PostGIS

PostGIS adds support for geographic objects to the PostgreSQL database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as spatial database for geographic information systems.

PostGIS follows the OpenGIS Simple Features Specification for SQL (SFSQL).

Geometry Objects

PostGIS supports the range of geometry objects:

Geometry Object	WKT Example
Point	POINT(0 0)
LineString	LINESTRING(0 0,1 1,1 2)
Polygon	POLYGON((0 0,4 0,4 4,0 4,0 0),(1 1, 2 1, 2 2, 1 2,1 1))
MultiPoint	MULTIPOINT(0 0,1 2)
MultiLineString	MULTILINESTRING((0 0,1 1,1 2),(2 3,3 2,5 4))
MiltiPolygon	MULTIPOLYGON(((0 0,4 0,4 4,0 4,0 0),(1 1,2 1,2 2,1 2,1 1)), ((-1 -1,-1 -2,-2 -2,-2 -1,-1 -1)))
GeometryCollection	GEOMETRYCOLLECTION(POINT(2 3),LINESTRING((2 3,3 4)))

These geometry objects are in accordance with the SFSQL specification mentioned above; the examples are given using the "Well Known Text" format - the same format used for selections in uDig (see Common Query Language )

Required PostGIS Tables

When you install the PostGIS extension into your database two tables will be created:

geometry_columns - records what spatial data is stored in which table; along with some information about the kind of geometry object and what SRID is expected
spatial_ref_sys - records definitions of spatial reference systems according to SRID

For uDig to recognize your database you will need to have at least one entry in your geometry_columns table.

PostGIS page
http://postgis.refractions.net/
http://www.postgresql.com/
Introduction to PostGIS (FOSS4G 2007 workshop)