Does the order of conditions in a WHERE clause affect MySQL performance?

Question

Say that I have a long, expensive query, packed with conditions, searching a large number of rows. I also have one particular condition, like a company id, that will limit t

Answer 1

Here is a demo showing the order of WHERE clause conditions can make a difference due to short-circuiting. It runs the following queries:

-- query #1
SELECT myint FROM mytable WHERE myint >= 3 OR myslowfunction('query #1', myint) = 1;

-- query #2
SELECT myint FROM mytable WHERE myslowfunction('query #2', myint) = 1 OR myint >= 3;

The only difference between these is the order of operands in the OR condition.

myslowfunction deliberately sleeps for a second and has the side effect of adding an entry to a log table each time it is run. Here are the results of what is logged when running the two queries:

myslowfunction called for query #1 with value 1
myslowfunction called for query #1 with value 2
myslowfunction called for query #2 with value 1
myslowfunction called for query #2 with value 2
myslowfunction called for query #2 with value 3
myslowfunction called for query #2 with value 4

The above shows that a slow function is executed more times when it appears on the left side of an OR condition when the other operand isn't always true.

So IMO the answer to the question:

Does the order of conditions in a WHERE clause affect MySQL performance?

is "Sometimes it can do."