Re: UNION ALL with ROW_NUMBER vs UNION (WAS: "Wow - has anyone used ROW_NUMBER() to get around using a UNION statement when UNION ALL doesn't work??")

From: Jeremy Schneider <jeremy.schneider_at_ardentperf.com>
Date: Fri, 12 Aug 2011 14:49:07 -0500
Message-ID: <CA+fnDAauY7eROTnaDTPJZEc8zFJG-k8v304e-CW=dXqKUntNMg_at_mail.gmail.com>

Aaaah, it's clear now. This has nothing to do with an optimization between UNION vs ROW_NUMBER -- it's that your plan completely changed when you switched the syntax -- most importantly the table join order. If you use the same join order with the UNION then you should see similar performance.

It seems that there are two table join orders with nearly identical cost. Something in the costing of the UNION statement causes the optimizer to pick the unlucky order, whereas with the UNION ALL it picks the more lucky one.

-Jeremy

On Fri, Aug 12, 2011 at 1:32 PM, Taylor, Chris David < ChrisDavid.Taylor_at_ingrambarge.com> wrote:

> First, disregard the part about the large TEMP tablespace usage. I think
> that was coming from the query _*before*_ I rewrote it to use WITH
> statements. I’m not seeing the large TEMP usage now.****
>
> ** **
>
> For each number below, I’ve included the dbms_xplan information, the row
> source operations, and the timings.****
>
> ** **
>
> #1 The original query using WITH statements and UNION****
>
> http://pastebin.com/embed_iframe.php?i=0Ht0V4T3****
>
> Time: 18 minutes, 38 secs (89,200 rows)****
>
> ** **
>
> #2 The original query CHANGED from UNION to UNION ALL (only change):****
>
> http://pastebin.com/embed_iframe.php?i=JybML3y8****
>
> Time: 1 minute, 03 secs (90,227 rows)****
>
> ** **
>
> #3 The query from #2 CHANGED to include the ROW_NUMBER function to give us
> the same results as #1:****
>
> http://pastebin.com/embed_iframe.php?i=75QJ2ShD****
>
> Time: 50 secs (89,200 rows)****
>
> ** **
>
> ** **
>
> ** **
>
> *Chris Taylor*****
>
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>
> Ingram Barge Company****
>
> Nashville, TN 37205****
>
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Jeremy Schneider
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Received on Fri Aug 12 2011 - 14:49:07 CDT