Re: index-only scans

On Thu, Aug 11, 2011 at 5:39 PM, Cédric Villemain
<cedric(dot)villemain(dot)debian(at)gmail(dot)com> wrote:
> 2011/8/11 Robert Haas <robertmhaas(at)gmail(dot)com>:
>> Please find attached a patch implementing a basic version of
>> index-only scans.  This patch is the work of my colleague Ibrar Ahmed
>> and myself, and also incorporates some code from previous patches
>> posted by Heikki Linnakanagas.
>
> Great!

Glad you like it...!

> Can this faux heap tuple be appended by the data from another index
> once it has been created ? ( if the query involves those 2 index)

I don't see how to make that work. In general, a query like "SELECT
a, b FROM foo WHERE a = 1 AND b = 1" can only use both indexes if we
use a bitmap index scan on each followed by a bitmapand and then a
bitmap heap scan. However, this patch only touches the index-scan
path, which only knows how to use one index for any given query.

Actually, I can see a possible way to allow an index-only type
optimization to be used for bitmap scans. As you scan the index, any
tuples that can be handled index-only get returned immediately; the
rest are thrown into a bitmap. Once you're done examining the index,
you then do a bitmap heap scan to get the tuples that couldn't be
handled index-only. This seems like it might be our best hope for a
"fast count(*)" type optimization, especially if you could combine it
with some method of scanning the index in physical order rather than
logical order.

But even that trick would only work with a single index. I'm not sure
there's any good way to assemble pieces of tuples from two different
indexes, at least not efficiently.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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