[postgis] SET ENABLE_SEQSCAN = OFF

[Frank Warmerdam]

Folks, 

I would like to know a little more about the effects of setting ENABLE_SEQSCAN
to OFF.  It seems like I do need to do this to have the GiST spatial index
kick in.  (It takes a query on 14lines out of 12000 VMAP0 coastlines for a 
small area from about 9s to a fraction of a second!)

I had originally though this was something I would do once at the same
time I applied the PostGIS types to a database, but I see it is specific
to a particular connection to the database, not persistent. 

The documentation warns that leaving this set to OFF can cause sub-optimal
performance for a variety of other query types.  How serious is this? 

Is there anyway we can force spatial indexes to be used but not affect
decisions on other index types?  This whole thing seems to make use of
spatial indexes substantially more problematic. 

However, the good news is that I have modified OGR to pass attribute
and spatial queries through to PostGIS, and the performance is great 
when selecting a small area out of a large dataset. 

Best regards,

---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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