The protocol has separate phases for startup and normal operation.
In the startup phase, the frontend opens a connection to the server
and authenticates itself to the satisfaction of the server. (This might
involve a single message, or multiple messages depending on the
authentication method being used.) If all goes well, the server then sends
status information to the frontend, and finally enters normal operation.
Except for the initial startup-request message, this part of the
protocol is driven by the server.
During normal operation, the frontend sends queries and
other commands to the backend, and the backend sends back query results
and other responses. There are a few cases (such as NOTIFY)
wherein the
backend will send unsolicited messages, but for the most part this portion
of a session is driven by frontend requests.
Termination of the session is normally by frontend choice, but can be
forced by the backend in certain cases. In any case, when the backend
closes the connection, it will roll back any open (incomplete) transaction
before exiting.
Within normal operation, SQL commands can be executed through either of
two sub-protocols. In the "simple query" protocol, the frontend
just sends a textual query string, which is parsed and immediately
executed by the backend. In the "extended query" protocol,
processing of queries is separated into multiple steps: parsing,
binding of parameter values, and execution. This offers flexibility
and performance benefits, at the cost of extra complexity.
Normal operation has additional sub-protocols for special operations
such as COPY.
All communication is through a stream of messages. The first byte of a
message identifies the message type, and the next four bytes specify the
length of the rest of the message (this length count includes itself, but
not the message-type byte). The remaining contents of the message are
determined by the message type. For historical reasons, the very first
message sent by the client (the startup message) has no initial
message-type byte.
To avoid losing synchronization with the message stream, both servers and
clients typically read an entire message into a buffer (using the byte
count) before attempting to process its contents. This allows easy
recovery if an error is detected while processing the contents. In
extreme situations (such as not having enough memory to buffer the
message), the receiver may use the byte count to determine how much
input to skip before it resumes reading messages.
Conversely, both servers and clients must take care never to send an
incomplete message. This is commonly done by marshaling the entire message
in a buffer before beginning to send it. If a communications failure
occurs partway through sending or receiving a message, the only sensible
response is to abandon the connection, since there is little hope of
recovering message-boundary synchronization.
In the extended-query protocol, execution of SQL commands is divided
into multiple steps. The state retained between steps is represented
by two types of objects: prepared statements and
portals. A prepared statement represents the result of
parsing, semantic analysis, and planning of a textual query string. A
prepared statement is not necessarily ready to execute, because it may
lack specific values for parameters. A portal represents
a ready-to-execute or already-partially-executed statement, with any
missing parameter values filled in. (For SELECT statements,
a portal is equivalent to an open cursor, but we choose to use a different
term since cursors don't handle non-SELECT statements.)
The overall execution cycle consists of a parse step,
which creates a prepared statement from a textual query string; a
bind step, which creates a portal given a prepared
statement and values for any needed parameters; and an
execute step that runs a portal's query. In the case of
a query that returns rows (SELECT, SHOW, etc),
the execute step can be told to fetch only
a limited number of rows, so that multiple execute steps may be needed
to complete the operation.
The backend can keep track of multiple prepared statements and portals
(but note that these exist only within a session, and are never shared
across sessions). Existing prepared statements and portals are
referenced by names assigned when they were created. In addition,
an "unnamed" prepared statement and portal exist. Although these
behave largely the same as named objects, operations on them are optimized
for the case of executing a query only once and then discarding it,
whereas operations on named objects are optimized on the expectation
of multiple uses.
Data of a particular data type might be transmitted in any of several
different formats. As of PostgreSQL 7.4
the only supported formats are "text" and "binary",
but the protocol makes provision for future extensions. The desired
format for any value is specified by a format code.
Clients may specify a format code for each transmitted parameter value
and for each column of a query result. Text has format code zero,
binary has format code one, and all other format codes are reserved
for future definition.
The text representation of values is whatever strings are produced
and accepted by the input/output conversion functions for the
particular data type. In the transmitted representation, there is
no trailing null character; the frontend must add one to received
values if it wants to process them as C strings.
(The text format does not allow embedded nulls, by the way.)
Binary representations for integers use network byte order (most
significant byte first). For other data types consult the documentation
or source code to learn about the binary representation. Keep in mind
that binary representations for complex data types may change across
server versions; the text format is usually the more portable choice.
