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

   Generally yes - the SQE does not create CTEs, but it does promote where
   clause conditions "up-stream" and filters early in the query execution. It
   even promotes conditions from join clauses to the where clause and
   vice-versa if it finds out that this gives an advantage. 
   But to analyze, where your query "burns" the most time, you should run it
   with Visual Explain or look up the query in the plan cache and find out,
   what is the really "expensive" stuff in your query. Normally you will find
   one or more table scans, that are done, because no matching indexes are
   found. 
   There are very rare cases, where you really get a performance boost from
   creating a CTE - CTEs are more a readability thing, not so much a
   performance thing. 
   If you really want to learn more about query performance I can only
   recommend: [1]SQL Performance Explained by Markus Winand - or his
   website [2]https://use-the-index-luke.com which holds more or less the
   same content. He concentrates on Db2 LUW but 99% of the information is
   also applying to Db2 for i. 
   HTH
   Daniel

     Am 06.08.2026 um 23:23 schrieb Dan Bale via MIDRANGE-L
     <[email protected]>:

     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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References

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   1. https://sql-performance-explained.com/
   2. https://use-the-index-luke.com/
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