Re: [HACKERS] row reuse while UPDATE and vacuum analyze problem

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: The Hermit Hacker <scrappy(at)hub(dot)org>
Cc: Oleg Bartunov <oleg(at)sai(dot)msu(dot)su>, pgsql-hackers(at)postgreSQL(dot)org, tgl(at)sss(dot)pgh(dot)pa(dot)us, vadim(at)krs(dot)ru
Subject: Re: [HACKERS] row reuse while UPDATE and vacuum analyze problem
Date: 1999-07-28 15:45:23
Message-ID: 199907281545.LAA11113@candle.pha.pa.us
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> On Wed, 28 Jul 1999, Oleg Bartunov wrote:
>
> > How update performance could be increased if:
> > 1. 'vacuum analyze' will analyze index file
> > 2. reuse row instead of inserting
>
> Just to clarify, 'reuse row' won't replace inserting (to the best of my
> knowledge), only reduce space wastage between vacuum's. Especially, again
> TTBOMK, with MVCC, where each "instance" of a row is serialized.
>
> Actually, there is a tought...if I understand the concept of MVCC, how is
> reusing a row going to work? My understanding is that I can "physically"
> have to copies of a row in a table, one newer then the other. So, if
> someone is running a SELECT while I'm doing an UPDATE, their SELECT will
> take the older version of hte row (the row at the time their SELECT
> started)...depending on how busy that table is, there will have to be some
> sort of mechanism for determining how 'stale' a row is, no?
>
> ie. on a *very* large table, with multiple SELECT/UPDATEs happening?

You would have to leave referenced rows alone. I think Vadim has this
covered.

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