From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
cc: Teodor Sigaev <teodor@sigaev.ru>,
        Michael Paquier <michael.paquier@gmail.com>,
        Robert Haas <robertmhaas@gmail.com>,
        Haribabu Kommi <kommi.haribabu@gmail.com>,
        Alvaro Herrera <alvherre@2ndquadrant.com>,
        Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Pluggable storage
In-reply-to: <9b5728fc-c266-5f71-9a69-7668ad231e05@2ndquadrant.com>
References: <20160824182201.GA232997@alvherre.pgsql>
 <20161013202642.ksiuyx72gu6wgupf@alvherre.pgsql>
 <CAJrrPGfG7BarSFb6KQkOOYVaC6vjQRgG45+J2+TwLHHaFKuwGA@mail.gmail.com>
 <CA+TgmoYQN9xtMopr4Db3UFCqpbfd5DJb0j5apSmWtkyuWRz0Xw@mail.gmail.com>
 <CAB7nPqR6TorHQ_mXmAiB_cL40Vf2+F6hsue-P9HVGoxVxoHyKA@mail.gmail.com>
 <8b2510f0-ab89-dbf1-5b1c-5d686a5e33b8@sigaev.ru>
 <9b5728fc-c266-5f71-9a69-7668ad231e05@2ndquadrant.com>
Comments: In-reply-to Tomas Vondra <tomas.vondra@2ndquadrant.com>
	message dated "Fri, 23 Jun 2017 16:07:44 +0200"
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Date: Fri, 23 Jun 2017 10:24:59 -0400
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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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