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Re: Zip Code Proximity search
Posted by: p dontthink
Date: June 25, 2006 09:32PM

@tri: You should tell people WHAT UNITS your query is in.

I just waded through this thread and a bunch of google search results and implemented a lookup where I am searching for things within a given radius. I coded it to do the full distance/radius calculation and comparison in MySQL *OR* to use the nifty idea from this thread of looking up a "sloppy rectangle" of results in MySQL and refining the results set in PHP... so I did a bit of testing:

My initial thought was that MySQL would be faster, but tests against a very small data set showed PHP to be a touch under twice as fast. Perhaps that will change drastically once I have a large dataset and a server under full load.

People's explanations here weren't too clear about constructing that rectangle, so here's something more concrete in PHP. Units used are miles, but you can change that easily enough by converting the 69.172 number to KM or whatever unit you want.

$sql = 'SELECT ... FROM items ... WHERE';

$distance_lookup_slop_miles = 5;

$delta_latitude = (($radius + $distance_lookup_slop_miles) / 69.172);
$lat1 = $latitude + $delta_latitude;
$lat2 = $latitude - $delta_latitude;
$delta_longitude = (($radius + $distance_lookup_slop_miles)
/ (cos($latitude) * 69.172));
$long1 = $longitude + $delta_longitude;
$long2 = $longitude - $delta_longitude;
$sql .= ' AND items.latitude <= ' . $lat1
. ' AND items.latitude >= ' . $lat2
. ' AND items.longitude <= ' . $long1
. ' AND items.longitude >= ' . $long2;

I also had to implemenet a SORT based on distance, which is independent of whether or not the lookup is based on distance. That means that unless I wanted to pull ALL rows out and then re-order them by distance (where distance is possibly NOT the first row in the ORDER BY clause - what a PHP nightmare THAT would be), then I have to include some of the distance math in the SQL. What I did for that was to simply trim down the math so that the "distance" it calculates is NOT the final real distance, but is just enough such that the sort will work. That looks something like this, again in PHP:

$sql = 'SELECT ... '
. ' ((items.latitude - ' . $latitude . ') * (items.latitude - '
. $latitude . ') + cos(items.latitude / 57.29578) * cos('
. $latitude . ' / 57.29578) * (items.longitude - '
. $longitude . ') * (items.longitude - ' . $longitude
. ')) as distance '
. ' FROM items ...'
. ' ORDER BY ..., distance, ...';

Just be careful not to use that "distance" number for anything but sorting. The full actual distance would look more like the following (this is what I used to compare the SQL and PHP-based lookups). As I said, it is slower than PHP under light load, but the resulting values are actually pretty accurate (again, in miles):

$sql = 'SELECT ... '
. ' (3958 * 3.1415926 * sqrt((items.latitude - ' . $latitude
. ') * (items.latitude - ' . $latitude
. ') + cos(items.latitude / 57.29578) * cos('
. $latitude . ' / 57.29578) * (items.longitude - '
. $longitude . ') * (items.longitude - ' . $longitude
. ')) / 180) as distance '
. ' FROM items ...'
. ' WHERE ...'
. ' HAVING distance <= ' . $radius;

Note here that you CANNOT use "distance" in the WHERE clause because column aliases are not allowed in the WHERE clause (see the manual). Thus, the HAVING clause.

Oh, and you should be asking me what calculation method I used in PHP, because that is important to know when you are making comparisons. I wrote a library that allows me to switch between Great Circle, Haversine, and Vincenty. I also added the ability to include a more accurate radial calculation that I found here:

http://wantingseed.com/sprout/2003/06/10/distance-between-two-points-on-earth/

Using that more accurate mean radius in the Great Circle or Haversine formulas got me results that were really darn close to the Vincenty results, but still noticably faster (OK, even thousands of nanoseconds is not noticable to a human...). So I settled on a fast but reasonably accurate (by using the better radial value) Great Circle calculation.

HTH

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