Check if DISTINCT is really necessary

OK, first things first, talk to the PHP devs to see if this situation is taken care of for you, which would then save a lot of pain and heartache!

Other than that, try with a group by instead of a distinct, see what the performance differences are - it may be that the group by is faster based on the indexes you have on the tables.

With the GROUP BY option, you can add a count(*) then talk to the php devs to ensure they know that if that result is 2 or more, then they need to handle it differently than if the result is a 1.

Have you tried replacing the order by with a group by and then having count(*) > 1 statement. That should show if there duplicated rows

Do a select count from each query and compare?

select count(*) from (
SELECT DISTINCT `tableA`.`ColA`, `tableA`.`ColB`, `tableB`.`ColA`, ...
FROM `tableA`
LEFT JOIN `tableB` ON `tableA`.`colC` = `tableB`.`colA`
WHERE <whereconditions>
) a

select count(*) from (
SELECT `tableA`.`ColA`, `tableA`.`ColB`, `tableB`.`ColA`, ...
FROM `tableA`
LEFT JOIN `tableB` ON `tableA`.`colC` = `tableB`.`colA`
WHERE <whereconditions>
) a