Meaning of “Select tables optimized away” in MySQL Explain plan

What is the meaning of Select tables optimized away in MySQL Explain plan?

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)

Note: explain plan output edited for legibility.

Answer

It means you have done a query that does nothing more than count the number of rows in a table, and that table is a MyISAM table. MyISAM tables are stored with a separate row count, so to do this query MySQL doesn’t need to look at any of the table row data at all. Instead it immediately returns the pre-calculated row count. Hence the table access is ‘optimized away’ and the query is lightning-fast.

The same won’t happen on other storage engines in MySQL such as InnoDB. But really, you want to be using InnoDB and not MyISAM in most cases for a variety of other reasons. (And even without the row count optimisation this kind of query is very, very fast.)

select count(comment_count) from wp_posts;

Is that what you really meant to do? That’s the same as just SELECT COUNT(*)... (assuming comment_count can’t be NULL, which it can’t be or you wouldn’t have got the optimisation). If you want a total of the comment_count​s you should be using SUM(comment_count), and you won’t get the ‘optimized away’ behaviour.