There are two ways to speedup UNIONs in a MySQL database. First use UNION ALL if at all possible, and second try to push down your conditions.

1. UNION ALL is much faster than UNION

How does a UNION work? Imagine you have two tables for shirts. The short_sleeve table looks like this:

blue
green
gray
black

And long_sleeve another that looks like this:

red
green
yellow
blue

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If you UNION those two tables, first MySQL will sort the combined set into a temp table like this:

black
blue
blue
gray
green
green
red
yellow

Once it’s done this sort, it can easily remove the duplicate blue & duplicate green for this resulting set:

black
blue
gray
green
red
yellow

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Why does it do this? UNION is defined that way in SQL. Duplicates must be removed and this is an efficient way for the MySQL engine to remove them. Combine results, sort, remove duplicates and return the set.

What if we did UNION ALL? The result would look like this:

blue
green
gray
black
red
green
yellow
blue

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It doesn’t have to sort, and doesn’t have to remove duplicates. If you imagine combining two 10 million row tables, and don’t have to sort, this speedup can be HUGE.

2. Use Push-down Conditions to speedup UNION in MySQL

Imagine with our example above the shirts have a design date, the year they were released. Yes we’re keeping this example very simple to illustrate the concept.

Here is the short_sleeve table:

blue		2013
green		2013
green		2012
gray		2011
black		2009
black		2011

And long_sleeve table looks like this:

red		2012
red		2013
green		2011
yellow	2010
blue		2011

For 2013 designs could combine them like this:

(SELECT type, release FROM short_sleeve)
UNION
(SELECT type, release FROM long_sleeve);
WHERE release >=2013;

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Here the WHERE clause works on this 11 record temp table:

black		2009
black		2011
blue		2011
blue		2013
gray		2011
green		2013
green		2012
green		2011
red		2012
red		2013
yellow	2010

But it would be much faster to move the WHERE inside each subquery like this:

(SELECT type, release FROM short_sleeve WHERE release >=2013)
UNION
(SELECT type, release FROM long_sleeve WHERE release >=2013);

That would be operating on a combined 3 record table. Faster to sort & remove duplicates. Smaller result sets cache better too, providing a pay forward dividend. That’s what performance optimization is all about!

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Remember multi-million row sets in each part of this query will quickly illustrate the optimization. We’re using very small results to make visualizing easier.

You can also use this optimization for ORDER BY and for LIMIT conditions. By reducing the number of records returned by EACH PART of the UNION, you reduce the work that happens at the stage where they are all combined.

If you’re seeing some UNION queries in your slow query log, I suggest you try this optimization out and see if you can tweak it.