[postgis-users] 3D index in postgis

[a1001800]

Thanks all 
I got it




From: Paul Ramsey
Sent: 2008-05-21 06:04:05
To: PostGIS Users Discussion
CC: 
Subject: Re: [postgis-users] 3D index in postgis

Bah humbug, right you are :)

P

On Tue, May 20, 2008 at 12:31 PM, Kevin Neufeld
<kneufeld at refractions.net > wrote:
> Paul Ramsey wrote:
> >
> > On Tue, May 20, 2008 at 8:40 AM, Kevin Neufeld  <kneufeld at refractions.net >
> > wrote:
> > >
> > > a1001800 wrote:
> > > >
> > > > Thanks Kevin,
> > > >
> > > > It looks like ~= not invoke the index.
> > >
> > > That's why I suggested the && operator as well.  It does use the index.
> > >
> > > > Do we have a way to deal with an index with third value?
> > > > For example, point (x, y) and userid
> > > >
> > > > I need to do a query like "select point(x,y) from table where point in
> > > > rectangle and userid=xxx"
> > >
> > > Sure.  Add the userid=xxx to your filter list like you were doing.
> > > ie.
> > > SELECT ST_MakePoint(x,y)
> > > FROM mytable
> > > WHERE geom &&  <insert rectangle geom here >
> > > AND userid = xxx;
> > >
> > > You can additionally add an ST_Contains() filter if you need to have
> > > your points exactly inside the rectangle.
> >
> > No, don't do that :)  "point && rectangle" is logically the same as
> > "st_contains(rectangle, point)" and somewhat cheaper.
> >
>
> :) I disagree Paul.  These are not logically the same - they are very close,
> but not the same.  The bounding box coordinates are stored using 4 bytes
> instead of the 8 bytes used to hold the rectangle's actual coordinates.  The
> bbox is rounded up to guarantee that the rectangle is contained entirely
> within it.  So, you could have a point that is contained within the bounding
> box but not the rectangle.
>
> Consider,
>
> SELECT
>   a.poly && b.point AS bbox,
>   ST_Contains(a.poly, b.point)
> FROM
>  (SELECT 'POLYGON((0 0, 0 1.0000001, 1 1.0000001, 1 0, 0 0))'::geometry AS
> poly) a,
>  (SELECT 'POINT(0.5 1.00000011)'::geometry AS point) b;
>
>  bbox | st_contains
> ------+----------
>  t    | f
> (1 row)
>
> -- Kevin
>
> > The key is knowing for 100% sure that your polygonal geometry will
> > *always* be a rectangle. If there's a chance it won't, you need the
> > st_contains.
> >
> > P
>
>
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