I am trying to get the diff between two nearly identical tables in postgresql. The current query I am running is:
SELECT * FROM tableA EXCEPT SELECT * FROM tableB;
and
SELECT * FROM tableB EXCEPT SELECT * FROM tableA;
Each of the above queries takes about 2 minutes to run (Its a large table)
I wanted to combine the two queries in hopes to save time, so I tried:
SELECT * FROM tableA EXCEPT SELECT * FROM tableB
UNION
SELECT * FROM tableB EXCEPT SELECT * FROM tableA;
And while it works, it takes 20 minutes to run!!! I would guess that it would at most take 4 minutes, the amount of time to run each query individually.
Is there some extra work UNION is doing that is making it take so long? Or is there any way I can speed this up (with or without the UNION)?
UPDATE: Running the query with UNION ALL takes 15 minutes, almost 4 times as long as running each one on its own, Am I correct in saying that UNION (all) is not going to speed this up at all?