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Building

The Makefiles have been edited to configure and build against libgeos, libproj, and libxml2 under ext/<arch>/. So long as
those dependencies are supported for a particular choice of <arch>, a simple `make` and `make install` should suffice.

Caveats - Greenplum v. Postgres

1. Analyze

Greenplum does not support ANALYZE functions for user-defined data types.

CREATE TYPE geometry (
    internallength = variable,
    input = ST_geometry_in,
    output = ST_geometry_out,
    send = ST_geometry_send,
    receive = ST_geometry_recv,
    delimiter = ':',
    analyze = ST_geometry_analyze,        <<< We don't support this.
    storage = main
);

ST_geometry_analyze, above, codes to the following API from PostgreSQL 8.2:

static void compute_geometry_stats(VacAttrStats *stats, AnalyzeAttrFetchFunc fetchfunc, int samplerows, double totalrows)

This difference will probably present itself in a number of areas that rely on certain pg_statistic behavior. I've
only seen one thus far: "select estimated_extent(<table>, <column>)" will cause a segfault at lwgeom_estimate.c:1470.
It assumes that ANALYZE would have been invoked and that pg_statistics would have been populated with the needed values
for the given <column>.

2. Triggers

If a Greenplum trigger invokes a user-defined function on a segment, said function cannot execute a SQL query (unless
it only accesses a pg_catalog relation.)

PostGIS relies on triggers for long transaction support; it seeks to expose the following functionality:

"""
-- place the lock
SELECT LockRow('test_locks', '1', 'auth1', now()::timestamp+'00:01');
SELECT LockRow('test_locks', '2', 'auth2', now()::timestamp+'00:01');
-- this should fail due to missing auth
UPDATE test_locks SET state = 'unauthorized' where id = 1;
BEGIN;
    -- Add authorization for row 1
    SELECT AddAuth('auth1');
    -- we're authorized for row 1
    UPDATE test_locks SET state = 'authorized' where id = 1;
END;
"""

LockRow will record the table, row id, and pseudo-authtoken in its meta-table, public.authorization_table. Futhermore,
it tries to place a trigger on that particular row, such that when that row is modified, a PostGIS function can consult
public.authorization_table to determine whether access is permitted. At this point, Greenplum throws errors citing that
the function cannot access the relation, authorization_table.

Since this is a row-level mechanism, in theory, there could be a way to keep each segments' auth checks on that 
segment itself, rather than accessing SQL on a cluster-level scope.

Note: Long transaction support is needed for the OpenGIS Web Feature Service specification.

3. Type modifiers

Greenplum don't not support type modifier for user defined types, the work around is
to use AddGeometryColumn for geometry and by pass it for geography type.

For example, following SQL won't work:

CREATE TABLE geometries(id INTEGER, geom geometry(LINESTRING));

Use below SQL instead:

CREATE TABLE geometries(id INTEGER);
SELECT AddGeometryColumn('public', 'geometries', 'geom', 0, 'LINESTRING', 2);