Re: Doubts about pushing LIMIT to MergeAppendPath

From: Antonin Houska <ah(at)cybertec(dot)at>
To: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Doubts about pushing LIMIT to MergeAppendPath
Date: 2018-11-02 07:16:53
Message-ID: 30407.1541143013@localhost
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Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:

> OK, so the reason is that when building child paths, we don't keep the
> pathkeys unless it matches the "interesting" pathkeys.
> So for example we may have an IndexPath, but with pathkeys=NIL if the
> index does not match the ORDER BY we need.

I don't agree that IndexPath will necessarily have pathkeys set to NIL in such
a case. Even if the index ordering does not match ORDER BY clause of the
query, the path can still be useful, typically for merge join.

Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt

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