Horton

Re: Early WIP/PoC for inlining CTEs

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrew Gierth <andrew@tao11.riddles.org.uk>
Cc: Michael Paquier <michael@paquier.xyz>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Andreas Karlsson <andreas@proxel.se>, Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@enterprisedb.com>, David Fetter <david@fetter.org>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2019-02-09T20:52:52Z
Lists: pgsql-hackers

Commits

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  1. Prevent inlining of multiply-referenced CTEs with outer recursive refs.

  2. Allow user control of CTE materialization, and change the default behavior.

  3. Split QTW_EXAMINE_RTES flag into QTW_EXAMINE_RTES_BEFORE/_AFTER.

  4. document when PREPARE uses generic plans

Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
> "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.

Sure, but this is exactly the sort of situation where we should offer
a way for the user to force either decision to be made.  I think it's
very unlikely that we'll ever be in a position to make a realistic
cost-based decision for that.  Actually planning it out both ways would
be horrendously expensive (and probably none too reliable anyway, given
how shaky ndistinct estimates tend to be); and we certainly don't have
enough info to make a smart choice without doing that.

			regards, tom lane