Will a function will be recalled when I use its alias in ORDER BY clause?

Question

Here is the simplified version of my query:

SELECT myfunc(u.id) score FROM users u
ORDER BY score
LIMIT 20

As you see, I've used score (which is the result of myfunc() function) in the ORDER BY clause. Now I want to know, will that function be recalled when I use it in ORDER BY clause? If yes, then how can I avoid that? Because that function needs lots of processing and recalling it twice would be like a nightmare.

No - ORDER BY is evaluated last: [See here](https://stackoverflow.com/questions/24127932/mysql-query-clause-execution-order) The query engine knows to wait to sort by the result of the function until it applies the function to each row. — Max Szczurek, Jul 26 '17 at 06:22
@RickJames Honestly I don't know what's `DETERMINISTIC` .. what do you mean exactly by that? — Martin AJ, Jul 26 '17 at 14:08
@RickJames no it is not a procedure. I don't use `CALL` to call it. I just use it into `select` statement. — Martin AJ, Jul 26 '17 at 14:27

score 0 · Answer 1 · answered Jul 26 '17 at 06:27

Probably not, because the data has already been prepared by the time the ordering is done. If you still have a doubt, test it by making your function do something visible between runs (increment a value in a table)

What you may find though, is that it runs a million times, once for each of a million rows, before limit 20 can be applied..

If your function is really slow, and its sensible to store the result upon insert rather than calculate it every time the query is run, do that

Will a function will be recalled when I use its alias in ORDER BY clause?

1 Answers1