Understanding "LOG: execute S_1: BEGIN " in PostgreSQL

Question

I turned on the Postgres logging with 'all' and logs show LOG: execute S_1: BEGIN.

What does S_1 mean?

Answer 1

The format of this log entry denotes the use of the extended query protocol.

From the linked doc:

In the extended protocol, the frontend first sends a Parse message, which contains a textual query string, optionally some information about data types of parameter placeholders, and the name of a destination prepared-statement object (an empty string selects the unnamed prepared statement)

The S_1 from the log corresponds to that name.

If the application uses the libpq C library or a layer based on it, libpq functions like PQprepare, PQexecPrepared or PQexecParams are built on the extended protocol. On the other hand, the older PQExec uses the simple query protocol only.