"fuzzy index" sounds like a more apt phrase than "lossy index" given Steve's description.
On 9/9/07, Ed McNierney <[EMAIL PROTECTED]> wrote: > Crystal - > > Steve's description is very good; while the GiST index is extremely helpful, > it is not "deterministic" (I'm sure there's a fancy database term that's more > accurate) like other indexes. What's being indexed is NOT the geometry > itself, but the bounding box of the geometry, so it's not the same as > indexing the actual data like you would with a text or numeric field. > > For all cases there the actual geometry of object A intersects the actual > geometry of B, their bounding boxes will also intersect, so the query (A && > B) will all objects that actually intersect. It will also, however, return > objects which do NOT intersect themselves, but whose bounding boxes intersect > - think of two diagonal parallel line segments. The test INTERSECTS(A, B) is > a much more time-consuming operation than the VERY fast (A && B) test, so it > is much faster to use (A && B) followed by INTERSECTS(A, B) in that order. > The first test will be executed first and will discard all objects whose > bounding boxes do not intersect. This is a "trivial rejection" test, quickly > tossing out the objects that can't possibly intersect. The more complex > INTERSECTS(A, B) test is then performed only on the subset of objects that > passes the first test, and will toss out any false positives where the > bounding boxes intersect but the objects themselves do not. > > - Ed > > Ed McNierney > Chief Mapmaker > Demand Media / TopoZone.com > 73 Princeton Street, Suite 305 > North Chelmsford, MA 01863 > Phone: 978-251-4242, Fax: 978-251-1396 > [EMAIL PROTECTED] > > > > -----Original Message----- > From: UMN MapServer Users List [mailto:[EMAIL PROTECTED] On Behalf Of Stephen > Woodbridge > Sent: Sunday, September 09, 2007 2:36 PM > To: [email protected] > Subject: Re: [UMN_MAPSERVER-USERS] lossy index > > Crystal Li wrote: > > Hi All, > > > > Would any one explain lossy indexes for me? in PostGIS Manual it talked > > about GiST indexes are assumed to be lossy.Thanks. > > > > Crystal > > I believe that this relates to the fact the gist indexes are based on > the bbox of the geometry, so that they loose the precise nature of the > geometry in favor of the faster bbox representation. > > So the use of gist indexes is a && b means that a might interact with b > and a moer precide test is needed to confirm that. So you see typical > queries doing: > > where a && b and intersects(a,b) > > HTH, > -Steve > -- Puneet Kishor http://punkish.eidesis.org/ Nelson Institute for Environmental Studies http://www.nelson.wisc.edu/ Open Source Geospatial Foundation (OSGeo) http://www.osgeo.org/ Summer 2007 S&T Policy Fellow, The National Academies http://www.nas.edu/
