On 5 May 2017 at 14:04, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Craig Ringer <craig.rin...@2ndquadrant.com> writes:
>> We're carefully maintaining this bizarre cognitive dissonance where we
>> justify the need for using this as a planner hint at the same time as
>> denying that we have a hint. That makes it hard to make progress here.
>> I think there's fear that we're setting some kind of precedent by
>> admitting what we already have.
> I think you're overstating the case.  It's clear that there's a
> significant subset of CTE functionality where there has to be an
> optimization fence.  The initial implementation basically took the
> easy way out by deeming *all* CTEs to be optimization fences.  Maybe
> we shouldn't have documented that behavior, but we did.  Now we're
> arguing about how much of a compatibility break it'd be to change that
> planner behavior.  I don't see any particular cognitive dissonance here,
> just disagreements about the extent to which backwards compatibility is
> more important than better query optimization.

How about we get the ball rolling on this in v10 and pull that part
out of the docs. If anything that'll buy us a bit more wiggle room to
change this in v11.

I've attached a proposed patch.

 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

Attachment: doc_caution_about_cte_changes_in_the_future.patch
Description: Binary data

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