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Can anyone tell me what is the purpose of geography_columns view and geometry_columns view in a PostGIS database?

My geography_columns view is empty so I can't see what the purpose of it is.

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geometry_columns is part of the Simple Features specification for OGC and ISO 19125-2:2004 Standards compliance. geography_columns is a non-Standard variation to support the geography type.

From http://postgis.net:

PostGIS follows the Open Geospatial Consortium’s “Simple Features for SQL Specification”

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Those views (tables in older versions) is there to help client software fast understand what it can expect of the datasets. Most of the information in those views is also in each geometry, like srid, number of dimensions and so on. But it would take a very long time for a client program to scan all the geometries to find that out.

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