Question

I have a query that goes something like this:

SELECT t1, t2,
IF(MATCH(t2) AGAINST ('input*' IN BOOLEAN MODE), 10, 0) AS matches,
IF(t2 LIKE '%input%', 2, 0) AS similar
FROM tbl
WHERE t2 LIKE '%input%'
ORDER BY (matches + similar) DESC
LIMIT 5

The query works fine, but the part I’m concerned about is whether or not MySQL is checking whether t2 is LIKE ‘%input%’ twice, or if it caches the first result (which would be cool!).

Thanks

Answer 1

Sorry, it does not.

As a poor man’s test, consider:

delimiter //
create function qqq() 
returns int no sql 
begin 
  set @x:=@x+1; 
  return 17; 
end //
delimiter ;

set @x := 0;
select qqq(), qqq(), qqq() > 1, qqq() > 1 from dual;
+-------+-------+-----------+-----------+
| qqq() | qqq() | qqq() > 1 | qqq() > 1 |
+-------+-------+-----------+-----------+
|    17 |    17 |         1 |         1 |
+-------+-------+-----------+-----------+

select @x;
+------+
| @x   |
+------+
|    4 |
+------+

While one can argue that this is a special case, since we’re invoking a stored routine, as opposed to built-in function, there is no difference at the moment. Everything is re-evaluated.

This may change in the future — there’s nothing to strictly prevent this optimization from happening on well known, deterministic functions. But sometimes this may be hard to diagnose by the optimizer.