On Fri, Mar 03, 2006 at 04:14:41PM +0100, Csaba Nagy wrote:
> > Ewe. How expensive is scanning an index compared to the heap? Does
> > anyone have figure on that in terms of I/O and time?
>
> See this post for an example:
> http://archives.postgresql.org/pgsql-performance/2006-02/msg00416.php
>
> For my 200 million table, scanning the pk index took ~ 4 hours. And then
> there are some more indexes...
>
> So if the index has to be scanned completely, that's still too much.
But how does a scan of the index compare to a scan of the table? For
example, if indexes are 1/5th the size of the table, you can
(theoretically) scan 5 indexes in the same amount of time it takes to
scan the heap. That indicates to me that even if we did have to scan all
indexes, a dirty page bitmap would still be a win over the current
situation. But it appears that it should be safe to do index lookups on
indexes that aren't expressions. And I believe that we could take steps
down the road to allow for index lookups on indexes that only used
functions that were known to be safe.
--
Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com
Pervasive Software http://pervasive.com work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461