So having spent the last few months with my head buried deep in a project at work, I finally managed to get back to my previous hacking on SQL/MED at the weekend after the kids went away for a week (good $DEITY it's quiet here)! Within a couple of hours, I had my half-baked Foreign Data Wrapper for MySQL up and running, and am now able to create foreign table objects in PostgreSQL 9.1 that map either directly to tables in a MySQL database, or to queries run on the MySQL server.
Here's an example:
-- Create the required functions for the FDW.
CREATE FUNCTION mysql_fdw_handler()
LANGUAGE C STRICT;
CREATE FUNCTION mysql_fdw_validator(text, oid)
LANGUAGE C STRICT;
-- Create the data wrapper or "transport".
CREATE FOREIGN DATA WRAPPER mysql_fdw
-- Create the foreign server, a pointer to the MySQL server.
CREATE SERVER mysql_svr
FOREIGN DATA WRAPPER mysql_fdw
OPTIONS (address '127.0.0.1', port '3306');
-- Create one or more foreign tables on the MySQL server. The first of
-- these maps to a remote table, whilst the second uses an SQL query.
CREATE FOREIGN TABLE employees (
OPTIONS (table 'hr.employees');
CREATE FOREIGN TABLE ex_staff (
OPTIONS (query 'SELECT * FROM hr.employees WHERE date_left IS NOT NULL');
- No attempt is currently made to push down quals (WHERE clauses) to the MySQL server, so every row MySQL finds is returned to PostgreSQL and filtered there. There's no defined API for this in PostgreSQL yet, and it's not immediately clear how to build something more complex than the simple example I used in my Redis FDW that would be required for a remote relational database. That said, you can build WHERE clauses into the foreign table definition of course.
- The MySQL C API doesn't seem to offer a simple way to either randomly access a result set, or at least reset the cursor to the first row, unless you copy the entire resultset to the client (PostgreSQL in this case). Because we need to be able to return to the first row if PostgreSQL calls the Rescan function, we therefore currently copy the entire resultset, rather than reading it from the server, on demand.
Aside from minor tweaks, this is probably about as far as I'll take this little project for now. I'll be talking about it at both Postgres Open 2011 in Chicago in September, and PGBR 2011 in São Paulo in November - hopefully I'll see you there.
The MySQL FDW source code is available on Github, and uses the PostgreSQL licence.