[postgis-users] relationship functions not working well

[William Kyngesburye]

On Dec 5, 2008, at 5:19 PM, Obe, Regina wrote:
> What does ST_Intersects + ST_Relate give you and timing.
>
> That's the one I was interested in if that is faster than
>
> && + ST_Relate
>
> In theory those 2 should give you the same answer.
>
>
Yes, I get the same features returned, but it's slower, as I expected:

large area: 15448 polys, 7924.840 ms

About as much slower, but a little better, as the difference between  
intersects time and && time - I took some more times to get an  
average, and the intersect time was a little faster than my first  
timing:

&& average: 380 ms

intersect average: 1100 ms

&& + relate average: 7200 ms

intersect + relate average: 7900 ms

>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net on behalf of  
> William Kyngesburye
> Sent: Fri 12/5/2008 5:34 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] relationship functions not working well
>
>
>  From what I read in the docs, Intersects first checks the bbox, then
> does the full intersect test.  While && just does a bbox test.
>
> && + st_relate gives me what I want, && only gets the touch-only
> neighboring polys I don't want (and seems to be more rigorous in that
> respect than intersects).
>
> Some times - the full database has 625396 polygons (that's all the
> SWBD files).
>
> * a 1 degree selection (an island tile)
>
> st_intersects only:  216 polys, 494.855 ms
>
> && only:             219 polys,  12.765 ms
>
> && plus st_relate:   207 polys, 189.360 ms
>
> * a 58x13 degree rectangle (many tiles empty - alaska region)
>
> st_intersects only:  15469 polys, 1460.344 ms
>
> && only:             15479 polys,  379.317 ms
>
> && plus st_relate:   15448 polys, 7217.592 ms
>
>
> This agrees with what I said about && doing a bbox test only.  And
> verifies my guess that larger areas would get real slow.
>

-----
William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
http://www.kyngchaos.com/

All generalizations are dangerous, even this one.