mysql explain 中的 ref|filter

在 MySQL 的 Explain 中，ref 表示使用哪个列或常数与索引进行比较，filter 表示额外的过滤条件，这些条件不使用索引进行比较，而是在结果集中进行过滤。

ref 列通常会显示使用了哪些索引列进行比较，如果 ref 的值为 NULL，则表示没有使用索引进行比较，这种情况下应该尽量避免。

filter 列通常会显示额外的过滤条件，这些条件可能涉及到非索引列或是函数操作，这些条件会在结果集中进行过滤，所以会影响查询效率。如果 filter 的值过大，则需要考虑优化查询语句或是调整数据结构。

相关问题

mysql explain filter

MySQL `EXPLAIN` is a command that is used to obtain information about how a SELECT query is executed by the MySQL optimizer. The `EXPLAIN` command provides the execution plan for a given SELECT query, which includes a detailed description of the access method, join type, and order of table access.

The `FILTER` option in MySQL `EXPLAIN` is used to show the conditions that are used to filter the rows in the result set. It is an optional clause that can be added to the `EXPLAIN` command to provide additional information about the query execution plan.

For example, consider the following query:

EXPLAIN SELECT * FROM employees WHERE salary > 50000 AND department = 'Sales';

The `EXPLAIN` command will provide the execution plan for the above query, and the `FILTER` option will show the conditions used to filter the rows in the result set. The output might look like this:

+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows | Filter      |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | employees | ref  | salary        | salary | 4       | NULL |    2 | salary > 50000 AND department = 'Sales' |
+----+-------------+-----------+------+---------------+------+---------+------+------+-------------+

In the above output, the `Filter` column shows the conditions used to filter the rows in the result set, which is `salary > 50000 AND department = 'Sales'`. This means that the MySQL engine will use the `salary` index to filter the rows with a salary greater than 50000, and then further filter the rows with department 'Sales'.