Re: Gsoc2012 idea, tablesample

On Tue, Apr 17, 2012 at 04:29:52PM -0400, Stephen Frost wrote:
> Josh,
> 
> * Josh Berkus (josh@agliodbs.com) wrote:
> > FWIW, the PostGIS folks would *really* love to have a TABLESAMPLE which
> > worked with geographic indexes.  This would be tremendously useful for
> > constructing low-resolution "zoom out" tiles on maps and similar.
> 
> I'm familiar with the concept of 'zoom out' tiles and PostGIS, but I
> don't actually see the connection between that and TABLESAMPLE.  Perhaps
> I'm missing something, but I've never seen a case where you create 'zoom
> out' tiles by just grabbing some portion of the data at random, as that
> would end up creating empty spots or missing pieces.

Actually a random sample would really be representative of the data
distribution. What the type analyzer gets is a sample and that sample
is what the estimator looks at to answer the question:
How many rows fall in this rectangle ?

You can see how well it works by passing your queries using && operator
to "EXPLAIN ANALYZE" and compare estimated/real.

I'm looking for a way to fetch random samples these days so I confirm
the need for a quick way to fetch the same sample that "analyze"
command fetches but at SQL level.

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