PostgreSQL 11 Administration Cookbook

How it works…

Our query has a WHERE condition that filters the rows we select from the foreign table. As in the postgres_fdw example from the previous recipe, Foreign Data Wrappers do the clever thing: the WHERE condition is pushed to the remote server, and only the matching rows are retrieved.

This is good in two ways: firstly, we delegate some work to another system, and secondly, we reduce the overall network traffic by not transferring unnecessary data.

We also notice that the WHERE condition is expressed in the PostgreSQL syntax; the Foreign Data Wrapper is able to translate it into whatever form is required by the remote system.