PostgreSQL OGR Foreign Data Wrapper
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.
- 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 install. Now you are ready to create a foreign table.
First install the
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).
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 -f Supported Formats: -> "PCIDSK" (read/write) -> "netCDF" (read/write) ... -> "HTTP" (readonly) > 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(Point, 4326), name varchar, age integer, height real, birthdate date ) SERVER myserver OPTIONS (layer 'pt_two');
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');
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(Point, 4326), 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');
Wraparound action! Handy for testing. Connect your database back to your database and watch the fur fly.
CREATE TABLE typetest ( fid serial primary key, 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(Point, 4326), num real, name varchar, clock time, calendar date, tstmp timestamp ) SERVER wraparound OPTIONS (layer 'typetest'); SELECT * FROM typetest_fdw;
Writeable FDW Tables
If the OGR driver you are using supports it, you can insert/update/delete records from your FDW tables.
For file-backed drivers, the user under which
postgres runs will need read/write access to the file being altered. For database-backed drivers, your connection needs a user with read/write permissions to the database.
By default, servers and tables are updateable if the OGR driver supports it, but you can turn off updateability at a server or table level using the
ALTER SERVER myserver OPTIONS (ADD updatable 'false'); ALTER FOREIGN TABLE mytable OPTIONS (ADD updatable 'false');
Writeable tables only work if you have included a
fid column in your table definition. By default, tables imported by
IMPORT FOREIGN SCHEMA or using the example SQL code from
ogr_fdw_info include a
Column Name Mapping
You can create an FDW table with any subset of columns from the OGR source you like, just by using the same column names as the source:
CREATE FOREIGN TABLE typetest_fdw_partial ( clock time, name varchar ) SERVER wraparound OPTIONS (layer 'typetest');
You can also explicitly map remote column names to different local names using the
CREATE FOREIGN TABLE typetest_fdw_mapped ( fid bigint, supertime time OPTIONS (column_name 'clock'), thebestnamething varchar OPTIONS (column_name 'name') ) SERVER wraparound OPTIONS (layer 'typetest');
Automatic Foreign Table Creation
This feature is only available with PostgreSQL 9.5 and higher
You can use the PostgreSQL
IMPORT FOREIGN SCHEMA command to import table definitions from an OGR data source.
Import All Tables
If you want to import all tables in the OGR data source use the special schema called "ogr_all".
CREATE SCHEMA fgdball; IMPORT FOREIGN SCHEMA ogr_all FROM SERVER fgdbtest INTO fgdball;
Import a Subset of Tables
Not all OGR data sources have a concept of schema, so we use the remote schema string as a prefix to match OGR layers. The matching is case sensitive, so make sure casing matches your layer names.
For example, the following will only import tables that start with CitiesIn. As long as you quote, you can handle true schemaed databases such as SQL Server or PostgreSQL by using something like "dbo."
CREATE SCHEMA fgdbcityinf; IMPORT FOREIGN SCHEMA "CitiesIn" FROM SERVER fgdbtest INTO fgdbcityinf;
You can also use PostgreSQL clauses
LIMIT TO and
EXCEPT to restrict the tables you are importing.
CREATE SCHEMA fgdbcitysub; -- import only layer called Cities IMPORT FOREIGN SCHEMA ogr_all LIMIT TO(cities) FROM server fgdbtest INTO fgdbcitysub ; -- import only layers not called Cities or Countries IMPORT FOREIGN SCHEMA ogr_all EXCEPT (cities, countries) FROM server fgdbtest INTO fgdbcitysub; -- With table laundering turned off, need to use exact layer names DROP SCHEMA IF EXISTS fgdbcitysub CASCADE; -- import with un-laundered table name IMPORT FOREIGN SCHEMA ogr_all LIMIT TO("Cities") FROM server fgdbtest INTO fgdbcitysub OPTIONS (launder_table_names 'false') ;
Mixed Case and Special Characters
In general, PostgreSQL prefers table names with simple numbers and letters, no punctuation or special characters.
By default, when
IMPORT FOREIGN SCHEMA is run on an OGR foreign data server, the table names and column names are "laundered" -- all upper case is converted to lowercase and special characters such as spaces and punctuation are replaced with "_".
Laundering is not desirable in all cases. You can override this behavior with two
IMPORT FOREIGN SCHEMA options specific to
To preserve casing and other funky characters in both column names and table names, do the following:
CREATE SCHEMA fgdbcitypreserve; IMPORT FOREIGN SCHEMA ogr_all FROM SERVER fgdbtest INTO fgdbpreserve OPTIONS ( launder_table_names 'false', launder_column_names 'false' ) ;
The behavior of your GDAL/OGR connection can be altered by passing GDAL
config_options to the connection when you set up the server. Most GDAL/OGR drivers have some specific behaviours that are controlled by configuration options. For example, the "ESRI Shapefile" driver includes a
SHAPE_ENCODING option that controls the character encoding applied to text data.
Since many Shapefiles are encoded using LATIN1, and most PostgreSQL databases are encoded in UTF-8, it is useful to specify the encoding to get proper handling of special characters like accents.
CREATE SERVER myserver_latin1 FOREIGN DATA WRAPPER ogr_fdw OPTIONS ( datasource '/tmp/test', format 'ESRI Shapefile', config_options 'SHAPE_ENCODING=LATIN1' );
Multiple config options can be passed at one time by supplying a space-separated list of options.
If you are using GDAL 2.0 or higher, you can also pass "open options" to your OGR foreign data wrapper, using the
open_options parameter. In GDAL 2.0, the global
SHAPE_ENCODING option has been superceded by a driver-specific
ENCODING option, which can be called like this:
CREATE SERVER myserver_latin1 FOREIGN DATA WRAPPER ogr_fdw OPTIONS ( datasource '/tmp/test', format 'ESRI Shapefile', open_options 'ENCODING=LATIN1' );
If you are getting odd behavior and you want to see what GDAL is doing behind the scenes, enable debug logging in your server:
CREATE SERVER myserver_latin1 FOREIGN DATA WRAPPER ogr_fdw OPTIONS ( datasource '/tmp/test', format 'ESRI Shapefile', config_options 'SHAPE_ENCODING=LATIN1 CPL_DEBUG=ON' );
GDAL-level messages will be logged at the PostgreSQL DEBUG2 level, so to see them when running a query, alter your
SET client_min_messages = debug2;
Once you've figured out your issue, don't forget to remove the
CPL_DEBUG option from your server definition, and set your messages back to NOTICE level.
SET client_min_messages = notice; ALTER SERVER myserver_latin1 OPTIONS (SET config_options 'SHAPE_ENCODING=LATIN1');