From: Tom Lane <tgl@sss.pgh.pa.us>
To: Edmund Horner <ejrh00@gmail.com>
cc: David Rowley <david.rowley@2ndquadrant.com>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Tid scan improvements
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Comments: In-reply-to Edmund Horner <ejrh00@gmail.com>
	message dated "Fri, 21 Dec 2018 21:04:58 +1300"
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Date: Fri, 21 Dec 2018 11:57:48 -0500
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Edmund Horner <ejrh00@gmail.com> writes:
> Ok.  I think that will simplify things.  So if I follow you correctly,
> we should do:

> 1. If has_useful_pathkeys is true: generate pathkeys (for CTID ASC),
> and use truncate_useless_pathkeys on them.
> 2. If we have tid quals or pathkeys, emit a TID scan path.

Check.

> For the (optional) backwards scan support patch, should we separately
> emit another path, in the reverse direction?

What indxpath.c does is, if has_useful_pathkeys is true, to generate
pathkeys both ways and then build paths if the pathkeys get past
truncate_useless_pathkeys.  That seems sufficient in this case too.
There are various heuristics about whether it's really useful to
consider both sort directions, but that intelligence is already
built into truncate_useless_pathkeys.  tid quals with no pathkeys
would be reason to generate a forward path, but not reason to
generate a reverse path, because then that would be duplicative.

			regards, tom lane