Re: Early WIP/PoC for inlining CTEs - Mailing list pgsql-hackers

From Andrew Gierth
Subject Re: Early WIP/PoC for inlining CTEs
Date
Msg-id 87d0o970th.fsf@news-spur.riddles.org.uk
Whole thread Raw
In response to Re: Early WIP/PoC for inlining CTEs  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: Early WIP/PoC for inlining CTEs  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-hackers
>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:

 Tom> After further reflection I really don't like Andrew's suggestion
 Tom> that we not document the rule that multiply-referenced CTEs won't
 Tom> be inlined by default. That would be giving up the principle that
 Tom> WITH calculations are not done multiple times by default, and I
 Tom> draw the line at that. It's an often-useful behavior as well as
 Tom> one that's been documented from day one, so I do not accept the
 Tom> argument that we might someday override it on the basis of nothing
 Tom> but planner cost estimates.

The case that springs to mind is when a CTE with grouping is then joined
multiple times in the main query with different conditions. If the
planner is able to deduce (e.g. via ECs) that restrictions on grouped
columns can be pushed into the CTE, then inlining the CTE multiple times
might be a significant win. But if that isn't possible, then inlining
multiple times might be a significant loss.

If such a query is made into a view, then (given your position) the
decision of whether to inline has to be made at view creation time,
which doesn't seem desirable (even if we have to put up with it for the
present).

-- 
Andrew (irc:RhodiumToad)


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