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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Teodor Sigaev <teodor@sigaev.ru>
Cc: Michael Paquier <michael.paquier@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Haribabu Kommi <kommi.haribabu@gmail.com>
Cc: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Pluggable storage
Date: Fri, 23 Jun 2017 10:24:59 -0400
Message-ID: <8786.1498227899@sss.pgh.pa.us> (raw)
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Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> It would be really great if you could explain why BitmapScans are 
> dubious, instead of just labeling them as dubious. (I guess you mean 
> Bitmap Heap Scans, right?)

The two things I'm aware of are (a) the assumption you noted, that
fetching tuples in TID sort order is a reasonably efficient thing,
and (b) the "offset" part of a TID can't exceed MaxHeapTuplesPerPage
--- see data structure in tidbitmap.c.  The latter issue means that
you don't really have a full six bytes to play with in a TID, only
about five.

I don't think (b) would be terribly hard to fix if we had a motivation to,
but I wonder whether there aren't other places that also know this about
TIDs.

			regards, tom lane


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