Use of data within indexes - Mailing list pgsql-performance

From Jacques Caron
Subject Use of data within indexes
Date
Msg-id 6.2.0.14.0.20050414113816.051f7e68@pop.interactivemediafactory.net
Whole thread Raw
Responses Re: Use of data within indexes  (Christopher Kings-Lynne <chriskl@familyhealth.com.au>)
List pgsql-performance
Hi,

Just wondering... Is Postgresql able to use data present within indexes
without looking up the table data?

To be more explicit, let's say I have table with two fields a and b. If I
have an index on (a,b) and I do a request like "SELECT b FROM table WHERE
a=x", will Postgresql use only the index, or will it need to also read the
table page for that (those) row(s)?

There might be a reason why this is not possible (I don't know if the
indexes have all necessary transaction ID information?) but otherwise this
could possibly provide an interesting performance gain for some operations,
in particular with some types of joins. Or maybe it already does it.

Any hint welcome!

Thanks,

Jacques.



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