Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly. You can read PostgreSQL as Postgres-XC except for version number, which is specific to each product.
This part is about extending the server functionality with user-defined functions, data types, triggers, etc. These are advanced topics which should probably be approached only after all the other user documentation about PostgreSQL has been understood. Later chapters in this part describe the server-side programming languages available in the PostgreSQL distribution as well as general issues concerning server-side programming languages. It is essential to read at least the earlier sections of Chapter 34 (covering functions) before diving into the material about server-side programming languages.
Note: XCONLY: The following description applies only to Postgres-XC.
In Postgres-XC, you should be careful where server means. This could be a coordinator where your statement is handled first, or could be a datnode where your statement is actually handled. Current version of Postgres-XC keeps this as transparent as possible but it is a good thing to keep this in mind.