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PostGIS - Geographic Information Systems Extensions to PostgreSQL ~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ VERSION: 0.2 (2001/06/19) 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. ./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.