what does “LANGUAGE 'plpgsql' VOLATILE” mean?

Question

When I create or update a function or procedure in a Postgres database I see LANGUAGE 'plpgsql' VOLATILE at the end of function.
What does this mean and what is its purpose?

Answer 1

From Postgres docs:

VOLATILE indicates that the function value can change even within a single table scan, so no optimizations can be made. Relatively few database functions are volatile in this sense; some examples are random(), currval(), timeofday(). But note that any function that has side-effects must be classified volatile, even if its result is quite predictable, to prevent calls from being optimized away; an example is setval().

Answer 2

Not least, LANGUAGE 'plpgsql' VOLATILE means that somebody didn't get the memo.

The language name in CREATE FUNCTION is an identifier and should not be quoted. Should be:

LANGUAGE plpgsql VOLATILE

Malpractice can lead to confusing errors. More details in this related answer:
PostgreSQL procedural language "C" not found