postgis/README.postgis

PostGIS - Geographic Information Systems Extensions to PostgreSQL
~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

VERSION: 0.5 (2001/07/20)

MORE INFORMATION: http://postgis.refractions.net

INTRODUCTION:
This distribution contains a module which implements GIS simple features, ties 
the features to rtree indexing, and provides some basic functions for accessing 
and analyzing geographic data.

Directory structure:

	./      Core source code, makefiles and install directions.
	./jdbc  Extensions to the PostgreSQL JDBC drivers to support
	        the GIS objects. 
	./doc   Documentation on the code, objects and functions 
	        provided. 
	./loader  A program to convert ESRI Shape files into SQL text
	        suitable for uploading into a PostGIS/PostgreSQL database.
	./examples  Small programs which demonstrate ways of accessing 
	        GIS data.
	        

INSTALLATION:

To install the module, move this directory to the "contrib" directory of your
PostgreSQL source installation. Alternately, edit the "top_buildir" in the 
Makefile and point it at your PostgreSQL source tree. You must have a 
PostgreSQL source tree, and you must have run succesfully built and installed
it for this to work.

Then run:
	
	make
	make install
	
Finally, load the function and object definitions into a database with psql 
(you must run this as a database user with system privledges):

	psql -f postgis.sql -d yourdatabase

Installation should be complete.

To run some regression tests, use 'make test'. The results will be output
to tests/output.log and compared to tests/expected.log.


USAGE:

Try the following example SQL statements:

CREATE TABLE geom_test ( gid int4, geom geometry, name varchar(25) );
INSERT INTO geom_test ( gid, geom, name ) 
  VALUES ( 1, 'POLYGON((0 0 0,0 5 0,5 5 0,5 0 0,0 0 0))', '3D Square');
INSERT INTO geom_test ( gid, geom, name ) 
  VALUES ( 2, 'LINESTRING(1 1 1,5 5 5,7 7 5)', '3D Line' );
INSERT INTO geom_test ( gid, geom, name )
  VALUES ( 3, 'MULTIPOINT(3 4,8 9)', '2D Aggregate Point' );
SELECT * from geom_test WHERE geom && 'BOX3D(2 2 0,3 3 0)'::box3d;

RTREE vs GIST:

PostgreSQL provides support for rtree and GiST index schemes. The rtree scheme 
is marginally faster, but does not support the indexing of objects greater 
than 8K in size. Unfortunately, GIS objects get larger than 8K moderately
often.  The GiST scheme offers indexing even on large objects, using a system
of "lossy" indexing where a large object is proxied by a smaller one in the 
index.  In the case of the PostGIS indexing system, all objects are proxied
in the index by their bounding boxes.

PostGIS fully supports GiST indexing, because it is the most widely useful for 
GIS indexing. RTree indexing is partially supported, and may be discontinued
in the future. We recommend you use GiST indexes.

You can build a GiST index with:

 CREATE INDEX <indexname> ON <tablename>  
     USING gist ( <geometryfield> gist_geometry_ops ) WITH ( islossy );

You can build an rtree index with:

 CREATE INDEX <indexname> ON <tablename> 
    USING rtree ( <geometryfield> rt_geometry_ops );

Note that PostgreSQL does not use the GiST or rtree indexes when performing 
searches unless *forced* to do so. This is not good behavior, but it seems
to stem from the fact that the developers are optimizing the planner while 
looking at btree indexing schemes. To force the system to use your spacial
indexes, use the command:

 SET ENABLE_SEQSCAN = OFF

Try doing an EXPLAIN on your query before and after the 'enable_seqscan'
command to see the different query plans.