[postgis-users] index-based inconsistencies

[Robert Hollingsworth]

using postgresql 8.3.4
using postgis 1.3.3
using geos 3.0.0
using proj 4.6.1

I'm encountering a couple of oddities which I'm pretty sure are related.

I have US Census Congressional districts modeled in PostGIS.

I've got a simple php program that uses ST_Within( ) to check a lat,lon against the congressional district polygons.  A small number of the districts are causing the query to return zero records incorrectly.

I've also got a simple php-mapscript with a LAYER to simply display all of these congressional districts.  When zoomed out around 1:400k and further out, these peculiar districts appear along side their neighbors.  When I zoom in closer, these same districts stop appearing.

I'm pretty sure this is a problem with the spatial indexes, which were created by shp2pgsql, because I converted the search program from ST_Within( ) to _ST_Within( ), which is advertised as skipping the indexing.  _ST_Within( ) is a bit slower, but finds these districts ok.

I've run VACUUM ANALYZE on the table, but this hasn't changed anything.

I haven't dug into the mapserver pg driver to see if there is any way for me to manipulate the generated query into skipping indexes.

I haven't yet dug into the records further to see if they have pathological bboxes or similar.

There are no holes in these polygons.

Anyone have any ideas on this?

thanks,
Robert H.
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