Re: SQL CTE of a UNION kills performance

Are you using DB2 SMP?

El mar., 28 de ago. de 2018 14:37, Buck Calabro <kc2hiz@xxxxxxxxx> escribió:

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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