sql - MySQL解释计划中 "Select tables optimized away"的含义

Question

MySQL Explain plan中Select tables optimization away是什么意思？

explain select count(comment_count) from wp_posts;

+----+-------------+---------------------------+-----------------------------+
| id | select_type | table,type,possible_keys, | Extra                       |
|    |             | key,key_len,ref,rows      |                             |
+----+-------------+---------------------------+-----------------------------+
| 1  | SIMPLE      | all NULLs                 | Select tables optimized away| 
+----+-------------+---------------------------+-----------------------------+
1 row in set (0.00 sec)

注意:explain plan 为便于阅读而编辑输出。

Answer 1

来自 MySQL 文档:

The query contained only aggregate functions (MIN(), MAX()) that were all resolved using an index, or COUNT(*) for MyISAM, and no GROUP BY clause. The optimizer determined that only one row should be returned.

基本上这意味着您的查询使用 MySQL 直接可用的数据，并且查询将在恒定时间内运行。