Import foreign schema support in Multicorn



3 minute read

Some of you may have noticed that support for the IMPORT FOREIGN SCHEMA statement has landed in the PostgreSQL source tree last july. This new command allows users to automatically map foreign tables to local ones.

Use-Case

Previously, if you wanted to use the postgres_fdw Foreign Data Wrapper to access data stored in a remote database you had to:

Create the extension

Create a server

Create a user mapping

For each remote table: Create a FOREIGN TABLE which structures matches the remote one



This last step is tedious, and error prone: you have to match the column names, in the right order, with the right type.

The IMPORT FOREIGN SCHEMA statements allows you to automatically create a foreign table object for each object available remotely.

Multicorn implementation

The API has been implemented in Multicorn for a few months, lingering in its own branch.

I just merged it back to the master branch, and this feature will land in an upcoming 1.2.0 release. In the meantime, test it !

The API

Its simple, like always with Multicorn. FDW just have to override the import_schema method:

@classmethod def import_schema ( self , schema , srv_options , options , restriction_type , restricts )

This method just has to build a list of TableDefinition objects:

return [ TableDefinition ( "table_name" , schema = None , columns = [ ColumnDefinition ( name = "column_name" , type_name = "integer" )])]

And thats it !

As for now, the only FDW shipped with Multicorn that does implement this API is the sqlalchemyfdw.

SQLAlchemyFDW test run

So, with this API in mind, I conducted a small test: trying to import an Oracle schema as well as a MS-SQLServer schema:

CREATE EXTENSION multicorn ; CREATE SERVER mssql_server FOREIGN DATA WRAPPER multicorn OPTIONS ( wrapper 'multicorn.sqlalchemyfdw.SqlAlchemyFdw' , drivername 'mssql+pymssql' , host 'myhost' , port '1433' , database 'testmulticorn' ); CREATE USER MAPPING FOR ronan SERVER mssql_server OPTIONS ( username 'user' , password 'password' ); CREATE SCHEMA mssql ; IMPORT FOREIGN SCHEMA dbo FROM SERVER mssql_server INTO mssql ; CREATE SERVER sqlite_server FOREIGN DATA WRAPPER multicorn OPTIONS ( wrapper 'multicorn.sqlalchemyfdw.SqlAlchemyFdw' , drivername 'sqlite' , database '/home/ronan/mydb.sqlite3' ); CREATE SCHEMA sqlite ; IMPORT FOREIGN SCHEMA main FROM SERVER sqlite_server INTO sqlite ; DELETE FROM mssql . t1 ; DELETE FROM sqlite . t1 ; INSERT INTO sqlite . t1 ( id , label ) VALUES ( 1 , DEFAULT ); SELECT * FROM sqlite . t1 ; CREATE SERVER oracle_server FOREIGN DATA WRAPPER multicorn OPTIONS ( wrapper 'multicorn.sqlalchemyfdw.SqlAlchemyFdw' , drivername 'oracle' , host 'another_host' , database 'testmulticorn' ); CREATE USER MAPPING FOR ronan SERVER oracle_server OPTIONS ( username 'user' , password 'password' ); CREATE SCHEMA oracle ; IMPORT FOREIGN SCHEMA ronan FROM SERVER oracle_server INTO oracle ;

And thats it ! Its sufficient to query tables from sqllite, oracle and MS-SQL from a single connection.

Once again, feel free to test it and to report any bugs you may find along the way !