Does MySQL Short Circuit the IF() function?

It depends.

IF doesn't short-circuit such that it can be used to avoid truncation warnings with GROUP_CONCAT, for example in:

set @@group_concat_max_len = 5;

select if(true or @var:=group_concat('warns if evaluated'), 'actual result', @var);

the result will be 'actual result' but you'll get a warning:

Warning (Code 1260): Row 1 was cut by GROUP_CONCAT()

which is the same warning you get with less trivial GROUP_CONCAT expressions, such as distinct keys, and without the IF at all.

With J. Jorgenson's help I came up with my own test case. His example does not try to short circuit in the condition evaluation, but using his idea I came up with my own test and verified that MySQL does indeed short-circuit the IF() condition check.

SET @var:=5;
SELECT IF(1 = 0 AND (@var:=10), 123, @var); #Expected output: 5
SELECT IF(1 = 1 AND (@var:=10), @var, 123); #Expected output: 10

On the second example, MySQL is properly short-circuiting: @var never gets set to 10.

Thanks for the help J. Jorgenson!

The answer is YES.
The IF(cond,expr_true,expr_false) within a mysql query is short-circuited.

Here a test, using @variables to prove the fact:

SET @var:=5;  
SELECT IF(1 = 0, (@var:=@var + 1), @var ); -- using ':=' operator to modify 'true' expr @var 
SELECT IF(1 = 1, @var, (@var:=@var + 1) ); -- using ':=' operator to modify 'false' expr @var 
SELECT @var;

The result is '5' from all three SELECT queries.

Had the IF() function NOT short circuited, the result would be a '5' from SELECT #1, and '6' from SELECT #2, and a '7' from the last "select @var".

This is because the 'true' expression is NEVER executed, in select #1 and nor is the false expression executed for select #2.

Note the ':=' operator is used to modify an @var, within an SQL query (select,from, and where clauses). You can get some really fancy/complex SQL from this. I've used @vars to apply 'procedural' logic within a SQL query.

-- J Jorgenson --