Does MySQL remove common subexpressions between SELECT clause and HAVING / GROUP BY

Question

I often see people answer MySQL questions with these queries:

SELECT DAY(date), other columns
FROM table
GROUP BY DAY(date);

SELECT somecolumn, COUNT(*)
FROM table
HAVING COUNT(*) > 1;

I always like to alias a column and refer to it in a sentence, GROUP BYor HAVINGfor example

SELECT DAY(date) AS day, other columns
FROM table
GROUP BY day;

SELECT somecolumn, COUNT(*) AS c
FROM table
HAVING c > 1;

Is MySQL smart enough to notice that the expressions in subsequent sentences are the same as in SELECT, and do it only once? I'm not sure how to check this - it EXPLAINdoes not show any difference, but it does not seem to show how grouping or filtering does it in the first place; this is mainly useful for optimizing associations and sentences WHERE.

I tend to be pessimistic about optimizing MySQL, so I would like to provide all my help.

Answer 1

, sleep(),
, : http://sqlfiddle.com/#!2/0bc1b/1

Select * FROM t;

| X |
|---|
| 1 |
| 2 |
| 2 |

SELECT x+sleep(1)
FROM t
GROUP BY x+sleep(1);

SELECT x+sleep(1) As name
FROM t
GROUP BY name;

3000 (3 ).
3 , 1 ,
, , .

Answer 2

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