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# PostgreSQL OGR Foreign Data Wrapper

## Motivation

OGR is the vector half of the [GDAL](http://www.gdal.org/) spatial data access library. It allows access to a [large number of GIS data formats](http://www.gdal.org/ogr_formats.html) using a [simple C API](http://www.gdal.org/ogr__api_8h.html) for data reading and writing. Since OGR exposes a simple table structure and PostgreSQL [foreign data wrappers](https://wiki.postgresql.org/wiki/Foreign_data_wrappers) allow access to table structures, the fit seems pretty perfect.

## Limitations

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.

## Basic Operation

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 -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');

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


## Examples

### WFS FDW

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 geoserver
      FOREIGN DATA WRAPPER ogr_fdw
      OPTIONS (
	datasource 'WFS:https://demo.geo-solutions.it/geoserver/wfs',
	format 'WFS' );

    CREATE FOREIGN TABLE topp_states (
      fid bigint,
      the_geom Geometry(MultiSurface,4326),
      gml_id varchar,
      state_name varchar,
      state_fips varchar,
      sub_region varchar,
      state_abbr varchar,
      land_km double precision,
      water_km double precision,
      persons double precision,
      families double precision,
      houshold double precision,
      male double precision,
      female double precision,
      workers double precision,
      drvalone double precision,
      carpool double precision,
      pubtrans double precision,
      employed double precision,
      unemploy double precision,
      service double precision,
      manual double precision,
      p_male double precision,
      p_female double precision,
      samp_pop double precision
    ) SERVER "geoserver"
    OPTIONS (layer 'topp:states');


### FGDB FDW

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(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');

Query away!

### PostgreSQL FDW

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;

## Advanced Features

### 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 `updateable` option:

    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 `fid` column.

### 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 `column_name` option:

    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](http://www.postgresql.org/docs/9.5/static/sql-importforeignschema.html).

#### 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](http://www.postgresql.org/docs/9.5/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS), 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 `ogr_fdw` servers: `launder_column_names` and `launder_table_names`.

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'
            ) ;


###  GDAL Options

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](http://www.gdal.org/drv_shapefile.html)" 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' );

### GDAL Debugging

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 `client_min_messages` setting.

    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');
    

