naming a column differently then its original one in foreign data wrapper of PostgreSQL

Question

I have two databases. In each database I have an employee table which differ from each other in the number of columns and column names.

Here is what the table looks in the first database:

 Emp_ID    Name        Father_Name      Family_Name     
 1         Max         Stjepan          Neg               
 2         John        Chris            James           

Here is what the second table looks like which will be in the second database:

 ID        Name        Father_Name      Family_Name   Blood_Type
 1         Max         Stjepan          Neg           ARH+
 2         John        Chris            James         ORH-

Now when creating the foreign table in Second Database I want to store the Emp_ID values from First Database with column name ID and also I want my foreign table to include the blood type from the table which is in the current database Second Database.

Shortly, how can I create a foreign table with different column names than the original table in remote database? It should also include the columns from first table which is in the current database.

Answer 1

You can name the columns of your foreign table whatever you want. For example, after an IMPORT FOREIGN SCHEMA ... you can do

ALTER FOREIGN TABLE myschema.mytable RENAME a_column TO another_column;

(For CREATE FOREIGN TABLE, you have to specify the different column name in the OPTIONS clause to the column: CREATE FOREIGN TABLE foreign.employee (emp_id integer OPTIONS (column_name 'id'), ...)

What you cannot do is mixing columns from local and foreign tables. To achieve something similar, you can create a view:

CREATE VIEW unified_employee AS
SELECT local.id AS emp_id,
       local.name,
       ...,
       foreign.blood_type
  FROM local_schema.employee AS local
  JOIN foreign_schema.employee AS foreign ON local.id = foreign.emp_id;

I swapped the tables in this example on purpose. Your example tables don't differ enough, so a simple view on the local employee table, where the id column is named emp_id would do the trick, without ever touching the other database.