How to improve MYSQL performance on UNION ALL?

Question

I am optimizing a query which involves a UNION ALL of two queries. Both Queries have been more than optimized and run at less than one second separately. However, when I perform the union of both, it takes around 30 seconds to calculate everything.

I won't bother you with the specific query, since they are optimized as they get, So let's call them Optimized_Query_1 and Optimized_Query_2

Number of rows from Optimized_Query_1 is roughly 100K

Number of rows from Optimized_Query_2 is roughly 200K

SELECT * FROM (
   Optimized_Query_1
UNION ALL
   Optimized_Query_2
) U 
ORDER BY START_TIME ASC

I do require for teh results to be in order, but I find that with or without the ORDER BY at the end the query takes as much time so shouldn't make no difference.

Apparently the UNION ALL creates a temporary table in memory, from where the final results are then given, is there any way to work around this?

Thanks

Answer 1

You can't optimize UNION ALL . It simply stacks the two results sets on top of each other. Compared to UNION where an extra step is required to remove duplicates, UNION ALL is a straight stacking of the two result sets. The ORDER BY is likely taking additional time.

You can try creating a VIEW out of this query.