Postgres frontend–backend protocol messages

This table was extracted from https://www.postgresql.org/docs/devel/static/protocol-message-formats.html

Chr	Message name	Who issue	Bind command.
B	Bind	F	Bind command.
C	Close	F	Close command.
f	CopyFail	F	COPY-failure indicator.
D	Describe	F	Describe command.
E	Execute	F	Execute command.
H	Flush	F	Flush command.
F	FunctionCall	F	function call.
p	GSSResponse	F	GSSAPI or SSPI response. Note that this is also used for SASL and password response messages. The exact message type can be deduced from the context.
P	Parse	F	Parse command.
p	PasswordMessage	F	password response. Note that this is also used for GSSAPI, SSPI and SASL response messages. The exact message type can be deduced from the context.
Q	Query	F	simple query.
p	SASLInitialResponse	F	initial SASL response. Note that this is also used for GSSAPI, SSPI and password response messages. The exact message type is deduced from the context.
p	SASLResponse	F	SASL response. Note that this is also used for GSSAPI, SSPI and password response messages. The exact message type can be deduced from the context.
S	Sync	F	Sync command.
X	Terminate	F	termination.
d	CopyData	F&B	COPY data.
c	CopyDone	F&B	COPY-complete indicator.
R	AuthenticationOk	B	authentication request.
R	AuthenticationKerberosV5	B	authentication request.
R	AuthenticationCleartextPassword	B	authentication request.
R	AuthenticationMD5Password	B	authentication request.
R	AuthenticationSCMCredential	B	authentication request.
R	AuthenticationGSS	B	authentication request.
R	AuthenticationSSPI	B	authentication request.
R	AuthenticationGSSContinue	B	authentication request.
R	AuthenticationSASL	B	authentication request.
R	AuthenticationSASLContinue	B	authentication request.
R	AuthenticationSASLFinal	B	authentication request.
K	BackendKeyData	B	cancellation key data. The frontend must save these values if it wishes to be able to issue CancelRequest messages later.
2	BindComplete	B	Bind-complete indicator.
3	CloseComplete	B	Close-complete indicator.
C	CommandComplete	B	command-completed response.
G	CopyInResponse	B	Start Copy In response. The frontend must now send copy-in data (if not prepared to do so, send a CopyFail message).
H	CopyOutResponse	B	Start Copy Out response. This message will be followed by copy-out data.
W	CopyBothResponse	B	Start Copy Both response. This message is used only for Streaming Replication.
D	DataRow	B	data row.
I	EmptyQueryResponse	B	response to an empty query string. (This substitutes for CommandComplete.)
E	ErrorResponse	B	error.
V	FunctionCallResponse	B	function call result.
v	NegotiateProtocolVersion	B	protocol version negotiation message.
n	NoData	B	no-data indicator.
N	NoticeResponse	B	notice.
A	NotificationResponse	B	notification response.
t	ParameterDescription	B	parameter description.
S	ParameterStatus	B	run-time parameter status report.
1	ParseComplete	B	Parse-complete indicator.
s	PortalSuspended	B	portal-suspended indicator. Note this only appears if an Execute message's row-count limit was reached.
Z	ReadyForQuery	B	ReadyForQuery is sent whenever the backend is ready for a new query cycle.
T	RowDescription	B	row description.

Other Messages

CancelRequest (F)

SSLRequest (F)

StartupMessage (F)