Re: Improving count(*)

Gregory Maxwell wrote:
> On 11/21/05, Jim C. Nasby <jnasby@pervasive.com> wrote:
> > What about Greg Stark's idea of combining Simon's idea of storing
> > per-heap-block xmin/xmax with using that information in an index scan?
> > ISTM that's the best of everything that's been presented: it allows for
> > faster index scans without adding a lot of visibility overhead to the
> > index heap, and it also allows VACUUM to hit only pages that need
> > vacuuming. Presumably this could also be used as the on-disk backing for
> > the FSM, or it could potentially replace the FSM.
> 
> This should be a big win all around, especially now since in memory
> bitmaps make it more likely that some classes of queries will be pure
> index.  I still think it would be useful to have a estimated_count()
> which switches to whatever method is needed to get a reasonably
> accurate count quickly (stats when there are no wheres we can't
> predict, sampling otherwise if the involved tables are large, and a
> normal count in other cases.)

Added to TODO:
* Add estimated_count(*) to return an estimate of COUNT(*)  This would use the planner ANALYZE statistatics to return
anestimated  count.
 

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