Spatial Databases - Desktop GIS: Mapping the Planet with Open Source Tools - page 93

The geometry_columns Table
The geometry_columns table describes the spatially enabled tables in
your database. Many application programs rely on the records in geom-
etry_columns to determine which tables are spatial tables. This table, as
well as spatial_ref_sys , is described in the OpenGIS Simple Features Spec-
ification for SQL. 7 We can view the structure of the geometry_columns
table using the \d command in the psql interactive terminal:

desktop_data= # \d geometry_columns
Table "public.geometry_columns"
Column | Type | Modifiers
-------------------+------------------------+-----------
f_table_catalog | character varying(256) | not null
f_table_schema | character varying(256) | not null
f_table_name | character varying(256) | not null
f_geometry_column | character varying(256) | not null
coord_dimension | integer | not null

srid | integer | not null

type | character varying(30) | not null

Indexes:
"geometry_columns_pk" PRIMARY KEY, btree (f_table_catalog, f_table_schema,
f_table_name, f_geometry_column)

The first three columns provide a fully qualified name for a table. Some
databases may use “catalog,” and since the geometry_columns table is a
standard, it is included in every OGC-compliant implementation. Post-
greSQL doesn't use the concept of a “catalog,” so this column will
always be blank. By default, all tables in PostgreSQL are placed in
the “public” schema. So for a PostGIS-enabled table, we will find the
f_table_schema and f_table_name populated with “public” and the table
name, respectively.

The f_geometry_column contains the name of the column in your spa-
tial table that contains the geometry. The coord_dimension contains the
dimension of the features (2, 3, or 4). The srid is the spatial reference ID
column and contains a number that is related to the srid column in the
spatial_ref_sys table. This defines the coordinate system for the table. The
type field contains information about the feature type contained in the
table and contains a keyword such as POINT, LINESTRING, POLYGON,
and the MULTI forms of each feature.

With this information, a client application (in other words, your desktop
GIS) can quickly determine which spatially enabled tables are available

7. http://www.opengeospatial.org