[postgis-devel] [PostGIS] #701: Support for 9.1 KNN GIST enhancements
PostGIS
trac at osgeo.org
Thu Feb 17 12:57:49 PST 2011
#701: Support for 9.1 KNN GIST enhancements
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Reporter: robe | Owner: pramsey
Type: enhancement | Status: new
Priority: medium | Milestone: PostGIS 2.0.0
Component: postgis | Version: trunk
Keywords: |
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Comment(by nicklas):
Hmm, If I read this part of the conversation right:
http://article.gmane.org/gmane.comp.db.postgresql.performance/28905
... it is as I have feared. There seems to be no way to use it for
anything but points.
If we use it against anything else (except a few exceptions) we will need
to do a recheck to see if we really have found the closest geometry and
not only the closest bounding box. But what we can get from the KNN gist
index is an ordered list. The number of items in the list we have to tell
beforehand. We can not come back from our recheck and say, We have reasons
to believe that there still might be some candidate out there for being
closest, give us some more.
To decide if we really know if we have the closest is no problem if we
have found a real mindistance (between the actual geometries, not the
bboxes) that is closer than the next bbox in the ordered list.
The problem is if the last bbox is closer than the rechecked closest, the
we don't know if the next one (outside our limit) in the ordered list is a
candidate.
Does it make sense.
I think that is very bad news, but maybe there is some way to find a
workaround.
/Nicklas
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/701#comment:3>
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