Last time we demonstrated how to use the ODBC Foreign Data wrapper, this time we'll continue our journey into Foreign Data Wrapper land by demonstrating what I'll call the File FDW family of Foreign Data Wrappers. There is one that usually comes packaged with PostgreSQL 9.1 which is called fdw_file but there are two other experimental ones I find very useful which are developed by Andrew Dunstan both of which Andrew demoed in PostgreSQL Foreign Data Wrappers and talked about a little bit Text files from a remote source. As people who have to deal with text data files day in and out, especially ones from mainframes, these satisfy a certain itch.
In this article, we'll just cover the
file_fdw one, but will follow up in subsequent articles, demonstrating the array and fixed length record ones.
Before we begin, We'll create a staging schema to throw all our foreign data tables.
CREATE SCHEMA staging;
We'll also create a folder on on the root of our postgres server called fdw_data where we will house all the flat files. Make sure the postgres service account has access to it.
We'll start with the
CREATE EXTENSION file_fdw;
CREATE SERVER file_fdw_server FOREIGN DATA WRAPPER file_fdw;
Note that the aircraft.txt has a header row. We'll save this to our fdw_data folder
CREATE FOREIGN TABLE staging.aircraft ( Model Char (12), Last_Change_Date VarChar(10), Region VarChar(2), Make VarChar(6), Aircraft_Group VarChar(6), Regis_Code VarChar(7), Design_Character VarChar(3), No_Engines VarChar(11), Type_Engine VarChar(2), Type_Landing_Gear VarChar(2), TC_Data_Sheet_Number VarChar(8), TC_Model VarChar(20) ) SERVER file_fdw_server OPTIONS (format 'csv',header 'true' , filename '/fdw_data/aircraft.txt', delimiter E'\t', null '');
You can change the location of a file as well. For example, the query above will not work if you are on windows. You'll need to specify the drive letter as well. You can do this:
ALTER FOREIGN TABLE staging.aircraft OPTIONS ( SET filename 'C:/fdw_data/aircraft.txt' );
Now to query our table, we simply do this:
SELECT model, region, make, last_change_date FROM staging.aircraft WHERE make LIKE 'BEECH%' ORDER BY last_change_date DESC LIMIT 5;
model | region | make | last_change_date --------------+--------+--------+------------------ B200CT | CE | BEECH | 12/18/2001 3000 | CE | BEECH | 12/17/2001 B300C | CE | BEECH | 10/11/2001 C12C | CE | BEECH | 10/11/2001 65A901 | CE | BEECH | 10/11/2001
Notice in the above definition that the Last_change_date field is set to varchar(10), but when we look at the data, it's in American date format and sorting is not using date sorting so records get sorted by month first. To force date sorting, we can make this changeWe could do this:
ALTER FOREIGN TABLE staging.aircraft ALTER COLUMN last_change_date type date;
And that works great if you are in the US since the default DateStyle is MDY and rerunning the query gives:
model | region | make | last_change_date -------------+--------+--------+------------------ U21A | CE | BEECH | 2004-01-28 B200CT | CE | BEECH | 2001-12-18 3000 | CE | BEECH | 2001-12-17 65A901 | CE | BEECH | 2001-10-11 C12C | CE | BEECH | 2001-10-11
However if your DateStyle is 'DMY' as it is in Europe and other places, you'll get an error when you go to query the data. We couldn't find a way to force the DateStyle property of a foreign table column aside from doing this:
before running a query on the table