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Protocol version 3 was introduced in PostgreSQL 7.4. There shouldn't be many clients or servers left out there without version 3 support. But as a courtesy, I kept just enough of the old protocol support that we can still send the "unsupported protocol version" error in v2 format, so that old clients can display the message properly. Likewise, libpq still understands v2 ErrorResponse messages when establishing a connection. The impetus to do this now is that I'm working on a patch to COPY FROM, to always prefetch some data. We cannot do that safely with the old protocol, because it requires parsing the input one byte at a time to detect the end-of-copy marker. Reviewed-by: Tom Lane, Alvaro Herrera, John Naylor Discussion: https://www.postgresql.org/message-id/9ec25819-0a8a-d51a-17dc-4150bb3cca3b%40iki.fi
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.