Date: Tue, 5 Feb 2019 22:35:12 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@lists.postgresql.org
Subject: Re: Refactoring IndexPath representation of index conditions
Message-ID: <20190206013512.GA3101@alvherre.pgsql>
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On 2019-Feb-02, Tom Lane wrote:

> In any case I think it makes things simpler and clearer, which is
> worth a good deal.

Looking at the patch, I agree -- this is clearer than what was there
before.

> Another idea that I looked into is to not create RestrictInfos for
> derived indexqual clauses, with the hope that that would further
> reduce the added overhead for the commuted-clause case.  However
> that crashed and burned when I found out that the extended-stats
> machinery punts when given a bare clause rather than a RestrictInfo.
> It could possibly be fixed to not do that, but it looks like the
> consequences would be extra lookups that'd probably cost more than
> we saved by omitting the RestrictInfo.  Also, having RestrictInfos
> means that we can cache selectivity estimates across multiple
> calls.  I'm not entirely sure how much that matters in this
> context, but it's probably not negligible.

Is it reasonable to give ext-stats the option to receive either a
"plain" clause or a RestrictInfo, and if the former have it construct
the RestrictInfo and return it?  It seems a pity to waste effort to
cater for ext-stats, only to be used in the rare case where any
ext-stats actually exist ... most of the time, it'd be wasted effort.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services