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## gist indexes for distance calculations

### Responses

### pgsql-performance by date

Hi everyone. I have a question, and it's well beyond me to even speculate about the inner workings of postgresql on this. I have a "places" table, and a "coordinates" column, of type POINT. If I want to find every place within, for example, a distance of 1 unit from an arbitrary point, I'll do: CREATE INDEX ON places USING gist (circle(coordinates, 1)); And then I'll fetch the desired rows like this: SELECT * FROM places WHERE circle(coordinates, 1) @> circle('(a,b)', 0); (where (a,b) is an arbitrary point) I'd like to know how this index works, though, as it seems to me the only way to have this kind of index to work is to calculate the distance of every point in a square of sides 2*1=2 units centered on (a, b). So, am I wrong to think it works like that? If it does work like that, could I have instead two columns of type FLOAT (xcoordinate and ycoordinate) and create traditional b-tree indexes on both of these, and then do something like: SELECT * FROM places WHERE xcoordinate >= (a-1) AND xcoordinate <= (a+1) AND ycoordinate >= (b-1) AND ycoordinate <= (b+1) And SQRT(POW(a-xcoordinate,2)+POW(b-ycoordinate,2))<=1; If you can also pinpoint me to where I can find this sort of information (index utilization and planning, performance tuning), I'd be very grateful. Thank you already, Marcelo Zabani.

- Re: gist indexes for distance calculations at 2010-09-30 19:16:36 from Tom Lane
- Re: gist indexes for distance calculations at 2010-10-01 02:50:14 from Marcelo Zabani
- Re: gist indexes for distance calculations at 2010-10-01 05:56:18 from Jesper Krogh
- Re: gist indexes for distance calculations at 2010-10-01 16:04:19 from Merlin Moncure

Next: From:Tom LaneDate:2010-09-30 19:16:36Subject: Re: gist indexes for distance calculationsPrevious: From: Willy-Bas LoosDate: 2010-09-30 16:41:55Subject: Re: turn off caching for performance test