Re: Who adds the "start transaction" and "commit" to the intended SQL statement in "autocommit" mode?

To rephrase the other responses, the client-defined setting has no inherent relationship to the concept of a PostgreSQL session.  How the client uses that setting is internal to the client and whatever abstraction(s) it provides the programmer.

So far as the server is concerned it is always auto (implicit) begin, and also auto (implicit) commit - absent receiving a BEGIN SQL Command in which case it disables implicit commit and (more or less) waits for a COMMIT or ROLLBACK before ending the transaction that it implicitly started.

David J.