Re: SQL CTE of a UNION kills performance -- MIDRANGE-L

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Subject: Re: SQL CTE of a UNION kills performance
From: Buck Calabro <kc2hiz@xxxxxxxxx>
Date: Tue, 28 Aug 2018 15:37:16 -0400
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On 8/28/2018 1:26 PM, Charles Wilt wrote:

I suspect that the DB is trying to materialize all the rows before
returning with the CTE...

Whereas w/o the CTE, you're seeing the results of *FIRSTIO as an
optimization goal...

Makes sense.

Try changing your goal to *ALLIO and see if the query becomes as slow as
the CTE.

That'd be QAQQINI, I guess, but it's a good diagnostic.

Using your hint about *FIRSTIO, I stuffed the CTE into a separate VIEW,
pulled it out of the problematic VIEW and boom - it's much, much faster.

Thank you for your insight!
--buck

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Follow-Ups:

Re: SQL CTE of a UNION kills performance , Charles Wilt
Re: SQL CTE of a UNION kills performance , Diego Kesselman

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