documentation help

[Paul Tilles <paul.tilles@xxxxxxxx>]

I just read the interesting article by Hans-Juergen Schoenig describing 
how to speed up GROUP BY and JOIN.  In the article, he mentions using an 
"optimization barrier" where the SQL is

WITH x AS

Can somebody tell me where in the postgres docs I can find information 
about this SQL?

TIA.

Paul Tilles