On 04/06/2010, at 6:20 AM, angus wrote:
The advantage of postgis is you don't query every record in your db
and calc the distance. You use one of those handy db tricks called
indexes.
It's not quite as bad as "every record in the db". You "square the
circle"
by multiplying your radius by 1.4, then query every record in a vertical
strip and a horizontal stripe of double that width, and join those. At
worst, that's a full index scan, but will often instead be two (very)
partial
scans and an in-memory join, like a hash-join. Not as good as using a
proper spatial index, but not too bad.
There are a lot of other postGIS query types for which there aren't
such good one-dimensional solutions however.
Clifford Heath.
Angus
On Jun 3, 4:31 pm, Nathan de Vries <nat...@atnan.com> wrote:
On 03/06/2010, at 1:45 PM, Keith Pitt wrote:
We wrote a plpgsql function that made it easier for us to find
locations within a certain radius.
This functionality is available via the PostgreSQL "earthdistance"
module. Your "km_between_lat_long" function can be replaced with:
earth_distance(ll_to_earth(lat1, lon1), ll_to_earth(lat2, lon2))
I'm unsure whether that would be faster or more accurate than what
you're doing, so you'd need to test it.
Cheers,
Nathan
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