Re: GiST index question: performance

Thanks Oleg - very interesting stuff you are working on.

You may recall I exchanged emails with you on openfts a little while
ago - my ISP that manages my Pg SQL server is (in my interests)
concerned about installing anything non-standard (read: unstable) onto
their server. I was able to get them to install your TSearch2 b/c it's
been proven many times, but I'm hesitant to even bring up Q3C since
it's less widely deployed.

The search method I proposed in my first email is not totally accurate
but just searching circles with radii using a GiST index and standard
Pg circle datatypes seems like a "close enough" solution for me (as
opposed to Q3C's conical search intersections with a spherical
projection). I realize that at higher latitudes my circles will be
elliptical but our needs are for approximations that are very fast
rather than accurate and the radii being searched are small relative to
the size of the sphere (I.e. when searching Nome, find everything in
+/- 40 miles and especially don't return Anchorage POI)..

It's an end user database, so if the query takes 500ms, that's really
too long. On the Q3C site, I see that your measure of speed is
processing many, many rows in 20 hours, which is a whole different
ballgame. :)

Do you have a thought as to whether GiST is going to be faster/more
efficient with Pg standard types of polygons or circles? I suppose I
should just test out both, and quit wasting your time. I'll certainly
repost to the list with whatever I uncover.

I really do appreciate the help you've provided.

Sincerely,

Steve

At 12:21 PM 3/5/2007, you wrote:
>On Mon, 5 Mar 2007, Steve Midgley wrote:
>
>>Hi,
>>
>>First off, can I say how much I love GiST? It's already solved a few
>>problems for me that seemed impossible to solve in real-time queries.
>>Thanks to everyone who works on that project!
>
>Thanks, Steve !
>
>>
>>I'm developing a geographic index based on a set of zip code
>>boundaries. Points of interest (POI) will fall within some boundaries
>>and not others. I need to search to find which POI are within a
>>specified boundary.
>
>You POI is what we call ConeSearch query in astronomy.
>Please, take a look on Q3C algorithm available from http://q3c.sf.net.
>Some information http://www.sai.msu.su/~megera/wiki/SkyPixelization
>
>This is what we use in our Virtual Observatory project and we're able
>to
>work with 10^9 objects on moderate hardware. It doesn't use GiST but
>special pixelization scheme allow to use standard Btree.
>
>>
>>I think have two options (see below) and I'm wondering if anyone has
>>an opinion or experience as to whether one or the other will have
>>substantially different performance characteristics. I can obviously
>>test when I get that far, but I'd prefer to try the anticipated
>>faster route first, if anyone has existing experience they can share:
>>
>>1) Index a series of circles of NN radius around each boundary marker
>>(lat/long point). Run a search on POI for those that fall within any
>>of the specified circles.
>>
>>2) Index a set of polygons that mark the "minimum area" around the
>>boundary markers in question. Run a search on POI that fall within
>>this single polygon.
>>
>>The polygon will have more points, but there will be more circles to
>>search - my understanding of GiST is limited so I'm not sure if
>>there's a performance benefit to searching many circles or a few
>>polygons.
>>
>>My tables are of this size:
>>
>># of POI: 50,000
>># of zip blocks (with and without regions): 217,000
>># of zip blocks in a given city (and hence in a given polygon): ~5
>>
>>Any thoughts or ideas?
>>
>>Thank you,
>>
>>Steve
>>
>>p.s. I could use a GIS system alongside of Postgres but performance
>>and efficiency are key to this system, and it seems to me that raw
>>GiST indexed SQL queries are going to be fastest and create the
>>lowest load on the server?
>>
>>
>>---------------------------(end of
>>broadcast)---------------------------
>>TIP 7: You can help support the PostgreSQL project by donating at
>>
>> http://www.postgresql.org/about/donate
>
> Regards,
> Oleg
>_____________________________________________________________
>Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
>Sternberg Astronomical Institute, Moscow University, Russia
>Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
>phone: +007(495)939-16-83, +007(495)939-23-83
>
>