Re: Index-only scans

Hi Heikki,
I was recollecting our conversation that, if the index-only quals
were implemented through indexes by storing snapshots, broken data-types may
not be supported. I feel this problem might exist, if we go on design a
IOT(Index Organized Tables) / Clustered Indexes. IOT is again a index which
stores snapshot info together with it.
So are we not going to have this feature ever in PG? What is our
stand in that case? I am asking this because i have an IOT implementation on
PG 8.2.

Thanks,
Gokul

On Tue, Jul 14, 2009 at 2:08 PM, Heikki Linnakangas <
heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:

> Simon Riggs wrote:
> > On Mon, 2009-07-13 at 10:16 +0300, Heikki Linnakangas wrote:
> >
> >> Implementing index-only scans requires a few changes:
> >
> > I would like to see a clear exposition of the use cases and an an
> > analysis of the costs and benefits of doing this. It sounds cool, but I
> > want to know it is cool before we spend time solving all of the juicy
> > problems.
>
> BTW, there's another trick that I'm *not* going to implement yet, which
> is to allow joins using data from indexes only, and fetching the rest of
> the columns after the join. For example:
>
> CREATE TABLE a (aid integer PRIMARY KEY, adata text);
> CREATE TABLE b (bid integer PRIMARY KEY, bdata text);
>
> SELECT aid, adata, bid, bdata FROM a, b WHERE aid = bid;
>
> If the join is very selective, IOW there's only a few matching rows, it
> is a lot more efficient to perform the join first using just the
> indexes, and fetch adata and bdata columns and check visibility for the
> matching rows only.
>
> You can get pretty close by clustering the tables, but the wider the
> tables the bigger the difference.
>
> --
> Heikki Linnakangas
> EnterpriseDB http://www.enterprisedb.com
>
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