From: | Hannu Krosing <hannuk(at)google(dot)com> |
---|---|
To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, Peter Geoghegan <pg(at)bowt(dot)ie>, Andres Freund <andres(at)anarazel(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Jeff Davis <pgsql(at)j-davis(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: MaxOffsetNumber for Table AMs |
Date: | 2021-05-06 01:32:28 |
Message-ID: | CAMT0RQRLO3+37niz_pU9uOi+CqVi0HzFgLuWdLzs_Bffp8KWhw@mail.gmail.com |
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On Thu, May 6, 2021 at 3:07 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Wed, May 5, 2021 at 3:43 PM Matthias van de Meent
> > Storage gains for index-oriented tables can become as large as the
> > size of the primary key by not having to store all primary key values
> > in both the index and the table; which can thus be around 100% of a
> > table in the least efficient cases of having a PK over all columns.
> >
> > Yes, this might be indeed only a 'small gain' for access latency, but
> > not needing to store another copy of your data (and keeping it in
> > cache, etc.) is a significant win in my book.
>
> This is a really good point. Also, if the table is ordered by a
> synthetic logical TID, range scans on the primary key will be less
> efficient than if the primary key is itself the TID. We have the
> ability to CLUSTER on an index for good reasons, and "Automatically
> maintain clustering on a table" has been on the todo list forever.
> It's hard to imagine this will ever be achieved with the current heap,
> though: the way to get there is to have a table AM for which this is
> an explicit goal.
But would this not have the downside that all the secondary indexes
will blow up as they now need to have the full table row as the TID ?
-----
Hannu Krosing
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