Re: Optimizer not using index on 120M row table

On Mon, Apr 07, 2003 at 11:49:27PM -0400, Neil Conway wrote:
> > On a related note, will pgsql do 'index covering', reading only the
> > index if it contains all the information a query needs?
>
> No -- in PostgreSQL, tuple visibility information is only stored in the
> heap, not in the index. So the heap tuple always needs to be examined,
> to determine if the specified tuple has been updated/deleted by some
> transaction.

'visibility information'? Is this related to the versioning that pgsql
uses instead of read locks? I know in other RDBMS's it's very useful to
have index covering because it eliminates needing to load the base pages
into memory at all, but I don't know pgqsl's layout well enough to know
how much this helps. Chapter 7 of the developers guide has info on page
files, but I haven't seen info on how index tuples are stored.
--
Jim C. Nasby (aka Decibel!) jim(at)nasby(dot)net
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