Why do two joins runs faster than one with an OR condition? - mysql

Question

I'm querying my database for a list of locations that are either a teacher's primary location or locations that the teacher has provided availability information for.
If I do two joins like this:
SELECT tea.*, avail_loc.*, pri_loc.*
FROM teachers AS tea
LEFT JOIN availability AS ava
ON(tea.teacher_id = ava.teacher_id AND ava.end_date > 1398706428)
LEFT JOIN locations AS avail_loc
ON(ava.location_id = avail_loc.location_id)
LEFT JOIN locations AS pri_loc
ON(tea.location_id = pri_loc.location_id)
WHERE tea.active = 1
My query takes .05 seconds. The problem is that I have to clean up the output in php because my locations are split between avail_loc (the alternate locations) and pri_loc (the primary location).
So, if I combine them into one join with an OR conditional, the query takes .8 seconds.
SELECT tea.*, loc.*
FROM teachers AS tea
LEFT JOIN availability AS ava
ON(tea.teacher_id = ava.teacher_id AND ava.end_date > 1398706428)
LEFT JOIN locations AS loc
ON(ava.location_id = loc.location_id OR tea.location_id = loc.location_id)
WHERE tea.active = 1
When I use EXPLAIN, the first query has indexes that match everything. When I run the second query, it's missing a join for my OR join.
Why are the two joins faster than the one with an OR? The resulting data is the same.

Answer 1

First, these two queries are not equivalent. If there is one match in each of the tables, then the first will return one row and the second will return two rows.
The answer to your question, though, is a matter of optimization. MySQL does a poor job of optimizing across or conditions. As you note, it misses the fact that different indexes could be used to satisfy each condition. To be honest, this is a problem with most database engines. If you want the effect of an or with better performance, then union all often works better:
SELECT tea.*, loc.*
FROM teachers AS tea
LEFT JOIN availability AS ava
ON(tea.teacher_id = ava.teacher_id AND ava.end_date > 1398706428)
LEFT JOIN locations AS loc
ON(tea.location_id = loc.location_id)
WHERE tea.active = 1
union all
SELECT tea.*, loc.*
FROM teachers AS tea
LEFT JOIN availability AS ava
ON(tea.teacher_id = ava.teacher_id AND ava.end_date > 1398706428)
LEFT JOIN locations AS loc
ON(ava.location_id = loc.location_id)
WHERE tea.active = 1;