From: Tom Lane <tgl@sss.pgh.pa.us>
To: Heikki Linnakangas <heikki.linnakangas@iki.fi>
cc: pgsql-hackers@postgresql.org
Subject: Re: [COMMITTERS] pgsql: Allow GiST distance function to return merely
 a lower-bound.
In-reply-to: <E1YtDlp-0002at-3o@gemulon.postgresql.org>
References: <E1YtDlp-0002at-3o@gemulon.postgresql.org>
Comments: In-reply-to Heikki Linnakangas <heikki.linnakangas@iki.fi>
 message dated "Fri, 15 May 2015 11:27:13 -0000"
Date: Sat, 23 May 2015 15:35:21 -0400
Message-ID: <31465.1432409721@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

Heikki Linnakangas <heikki.linnakangas@iki.fi> writes:
> Allow GiST distance function to return merely a lower-bound.

I wondered how come this patch did not touch nodeIndexonlyscan.c.

After some investigation, it seems the only reason that this patch even
appears to work is that none of the built-in or contrib opclasses support
both lossy ordering operators and GIST fetch functions.  Otherwise we
would do an index-only scan and the executor would simply ignore the
recheck flag.

I doubt we can ship this in this state; even if the core code doesn't
exercise the problematic combination, surely third-party opclasses
will want to?

I thought about hacking the planner to not select index-only scans,
but there's no way for it to know whether the opclass might return
recheck = true at runtime.  I think the only real fix is to actually
propagate all the changes in nodeIndexscan.c into nodeIndexonlyscan.c.
Testing it would be problematic without a suitable opclass, though :-(

A short-term hack might be to throw a "not implemented" error in
nodeIndexonlyscan.c if it sees the distance-recheck flag set.

			regards, tom lane


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