AND statements

[Shawazi]

I have a tricky question. If there is a MySQL query that has

WHERE a=1 AND b=1

Will it still go to evaluate "b=1" if it finds "a is not equal to 1" first. If so, the order of WHERE statements can impact performance so I'm trying to find out.

[NogDog]

I do not know for sure how MySQL handles it, but every programming language I've ever used goes left to right through a series of ANDs and stops as soon as one comparison evaluates to false.

[Shawazi]

that's what i thought i just wish i could research it easier... typing in any of the words dealing with this is just ridiculous ha

[bpat1434]

I believe that MySQL evaluates from Left to Right, so first rows are filter based upon "a" then that temporary table is filtered against "b" and the result is given.

You can always define which way will be done by using a sub-query:

SELECT * FROM (SELECT * FROM `table` WHERE a=1) WHERE b=1

Essentially you're forcing MySQL to first generate a temp table of items where a=1, then you're filtering on that temp table where b=1. You could also switch it if you needed.

And for your own edification, here is the MySQL WHERE optimization entry.