EXPLAIN EXTENDED

From Stack Overflow:

I have a query where I have a custom developed UDF that is used to calculate whether or not certain points are within a polygon (first query in UNION) or circular (second query in UNION) shape.

SELECT  a.geo_boundary_id, …
FROM     geo_boundary_vertex a, …
…
GROUP BY
a.geo_boundary_id
UNION
SELECT  b.geo_boundary_id, …
FROM     geo_boundary b, …
…
GROUP BY
b.geo_boundary_id

When I run an explain for the query I get filesort for both queries within the UNION.

Now, I can split the queries up and use the ORDER BY NULL trick to get rid of the filesort however when I attempt to add that to the end of a UNION it doesn't work.

How do I get rid of the filesort?

In MySQL, GROUP BY also implies ORDER BY on the same set of expressions in the same order. That's why it adds an additional filesort operation to sort the resultset if it does not come out naturally sorted (say, from an index).

This is not always a desired behavior, and MySQL manual suggests adding ORDER BY NULL to the queries where sorting is not required. This can improve performance of the queries.

Let's create a sample table and see:

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