Re: Is the query engine always smart enough to know when to generate an internal CTE on a query?

Hi Dan

I think others have mentioned this, and I'll stay say it - run both 
versions - with and without CTE - in Visual Explain. First see if there 
are table scans in the "without" version, then see if you can see why - 
I know you know this stuff, too. See if there are indexes recommended, 
of course. Are the WHEREs internal to that main table? I'm inclined to 
say both will be similar.

Now using a CTE, that always seems similar to having a view that is the 
same thing - views are executed at run-time but they simplify the 
syntax, same as one of the replies said, to segregate complexity - I 
made that phrase up, I think!  :)

Anyhow, good luck, it'll be interesting to hear your results.

*Regards*

*Vern Hamberg*

IBM Champion 2025 <cid:[email protected]> CAAC 
(COMMON Americas Advisory Council) IBM Influencer 2023


On 8/7/2026 8:14 AM, Peter Lunde wrote:
> Hi Dan,
>
> I think an explicit CTE is worth a try.
>
> Best Regards,
>
> Peter.
> 647-456-3124
> [email protected]
>
> ________________________________
> From: MIDRANGE-L<[email protected]> on behalf of Dan Bale via MIDRANGE-L<[email protected]>
> Sent: Thursday, August 06, 2026 17:23
> To: Midrange Systems Technical Discussion<[email protected]>
> Cc: Dan Bale<[email protected]>
> Subject: Is the query engine always smart enough to know when to generate an internal CTE on a query?
>
> We've got a long-running query that has a tendency to get canceled because it hits our temporary storage threshold.
>
> The current table has a lot of conditions in the WHERE clause on the primary table that greatly narrow the rows from that table.  There are also 15 left joins on that primary table.  I've thought about modifying the query to use a CTE on the primary table and move the conditions in the WHERE clause to the CTE.
>
> I'm just wondering if the query engine optimizes this anyway without the explicit CTE.  Thoughts?
>
> - Dan Bale
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