Yesterday there was a post which hit Hacker News that talked about using SQL to access Mongo. While this is powerful I think much of the true value was entirely missed within the post.
SQL is an expressive language, though people are often okay with accessing Mongo data through its own ORM. The real value is that you could actually query the data from within Postgres then join across your data stores, without having to do some ETL process to move data around. Think… joining sales data from Postgres with user reviews stored in Mongo or searching for visits to a website (retained in redis) against purchases by user in Postgres.
The mechanism pointed out was a MongoDB Foreign Data Wrapper. A Foreign Data Wrapper or FDW essentially lets you connect to an external datastore from within a Postgres database. In addition to the Mongo FDW released the other day there’s many others. For example Postgres 9.0 and up ships with one called db_link, which lets you query and join across two different Postgres databases. Beyond that there’s support for a variety of other data stores including some you may have never expected:
Lets look at actually getting the Redis one running then see what some of the power of it really looks like. First we have to get the code then build it: