| id | select_type | table     | partitions | type | possible_keys              | key                        | key_len | ref   | rows | filtered | Extra |
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | any_table | NULL       | ref  | any_table_any_column_2_IDX | any_table_any_column_2_IDX | 768     | const |    1 |   100.00 | NULL  |
+----+-------------+-----------+------------+------+----------------------------+----------------------------+---------+-------+------+----------+-------+

但是整数文字没有排序规则，所以你会收到警告，并且无法使用索引。 EXPLAIN 显示 type: ALL，因此它将执行 table-scan，如果您查询包含很多行的 table，那么性能会很差。

mysql> explain select * from any_table where any_column_2 = 12345;
+----+-------------+-----------+------------+------+----------------------------+------+---------+------+------+----------+-------------+
| id | select_type | table     | partitions | type | possible_keys              | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-----------+------------+------+----------------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | any_table | NULL       | ALL  | any_table_any_column_2_IDX | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+-----------+------------+------+----------------------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

mysql> show warnings;
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                                                                                                                                     |
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Warning | 1739 | Cannot use ref access on index 'any_table_any_column_2_IDX' due to type or collation conversion on field 'any_column_2'                                                                                     |
| Warning | 1739 | Cannot use range access on index 'any_table_any_column_2_IDX' due to type or collation conversion on field 'any_column_2'                                                                                   |
| Note    | 1003 | /* select#1 */ select `test2`.`any_table`.`any_column_1` AS `any_column_1`,`test2`.`any_table`.`any_column_2` AS `any_column_2` from `test2`.`any_table` where (`test2`.`any_table`.`any_column_2` = 12345) |
+---------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

查询具有字符串数据类型的 int 列对 mysql 查询有任何性能影响吗？

Does querying int column with string datatype have any performance impact in mysql queries?

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