[postgis-users] What's the Purpose of the Geometry_Columns Table?

Stephen Frost sfrost at snowman.net
Tue Sep 5 08:38:15 PDT 2006

* TECHER David (davidtecher at yahoo.fr) wrote:
> Stephen Frost a écrit :
> >At first glance it looks like the geometry_columns could be based off
> >the pg_class, pg_attribute, and pg_type system tables by just looking
> >for tables which have columns of type 'geometry'.  I see that
> >geometry_columns also contains the number of dimensions, the srid, and
> >the type.  Are all three needed? 
> >
> Yes they are! Make a error with mapserver from a postgis connection and
> U will see why U need this!
> Make a postgis connection with the same data but with different srids!
> and so on...
> There are lost of examples to see why we need these infos!

Alright, that was less than useful...  I realize the information is
needed, I was asking if all three were required or if some of them could
be derived from other information (ie: could the number of dimensions be
derived from the 'TYPE'?  Or from the SRID?).

Having looked through the Postgres system tables a bit more though, why
aren't we just using 'atttypmod'?  Is is possible for custom types to
use that (I would have thought so...)?  It should be possible to encode
the dimensions, srid and type so that all the funny functions for adding
geometry columns could go away, along with the check constraints


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