This section provides a quick checklist of changes, for the benefit of developers trying to update existing client libraries to protocol 3.0.
The initial startup packet uses a flexible list-of-strings format instead of a fixed format. Notice that session default values for run-time parameters can now be specified directly in the startup packet. (Actually, you could do that before using the options field, but given the limited width of options and the lack of any way to quote whitespace in the values, it wasn't a very safe technique.)
All messages now have a length count immediately following the message type byte (except for startup packets, which have no type byte). Also note that PasswordMessage now has a type byte.
ErrorResponse and NoticeResponse ('E' and 'N') messages now contain multiple fields, from which the client code can assemble an error message of the desired level of verbosity. Note that individual fields will typically not end with a newline, whereas the single string sent in the older protocol always did.
The ReadyForQuery ('Z') message includes a transaction status indicator.
The distinction between BinaryRow and DataRow message types is gone; the single DataRow message type serves for returning data in all formats. Note that the layout of DataRow has changed to make it easier to parse. Also, the representation of binary values has changed: it is no longer directly tied to the server's internal representation.
There is a new "extended query" sub-protocol, which adds the frontend message types Parse, Bind, Execute, Describe, Close, Flush, and Sync, and the backend message types ParseComplete, BindComplete, PortalSuspended, ParameterDescription, NoData, and CloseComplete. Existing clients do not have to concern themselves with this sub-protocol, but making use of it might allow improvements in performance or functionality.
COPY data is now encapsulated into CopyData and CopyDone messages. There is a well-defined way to recover from errors during COPY. The special "\." last line is not needed anymore, and is not sent during COPY OUT. (It is still recognized as a terminator during COPY IN, but its use is deprecated and will eventually be removed.) Binary COPY is supported. The CopyInResponse and CopyOutResponse messages include fields indicating the number of columns and the format of each column.
The layout of FunctionCall and FunctionCallResponse messages has changed. FunctionCall can now support passing NULL arguments to functions. It also can handle passing parameters and retrieving results in either text or binary format. There is no longer any reason to consider FunctionCall a potential security hole, since it does not offer direct access to internal server data representations.
The backend sends ParameterStatus ('S') messages during connection startup for all parameters it considers interesting to the client library. Subsequently, a ParameterStatus message is sent whenever the active value changes for any of these parameters.
The RowDescription ('T') message carries new table OID and column number fields for each column of the described row. It also shows the format code for each column.
The CursorResponse ('P') message is no longer generated by the backend.
The NotificationResponse ('A') message has an additional string field, which can carry a "payload" string passed from the NOTIFY event sender.
The EmptyQueryResponse ('I') message used to include an empty string parameter; this has been removed.