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Re: Gsoc2012 idea, tablesample

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Gsoc2012 idea, tablesample
Date: 2012-04-17 20:29:52
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Lists: pgsql-hackers

* Josh Berkus (josh(at)agliodbs(dot)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.

My experience has been with running an algorithm on each record in the
data set to reduce the number of points on a given record (which ends up
reducing the size of the record, etc).  You could also filter out
records which wouldn't be visible due to the small 'size' of the record.
Again, neither of those would benefit from a TABLESAMPLE command.
Perhaps they're thinking it's going to use a GIST index to only pull out
records matching a certain condition..?  Except we have WHERE for that..



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