Comments on: points in an area in mysql

By: Strom

Strom — Sun, 31 Aug 2008 16:26:58 +0000

I use this solution:
CREATE TABLE points (y INT , x INT, y INT)..

id = y * (yMax) + x;

SELECT FROM points WHERE id IN (xxx);

wery fast …

By: judy

judy — Tue, 03 Jun 2008 20:46:57 +0000

s prazdnikov vasl

By: Markku Uttula

Markku Uttula — Tue, 08 Apr 2008 02:27:07 +0000

In case it isn’t clear to others, calculating a square root is a rather “expensive” operation for the database engine – not to mention that in this case it is entirely unnecessary. If we wanted to know “how far” each point is from our reference point, the calculation would be necessary. However, since we only want to find out if it is “nearer than” the given radius, we can actually factor out the calculation of distance, and based on Pythagoras’s theorem decide that if “x to the second power”+”y to the second power” is less than “radius to the second power”, the point is in fact within the circle we’re interested in.

Damn I wish I’d paid attention in my maths classes while in school. These tidbits come in handy from time to time, and I really hope I’d have managed to get more of them in my head when they were given for free

By: Kae Verens

Kae Verens — Mon, 07 Apr 2008 20:36:14 +0000

.226 – nice spot! I should have noticed that optimisation as well. That’s what I get for testing at 3am. Almost down to 50% of the time for the unoptimised version.

I have a little work to do now, but I’ll try the center square idea (the last one I mentioned) when I’m finished, to see if it offers any further optimisation. My guess is that it will only shave a few ms off it.

By: Markku Uttula

Markku Uttula — Mon, 07 Apr 2008 19:35:53 +0000

I’d be interested in knowing how (on your system) would this work:

time echo “SELECT COUNT(x) FROM (SELECT x,y FROM points WHERE ABS(x)<=100000 AND ABS(y)<=100000) AS sub1 WHERE (x*x+y*y)<(100000*100000)” | mysql -uusername -ppassword geodb