OGR is the vector half of the GDAL spatial data access library. It allows access to a large number of GIS data formats using a simple C API for data reading and writing. Since OGR exposes a simple table structure and PostgreSQL foreign data wrappers allow access to table structures, the fit seems pretty perfect.
This implementation currently has the following limitations:
- PostgreSQL 9.3+ This wrapper does not support the FDW implementations in older versions of PostgreSQL.
- Tables are read-only. Foreign data wrappers support read/write and many OGR drivers support read/write, so this limitation can be removed given some development time.
- Only non-spatial query restrictions are pushed down to the OGR driver. PostgreSQL foreign data wrappers support delegating portions of the SQL query to the underlying data source, in this case OGR. This implementation currently pushes down only non-spatial query restrictions, and only for the small subset of comparison operators (>, <, <=, >=, =) supported by OGR.
- Spatial restrictions are not pushed down. OGR can handle basic bounding box restrictions and even (for some drivers) more explicit intersection restrictions, but those are not passed to the OGR driver yet.
- OGR connections every time Rather than pooling OGR connections, each query makes (and disposes of) two new ones, which seems to be the largest performance drag at the moment for restricted (small) queries.
- All columns are retrieved every time. PostgreSQL foreign data wrappers don't require all columns all the time, and some efficiencies can be gained by only requesting the columns needed to fulfill a query. This would be a minimal efficiency improvement, but can be removed given some development time, since the OGR API supports returning a subset of columns.
In order to access geometry data from OGR, the PostGIS extension has to be installed: if it is not installed, geometry will be represented as bytea columns, with well-known binary (WKB) values.
To build the wrapper, make sure you have the GDAL library and development packages (is gdal-config on your path?) installed, as well as the PostgreSQL development packages (is pg_config on your path?)
Build the wrapper with make and make install. Now you are ready to create a foreign table.
First install the postgis and ogr_fdw extensions in your database.
-- Install the required extensions
CREATE EXTENSION postgis;
CREATE EXTENSION ogr_fdw;
For a test data set, copy the pt_two example shape file from the data directory to a location where the PostgreSQL server can read it (like /tmp/test/ for example).
Use the ogr_fdw_info tool to read an OGR data source and output a server and table definition for a particular layer. (You can write these manually, but the utility makes it a little more foolproof.)
> ogr_fdw_info -s /tmp/test
Layers:
pt_two
> ogr_fdw_info -s /tmp/test -l pt_two
CREATE SERVER myserver
FOREIGN DATA WRAPPER ogr_fdw
OPTIONS (
datasource '/tmp/test',
format 'ESRI Shapefile' );
CREATE FOREIGN TABLE pt_two (
fid integer,
geom geometry,
name varchar,
age integer,
height real,
birthdate date )
SERVER myserver
OPTIONS ( layer 'pt_two' );
Copy the CREATE SERVER and CREATE FOREIGN SERVER SQL commands into the database and you'll have your foreign table definition.
Foreign table "public.pt_two"
Column | Type | Modifiers | FDW Options
----------+-------------------+-----------+-------------
fid | integer | |
geom | geometry | |
name | character varying | |
age | integer | |
height | real | |
birthday | date | |
Server: tmp_shape
FDW Options: (layer 'pt_two')
And you can query the table directly, even though it's really just a shape file.
> SELECT * FROM pt_two;
fid | geom | name | age | height | birthday
-----+--------------------------------------------+-------+-----+--------+------------
0 | 0101000000C00497D1162CB93F8CBAEF08A080E63F | Peter | 45 | 5.6 | 1965-04-12
1 | 010100000054E943ACD697E2BFC0895EE54A46CF3F | Paul | 33 | 5.84 | 1971-03-25
Since we can access any OGR data source as a table, how about a public WFS server?
CREATE EXTENSION postgis;
CREATE EXTENSION ogr_fdw;
CREATE SERVER opengeo
FOREIGN DATA WRAPPER ogr_fdw
OPTIONS (
datasource 'WFS:http://demo.opengeo.org/geoserver/wfs',
format 'WFS' );
CREATE FOREIGN TABLE topp_states (
fid integer,
geom geometry,
gml_id varchar,
state_name varchar,
state_fips varchar,
sub_region varchar,
state_abbr varchar,
land_km real,
water_km real,
persons real,
families real,
houshold real,
male real,
female real,
workers real,
drvalone real,
carpool real,
pubtrans real,
employed real,
unemploy real,
service real,
manual real,
p_male real,
p_female real,
samp_pop real )
SERVER opengeo
OPTIONS ( layer 'topp:states' );
Unzip the Querying.zip file from the data directory to get a Querying.gdb file, and put it somewhere public (like /tmp). Now run the ogr_fdw_info tool on it to get a table definition.
CREATE SERVER fgdbtest
FOREIGN DATA WRAPPER ogr_fdw
OPTIONS (
datasource '/tmp/Querying.gdb',
format 'OpenFileGDB' );
CREATE FOREIGN TABLE cities (
fid integer,
geom geometry,
city_fips varchar,
city_name varchar,
state_fips varchar,
state_name varchar,
state_city varchar,
type varchar,
capital varchar,
elevation integer,
pop1990 integer,
popcat integer )
SERVER fgdbtest
OPTIONS ( layer 'Cities' );
Query away!
Wraparound action! Handy for testing. Connect your database back to your database and watch the fur fly.
CREATE TABLE typetest (
id integer,
geom geometry(point, 4326),
num real,
name varchar,
clock time,
calendar date,
tstmp timestamp
);
INSERT INTO typetest
VALUES (1, 'SRID=4326;POINT(-126 46)', 4.5, 'Paul', '09:34:23', 'June 1, 2013', '12:34:56 December 14, 1823');
INSERT INTO typetest
VALUES (2, 'SRID=4326;POINT(-126 46)', 4.8, 'Peter', '14:34:53', 'July 12, 2011', '1:34:12 December 24, 1923');
CREATE SERVER wraparound
FOREIGN DATA WRAPPER ogr_fdw
OPTIONS (
datasource 'Pg:dbname=fdw user=postgres',
format 'PostgreSQL' );
CREATE FOREIGN TABLE typetest_fdw (
fid integer,
geom geometry,
id integer,
num real,
name varchar,
clock time,
calendar date,
tstmp timestamp )
SERVER wraparound
OPTIONS ( layer 'typetest' );
SELECT * FROM typetest_fdw;
Enjoy!
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