From: | "Dmitry Koterov" <dmitry(at)koterov(dot)ru> |
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To: | "David Rowley" <dgrowley(at)gmail(dot)com> |
Cc: | "Postgres General" <pgsql-general(at)postgresql(dot)org> |
Subject: | Re: Is index enough to do simple fetch, or table is always used too? |
Date: | 2008-12-14 20:35:46 |
Message-ID: | d7df81620812141235h49fae33do951748f86195fd37@mail.gmail.com |
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Lists: | pgsql-general |
On Sun, Dec 14, 2008 at 3:36 PM, David Rowley <dgrowley(at)gmail(dot)com> wrote:
> 2008/12/14 Dmitry Koterov <dmitry(at)koterov(dot)ru>:
> > The question: if the table "tbl" scanned to fetch "id" and calculate
> > md5(id), or the value of "id" is brought directly from "idx" index with
> no
> > table data access at all? The second behaviour is logical: why should we
> > access the table if all the needed data is already in the index entry...
>
> In fact not all the required information is in the index. Postgresql
> uses Multi-version-concurrency-control, which means there may be multi
> versions of the same row. Postgresql must hit the heap (table) no get
> the visibility information.
>
But isn't an index data is also multi-version?
If no, how could a single-versioned index be used to fetch the data from a
past snapshot?
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