1 <!-- doc/src/sgml/protocol.sgml -->
3 <chapter id="protocol">
4 <title>Frontend/Backend Protocol</title>
6 <indexterm zone="protocol">
7 <primary>protocol</primary>
8 <secondary>frontend-backend</secondary>
12 <productname>PostgreSQL</productname> uses a message-based protocol
13 for communication between frontends and backends (clients and servers).
14 The protocol is supported over <acronym>TCP/IP</acronym> and also over
15 Unix-domain sockets. Port number 5432 has been registered with IANA as
16 the customary TCP port number for servers supporting this protocol, but
17 in practice any non-privileged port number can be used.
21 This document describes version 3.0 of the protocol, implemented in
22 <productname>PostgreSQL</productname> 7.4 and later. For descriptions
23 of the earlier protocol versions, see previous releases of the
24 <productname>PostgreSQL</productname> documentation. A single server
25 can support multiple protocol versions. The initial
26 startup-request message tells the server which protocol version the
27 client is attempting to use, and then the server follows that protocol
32 In order to serve multiple clients efficiently, the server launches
33 a new <quote>backend</> process for each client.
34 In the current implementation, a new child
35 process is created immediately after an incoming connection is detected.
36 This is transparent to the protocol, however. For purposes of the
37 protocol, the terms <quote>backend</> and <quote>server</> are
38 interchangeable; likewise <quote>frontend</> and <quote>client</>
42 <sect1 id="protocol-overview">
43 <title>Overview</title>
46 The protocol has separate phases for startup and normal operation.
47 In the startup phase, the frontend opens a connection to the server
48 and authenticates itself to the satisfaction of the server. (This might
49 involve a single message, or multiple messages depending on the
50 authentication method being used.) If all goes well, the server then sends
51 status information to the frontend, and finally enters normal operation.
52 Except for the initial startup-request message, this part of the
53 protocol is driven by the server.
57 During normal operation, the frontend sends queries and
58 other commands to the backend, and the backend sends back query results
59 and other responses. There are a few cases (such as <command>NOTIFY</>)
61 backend will send unsolicited messages, but for the most part this portion
62 of a session is driven by frontend requests.
66 Termination of the session is normally by frontend choice, but can be
67 forced by the backend in certain cases. In any case, when the backend
68 closes the connection, it will roll back any open (incomplete) transaction
73 Within normal operation, SQL commands can be executed through either of
74 two sub-protocols. In the <quote>simple query</> protocol, the frontend
75 just sends a textual query string, which is parsed and immediately
76 executed by the backend. In the <quote>extended query</> protocol,
77 processing of queries is separated into multiple steps: parsing,
78 binding of parameter values, and execution. This offers flexibility
79 and performance benefits, at the cost of extra complexity.
83 Normal operation has additional sub-protocols for special operations
84 such as <command>COPY</>.
87 <sect2 id="protocol-message-concepts">
88 <title>Messaging Overview</title>
91 All communication is through a stream of messages. The first byte of a
92 message identifies the message type, and the next four bytes specify the
93 length of the rest of the message (this length count includes itself, but
94 not the message-type byte). The remaining contents of the message are
95 determined by the message type. For historical reasons, the very first
96 message sent by the client (the startup message) has no initial
101 To avoid losing synchronization with the message stream, both servers and
102 clients typically read an entire message into a buffer (using the byte
103 count) before attempting to process its contents. This allows easy
104 recovery if an error is detected while processing the contents. In
105 extreme situations (such as not having enough memory to buffer the
106 message), the receiver can use the byte count to determine how much
107 input to skip before it resumes reading messages.
111 Conversely, both servers and clients must take care never to send an
112 incomplete message. This is commonly done by marshaling the entire message
113 in a buffer before beginning to send it. If a communications failure
114 occurs partway through sending or receiving a message, the only sensible
115 response is to abandon the connection, since there is little hope of
116 recovering message-boundary synchronization.
120 <sect2 id="protocol-query-concepts">
121 <title>Extended Query Overview</title>
124 In the extended-query protocol, execution of SQL commands is divided
125 into multiple steps. The state retained between steps is represented
126 by two types of objects: <firstterm>prepared statements</> and
127 <firstterm>portals</>. A prepared statement represents the result of
128 parsing and semantic analysis of a textual query string.
129 A prepared statement is not in itself ready to execute, because it might
130 lack specific values for <firstterm>parameters</>. A portal represents
131 a ready-to-execute or already-partially-executed statement, with any
132 missing parameter values filled in. (For <command>SELECT</> statements,
133 a portal is equivalent to an open cursor, but we choose to use a different
134 term since cursors don't handle non-<command>SELECT</> statements.)
138 The overall execution cycle consists of a <firstterm>parse</> step,
139 which creates a prepared statement from a textual query string; a
140 <firstterm>bind</> step, which creates a portal given a prepared
141 statement and values for any needed parameters; and an
142 <firstterm>execute</> step that runs a portal's query. In the case of
143 a query that returns rows (<command>SELECT</>, <command>SHOW</>, etc),
144 the execute step can be told to fetch only
145 a limited number of rows, so that multiple execute steps might be needed
146 to complete the operation.
150 The backend can keep track of multiple prepared statements and portals
151 (but note that these exist only within a session, and are never shared
152 across sessions). Existing prepared statements and portals are
153 referenced by names assigned when they were created. In addition,
154 an <quote>unnamed</> prepared statement and portal exist. Although these
155 behave largely the same as named objects, operations on them are optimized
156 for the case of executing a query only once and then discarding it,
157 whereas operations on named objects are optimized on the expectation
162 <sect2 id="protocol-format-codes">
163 <title>Formats and Format Codes</title>
166 Data of a particular data type might be transmitted in any of several
167 different <firstterm>formats</>. As of <productname>PostgreSQL</> 7.4
168 the only supported formats are <quote>text</> and <quote>binary</>,
169 but the protocol makes provision for future extensions. The desired
170 format for any value is specified by a <firstterm>format code</>.
171 Clients can specify a format code for each transmitted parameter value
172 and for each column of a query result. Text has format code zero,
173 binary has format code one, and all other format codes are reserved
174 for future definition.
178 The text representation of values is whatever strings are produced
179 and accepted by the input/output conversion functions for the
180 particular data type. In the transmitted representation, there is
181 no trailing null character; the frontend must add one to received
182 values if it wants to process them as C strings.
183 (The text format does not allow embedded nulls, by the way.)
187 Binary representations for integers use network byte order (most
188 significant byte first). For other data types consult the documentation
189 or source code to learn about the binary representation. Keep in mind
190 that binary representations for complex data types might change across
191 server versions; the text format is usually the more portable choice.
196 <sect1 id="protocol-flow">
197 <title>Message Flow</title>
200 This section describes the message flow and the semantics of each
201 message type. (Details of the exact representation of each message
202 appear in <xref linkend="protocol-message-formats">.) There are
203 several different sub-protocols depending on the state of the
204 connection: start-up, query, function call,
205 <command>COPY</command>, and termination. There are also special
206 provisions for asynchronous operations (including notification
207 responses and command cancellation), which can occur at any time
208 after the start-up phase.
212 <title>Start-up</title>
215 To begin a session, a frontend opens a connection to the server and sends
216 a startup message. This message includes the names of the user and of the
217 database the user wants to connect to; it also identifies the particular
218 protocol version to be used. (Optionally, the startup message can include
219 additional settings for run-time parameters.)
220 The server then uses this information and
221 the contents of its configuration files (such as
222 <filename>pg_hba.conf</filename>) to determine
223 whether the connection is provisionally acceptable, and what additional
224 authentication is required (if any).
228 The server then sends an appropriate authentication request message,
229 to which the frontend must reply with an appropriate authentication
230 response message (such as a password).
231 For all authentication methods except GSSAPI and SSPI, there is at most
232 one request and one response. In some methods, no response
233 at all is needed from the frontend, and so no authentication request
234 occurs. For GSSAPI and SSPI, multiple exchanges of packets may be needed
235 to complete the authentication.
239 The authentication cycle ends with the server either rejecting the
240 connection attempt (ErrorResponse), or sending AuthenticationOk.
244 The possible messages from the server in this phase are:
248 <term>ErrorResponse</term>
251 The connection attempt has been rejected.
252 The server then immediately closes the connection.
258 <term>AuthenticationOk</term>
261 The authentication exchange is successfully completed.
267 <term>AuthenticationKerberosV5</term>
270 The frontend must now take part in a Kerberos V5
271 authentication dialog (not described here, part of the
272 Kerberos specification) with the server. If this is
273 successful, the server responds with an AuthenticationOk,
274 otherwise it responds with an ErrorResponse.
280 <term>AuthenticationCleartextPassword</term>
283 The frontend must now send a PasswordMessage containing the
284 password in clear-text form. If
285 this is the correct password, the server responds with an
286 AuthenticationOk, otherwise it responds with an ErrorResponse.
292 <term>AuthenticationMD5Password</term>
295 The frontend must now send a PasswordMessage containing the
296 password (with username) encrypted via MD5, then encrypted
297 again using the 4-byte random salt specified in the
298 AuthenticationMD5Password message. If this is the correct
299 password, the server responds with an AuthenticationOk,
300 otherwise it responds with an ErrorResponse. The actual
301 PasswordMessage can be computed in SQL as <literal>concat('md5',
302 md5(concat(md5(concat(password, username)), random-salt)))</>.
303 (Keep in mind the <function>md5()</> function returns its
304 result as a hex string.)
310 <term>AuthenticationSCMCredential</term>
313 This response is only possible for local Unix-domain connections
314 on platforms that support SCM credential messages. The frontend
315 must issue an SCM credential message and then send a single data
316 byte. (The contents of the data byte are uninteresting; it's
317 only used to ensure that the server waits long enough to receive
318 the credential message.) If the credential is acceptable,
319 the server responds with an
320 AuthenticationOk, otherwise it responds with an ErrorResponse.
321 (This message type is only issued by pre-9.1 servers. It may
322 eventually be removed from the protocol specification.)
328 <term>AuthenticationGSS</term>
331 The frontend must now initiate a GSSAPI negotiation. The frontend
332 will send a PasswordMessage with the first part of the GSSAPI
333 data stream in response to this. If further messages are needed,
334 the server will respond with AuthenticationGSSContinue.
340 <term>AuthenticationSSPI</term>
343 The frontend must now initiate a SSPI negotiation. The frontend
344 will send a PasswordMessage with the first part of the SSPI
345 data stream in response to this. If further messages are needed,
346 the server will respond with AuthenticationGSSContinue.
352 <term>AuthenticationGSSContinue</term>
355 This message contains the response data from the previous step
356 of GSSAPI or SSPI negotiation (AuthenticationGSS, AuthenticationSSPI
357 or a previous AuthenticationGSSContinue). If the GSSAPI
358 or SSPI data in this message
359 indicates more data is needed to complete the authentication,
360 the frontend must send that data as another PasswordMessage. If
361 GSSAPI or SSPI authentication is completed by this message, the server
362 will next send AuthenticationOk to indicate successful authentication
363 or ErrorResponse to indicate failure.
372 If the frontend does not support the authentication method
373 requested by the server, then it should immediately close the
378 After having received AuthenticationOk, the frontend must wait
379 for further messages from the server. In this phase a backend process
380 is being started, and the frontend is just an interested bystander.
381 It is still possible for the startup attempt
382 to fail (ErrorResponse), but in the normal case the backend will send
383 some ParameterStatus messages, BackendKeyData, and finally ReadyForQuery.
387 During this phase the backend will attempt to apply any additional
388 run-time parameter settings that were given in the startup message.
389 If successful, these values become session defaults. An error causes
390 ErrorResponse and exit.
394 The possible messages from the backend in this phase are:
398 <term>BackendKeyData</term>
401 This message provides secret-key data that the frontend must
402 save if it wants to be able to issue cancel requests later.
403 The frontend should not respond to this message, but should
404 continue listening for a ReadyForQuery message.
410 <term>ParameterStatus</term>
413 This message informs the frontend about the current (initial)
414 setting of backend parameters, such as <xref
415 linkend="guc-client-encoding"> or <xref linkend="guc-datestyle">.
416 The frontend can ignore this message, or record the settings
417 for its future use; see <xref linkend="protocol-async"> for
418 more details. The frontend should not respond to this
419 message, but should continue listening for a ReadyForQuery
426 <term>ReadyForQuery</term>
429 Start-up is completed. The frontend can now issue commands.
435 <term>ErrorResponse</term>
438 Start-up failed. The connection is closed after sending this
445 <term>NoticeResponse</term>
448 A warning message has been issued. The frontend should
449 display the message but continue listening for ReadyForQuery
458 The ReadyForQuery message is the same one that the backend will
459 issue after each command cycle. Depending on the coding needs of
460 the frontend, it is reasonable to consider ReadyForQuery as
461 starting a command cycle, or to consider ReadyForQuery as ending the
462 start-up phase and each subsequent command cycle.
467 <title>Simple Query</title>
470 A simple query cycle is initiated by the frontend sending a Query message
471 to the backend. The message includes an SQL command (or commands)
472 expressed as a text string.
473 The backend then sends one or more response
474 messages depending on the contents of the query command string,
475 and finally a ReadyForQuery response message. ReadyForQuery
476 informs the frontend that it can safely send a new command.
477 (It is not actually necessary for the frontend to wait for
478 ReadyForQuery before issuing another command, but the frontend must
479 then take responsibility for figuring out what happens if the earlier
480 command fails and already-issued later commands succeed.)
484 The possible response messages from the backend are:
488 <term>CommandComplete</term>
491 An SQL command completed normally.
497 <term>CopyInResponse</term>
500 The backend is ready to copy data from the frontend to a
501 table; see <xref linkend="protocol-copy">.
507 <term>CopyOutResponse</term>
510 The backend is ready to copy data from a table to the
511 frontend; see <xref linkend="protocol-copy">.
517 <term>RowDescription</term>
520 Indicates that rows are about to be returned in response to
521 a <command>SELECT</command>, <command>FETCH</command>, etc query.
522 The contents of this message describe the column layout of the rows.
523 This will be followed by a DataRow message for each row being returned
533 One of the set of rows returned by
534 a <command>SELECT</command>, <command>FETCH</command>, etc query.
540 <term>EmptyQueryResponse</term>
543 An empty query string was recognized.
549 <term>ErrorResponse</term>
552 An error has occurred.
558 <term>ReadyForQuery</term>
561 Processing of the query string is complete. A separate
562 message is sent to indicate this because the query string might
563 contain multiple SQL commands. (CommandComplete marks the
564 end of processing one SQL command, not the whole string.)
565 ReadyForQuery will always be sent, whether processing
566 terminates successfully or with an error.
572 <term>NoticeResponse</term>
575 A warning message has been issued in relation to the query.
576 Notices are in addition to other responses, i.e., the backend
577 will continue processing the command.
586 The response to a <command>SELECT</> query (or other queries that
587 return row sets, such as <command>EXPLAIN</> or <command>SHOW</>)
588 normally consists of RowDescription, zero or more
589 DataRow messages, and then CommandComplete.
590 <command>COPY</> to or from the frontend invokes special protocol
591 as described in <xref linkend="protocol-copy">.
592 All other query types normally produce only
593 a CommandComplete message.
597 Since a query string could contain several queries (separated by
598 semicolons), there might be several such response sequences before the
599 backend finishes processing the query string. ReadyForQuery is issued
600 when the entire string has been processed and the backend is ready to
601 accept a new query string.
605 If a completely empty (no contents other than whitespace) query string
606 is received, the response is EmptyQueryResponse followed by ReadyForQuery.
610 In the event of an error, ErrorResponse is issued followed by
611 ReadyForQuery. All further processing of the query string is aborted by
612 ErrorResponse (even if more queries remained in it). Note that this
613 might occur partway through the sequence of messages generated by an
618 In simple Query mode, the format of retrieved values is always text,
619 except when the given command is a <command>FETCH</> from a cursor
620 declared with the <literal>BINARY</> option. In that case, the
621 retrieved values are in binary format. The format codes given in
622 the RowDescription message tell which format is being used.
626 A frontend must be prepared to accept ErrorResponse and
627 NoticeResponse messages whenever it is expecting any other type of
628 message. See also <xref linkend="protocol-async"> concerning messages
629 that the backend might generate due to outside events.
633 Recommended practice is to code frontends in a state-machine style
634 that will accept any message type at any time that it could make sense,
635 rather than wiring in assumptions about the exact sequence of messages.
639 <sect2 id="protocol-flow-ext-query">
640 <title>Extended Query</title>
643 The extended query protocol breaks down the above-described simple
644 query protocol into multiple steps. The results of preparatory
645 steps can be re-used multiple times for improved efficiency.
646 Furthermore, additional features are available, such as the possibility
647 of supplying data values as separate parameters instead of having to
648 insert them directly into a query string.
652 In the extended protocol, the frontend first sends a Parse message,
653 which contains a textual query string, optionally some information
654 about data types of parameter placeholders, and the
655 name of a destination prepared-statement object (an empty string
656 selects the unnamed prepared statement). The response is
657 either ParseComplete or ErrorResponse. Parameter data types can be
658 specified by OID; if not given, the parser attempts to infer the
659 data types in the same way as it would do for untyped literal string
665 A parameter data type can be left unspecified by setting it to zero,
666 or by making the array of parameter type OIDs shorter than the
667 number of parameter symbols (<literal>$</><replaceable>n</>)
668 used in the query string. Another special case is that a parameter's
669 type can be specified as <type>void</> (that is, the OID of the
670 <type>void</> pseudotype). This is meant to allow parameter symbols
671 to be used for function parameters that are actually OUT parameters.
672 Ordinarily there is no context in which a <type>void</> parameter
673 could be used, but if such a parameter symbol appears in a function's
674 parameter list, it is effectively ignored. For example, a function
675 call such as <literal>foo($1,$2,$3,$4)</> could match a function with
676 two IN and two OUT arguments, if <literal>$3</> and <literal>$4</>
677 are specified as having type <type>void</>.
683 The query string contained in a Parse message cannot include more
684 than one SQL statement; else a syntax error is reported. This
685 restriction does not exist in the simple-query protocol, but it
686 does exist in the extended protocol, because allowing prepared
687 statements or portals to contain multiple commands would complicate
693 If successfully created, a named prepared-statement object lasts till
694 the end of the current session, unless explicitly destroyed. An unnamed
695 prepared statement lasts only until the next Parse statement specifying
696 the unnamed statement as destination is issued. (Note that a simple
697 Query message also destroys the unnamed statement.) Named prepared
698 statements must be explicitly closed before they can be redefined by
699 another Parse message, but this is not required for the unnamed statement.
700 Named prepared statements can also be created and accessed at the SQL
701 command level, using <command>PREPARE</> and <command>EXECUTE</>.
705 Once a prepared statement exists, it can be readied for execution using a
706 Bind message. The Bind message gives the name of the source prepared
707 statement (empty string denotes the unnamed prepared statement), the name
708 of the destination portal (empty string denotes the unnamed portal), and
709 the values to use for any parameter placeholders present in the prepared
711 supplied parameter set must match those needed by the prepared statement.
712 (If you declared any <type>void</> parameters in the Parse message,
713 pass NULL values for them in the Bind message.)
714 Bind also specifies the format to use for any data returned
715 by the query; the format can be specified overall, or per-column.
716 The response is either BindComplete or ErrorResponse.
721 The choice between text and binary output is determined by the format
722 codes given in Bind, regardless of the SQL command involved. The
723 <literal>BINARY</> attribute in cursor declarations is irrelevant when
724 using extended query protocol.
729 Query planning typically occurs when the Bind message is processed.
730 If the prepared statement has no parameters, or is executed repeatedly,
731 the server might save the created plan and re-use it during subsequent
732 Bind messages for the same prepared statement. However, it will do so
733 only if it finds that a generic plan can be created that is not much
734 less efficient than a plan that depends on the specific parameter values
735 supplied. This happens transparently so far as the protocol is concerned.
739 If successfully created, a named portal object lasts till the end of the
740 current transaction, unless explicitly destroyed. An unnamed portal is
741 destroyed at the end of the transaction, or as soon as the next Bind
742 statement specifying the unnamed portal as destination is issued. (Note
743 that a simple Query message also destroys the unnamed portal.) Named
744 portals must be explicitly closed before they can be redefined by another
745 Bind message, but this is not required for the unnamed portal.
746 Named portals can also be created and accessed at the SQL
747 command level, using <command>DECLARE CURSOR</> and <command>FETCH</>.
751 Once a portal exists, it can be executed using an Execute message.
752 The Execute message specifies the portal name (empty string denotes the
754 a maximum result-row count (zero meaning <quote>fetch all rows</>).
755 The result-row count is only meaningful for portals
756 containing commands that return row sets; in other cases the command is
757 always executed to completion, and the row count is ignored.
759 responses to Execute are the same as those described above for queries
760 issued via simple query protocol, except that Execute doesn't cause
761 ReadyForQuery or RowDescription to be issued.
765 If Execute terminates before completing the execution of a portal
766 (due to reaching a nonzero result-row count), it will send a
767 PortalSuspended message; the appearance of this message tells the frontend
768 that another Execute should be issued against the same portal to
769 complete the operation. The CommandComplete message indicating
770 completion of the source SQL command is not sent until
771 the portal's execution is completed. Therefore, an Execute phase is
772 always terminated by the appearance of exactly one of these messages:
773 CommandComplete, EmptyQueryResponse (if the portal was created from
774 an empty query string), ErrorResponse, or PortalSuspended.
778 At completion of each series of extended-query messages, the frontend
779 should issue a Sync message. This parameterless message causes the
780 backend to close the current transaction if it's not inside a
781 <command>BEGIN</>/<command>COMMIT</> transaction block (<quote>close</>
782 meaning to commit if no error, or roll back if error). Then a
783 ReadyForQuery response is issued. The purpose of Sync is to provide
784 a resynchronization point for error recovery. When an error is detected
785 while processing any extended-query message, the backend issues
786 ErrorResponse, then reads and discards messages until a Sync is reached,
787 then issues ReadyForQuery and returns to normal message processing.
788 (But note that no skipping occurs if an error is detected
789 <emphasis>while</> processing Sync — this ensures that there is one
790 and only one ReadyForQuery sent for each Sync.)
795 Sync does not cause a transaction block opened with <command>BEGIN</>
796 to be closed. It is possible to detect this situation since the
797 ReadyForQuery message includes transaction status information.
802 In addition to these fundamental, required operations, there are several
803 optional operations that can be used with extended-query protocol.
807 The Describe message (portal variant) specifies the name of an existing
808 portal (or an empty string for the unnamed portal). The response is a
809 RowDescription message describing the rows that will be returned by
810 executing the portal; or a NoData message if the portal does not contain a
811 query that will return rows; or ErrorResponse if there is no such portal.
815 The Describe message (statement variant) specifies the name of an existing
816 prepared statement (or an empty string for the unnamed prepared
817 statement). The response is a ParameterDescription message describing the
818 parameters needed by the statement, followed by a RowDescription message
819 describing the rows that will be returned when the statement is eventually
820 executed (or a NoData message if the statement will not return rows).
821 ErrorResponse is issued if there is no such prepared statement. Note that
822 since Bind has not yet been issued, the formats to be used for returned
823 columns are not yet known to the backend; the format code fields in the
824 RowDescription message will be zeroes in this case.
829 In most scenarios the frontend should issue one or the other variant
830 of Describe before issuing Execute, to ensure that it knows how to
831 interpret the results it will get back.
836 The Close message closes an existing prepared statement or portal
837 and releases resources. It is not an error to issue Close against
838 a nonexistent statement or portal name. The response is normally
839 CloseComplete, but could be ErrorResponse if some difficulty is
840 encountered while releasing resources. Note that closing a prepared
841 statement implicitly closes any open portals that were constructed
846 The Flush message does not cause any specific output to be generated,
847 but forces the backend to deliver any data pending in its output
848 buffers. A Flush must be sent after any extended-query command except
849 Sync, if the frontend wishes to examine the results of that command before
850 issuing more commands. Without Flush, messages returned by the backend
851 will be combined into the minimum possible number of packets to minimize
857 The simple Query message is approximately equivalent to the series Parse,
858 Bind, portal Describe, Execute, Close, Sync, using the unnamed prepared
859 statement and portal objects and no parameters. One difference is that
860 it will accept multiple SQL statements in the query string, automatically
861 performing the bind/describe/execute sequence for each one in succession.
862 Another difference is that it will not return ParseComplete, BindComplete,
863 CloseComplete, or NoData messages.
869 <title>Function Call</title>
872 The Function Call sub-protocol allows the client to request a direct
873 call of any function that exists in the database's
874 <structname>pg_proc</structname> system catalog. The client must have
875 execute permission for the function.
880 The Function Call sub-protocol is a legacy feature that is probably best
881 avoided in new code. Similar results can be accomplished by setting up
882 a prepared statement that does <literal>SELECT function($1, ...)</>.
883 The Function Call cycle can then be replaced with Bind/Execute.
888 A Function Call cycle is initiated by the frontend sending a
889 FunctionCall message to the backend. The backend then sends one
890 or more response messages depending on the results of the function
891 call, and finally a ReadyForQuery response message. ReadyForQuery
892 informs the frontend that it can safely send a new query or
897 The possible response messages from the backend are:
901 <term>ErrorResponse</term>
904 An error has occurred.
910 <term>FunctionCallResponse</term>
913 The function call was completed and returned the result given
915 (Note that the Function Call protocol can only handle a single
916 scalar result, not a row type or set of results.)
922 <term>ReadyForQuery</term>
925 Processing of the function call is complete. ReadyForQuery
926 will always be sent, whether processing terminates
927 successfully or with an error.
933 <term>NoticeResponse</term>
936 A warning message has been issued in relation to the function
937 call. Notices are in addition to other responses, i.e., the
938 backend will continue processing the command.
946 <sect2 id="protocol-copy">
947 <title>COPY Operations</title>
950 The <command>COPY</> command allows high-speed bulk data transfer
951 to or from the server. Copy-in and copy-out operations each switch
952 the connection into a distinct sub-protocol, which lasts until the
953 operation is completed.
957 Copy-in mode (data transfer to the server) is initiated when the
958 backend executes a <command>COPY FROM STDIN</> SQL statement. The backend
959 sends a CopyInResponse message to the frontend. The frontend should
960 then send zero or more CopyData messages, forming a stream of input
961 data. (The message boundaries are not required to have anything to do
962 with row boundaries, although that is often a reasonable choice.)
963 The frontend can terminate the copy-in mode by sending either a CopyDone
964 message (allowing successful termination) or a CopyFail message (which
965 will cause the <command>COPY</> SQL statement to fail with an
966 error). The backend then reverts to the command-processing mode it was
967 in before the <command>COPY</> started, which will be either simple or
968 extended query protocol. It will next send either CommandComplete
969 (if successful) or ErrorResponse (if not).
973 In the event of a backend-detected error during copy-in mode (including
974 receipt of a CopyFail message), the backend will issue an ErrorResponse
975 message. If the <command>COPY</> command was issued via an extended-query
976 message, the backend will now discard frontend messages until a Sync
977 message is received, then it will issue ReadyForQuery and return to normal
978 processing. If the <command>COPY</> command was issued in a simple
979 Query message, the rest of that message is discarded and ReadyForQuery
980 is issued. In either case, any subsequent CopyData, CopyDone, or CopyFail
981 messages issued by the frontend will simply be dropped.
985 The backend will ignore Flush and Sync messages received during copy-in
986 mode. Receipt of any other non-copy message type constitutes an error
987 that will abort the copy-in state as described above. (The exception for
988 Flush and Sync is for the convenience of client libraries that always
989 send Flush or Sync after an Execute message, without checking whether
990 the command to be executed is a <command>COPY FROM STDIN</>.)
994 Copy-out mode (data transfer from the server) is initiated when the
995 backend executes a <command>COPY TO STDOUT</> SQL statement. The backend
996 sends a CopyOutResponse message to the frontend, followed by
997 zero or more CopyData messages (always one per row), followed by CopyDone.
998 The backend then reverts to the command-processing mode it was
999 in before the <command>COPY</> started, and sends CommandComplete.
1000 The frontend cannot abort the transfer (except by closing the connection
1001 or issuing a Cancel request),
1002 but it can discard unwanted CopyData and CopyDone messages.
1006 In the event of a backend-detected error during copy-out mode,
1007 the backend will issue an ErrorResponse message and revert to normal
1008 processing. The frontend should treat receipt of ErrorResponse as
1009 terminating the copy-out mode.
1013 It is possible for NoticeResponse and ParameterStatus messages to be
1014 interspersed between CopyData messages; frontends must handle these cases,
1015 and should be prepared for other asynchronous message types as well (see
1016 <xref linkend="protocol-async">). Otherwise, any message type other than
1017 CopyData or CopyDone may be treated as terminating copy-out mode.
1021 There is another Copy-related mode called Copy-both, which allows
1022 high-speed bulk data transfer to <emphasis>and</> from the server.
1023 Copy-both mode is initiated when a backend in walsender mode
1024 executes a <command>START_REPLICATION</command> statement. The
1025 backend sends a CopyBothResponse message to the frontend. Both
1026 the backend and the frontend may then send CopyData messages
1027 until the connection is terminated. See <xref
1028 linkend="protocol-replication">.
1032 The CopyInResponse, CopyOutResponse and CopyBothResponse messages
1033 include fields that inform the frontend of the number of columns
1034 per row and the format codes being used for each column. (As of
1035 the present implementation, all columns in a given <command>COPY</>
1036 operation will use the same format, but the message design does not
1042 <sect2 id="protocol-async">
1043 <title>Asynchronous Operations</title>
1046 There are several cases in which the backend will send messages that
1047 are not specifically prompted by the frontend's command stream.
1048 Frontends must be prepared to deal with these messages at any time,
1049 even when not engaged in a query.
1050 At minimum, one should check for these cases before beginning to
1051 read a query response.
1055 It is possible for NoticeResponse messages to be generated due to
1056 outside activity; for example, if the database administrator commands
1057 a <quote>fast</> database shutdown, the backend will send a NoticeResponse
1058 indicating this fact before closing the connection. Accordingly,
1059 frontends should always be prepared to accept and display NoticeResponse
1060 messages, even when the connection is nominally idle.
1064 ParameterStatus messages will be generated whenever the active
1065 value changes for any of the parameters the backend believes the
1066 frontend should know about. Most commonly this occurs in response
1067 to a <command>SET</> SQL command executed by the frontend, and
1068 this case is effectively synchronous — but it is also possible
1069 for parameter status changes to occur because the administrator
1070 changed a configuration file and then sent the
1071 <systemitem>SIGHUP</systemitem> signal to the server. Also,
1072 if a <command>SET</command> command is rolled back, an appropriate
1073 ParameterStatus message will be generated to report the current
1078 At present there is a hard-wired set of parameters for which
1079 ParameterStatus will be generated: they are
1080 <varname>server_version</>,
1081 <varname>server_encoding</>,
1082 <varname>client_encoding</>,
1083 <varname>application_name</>,
1084 <varname>is_superuser</>,
1085 <varname>session_authorization</>,
1086 <varname>DateStyle</>,
1087 <varname>IntervalStyle</>,
1088 <varname>TimeZone</>,
1089 <varname>integer_datetimes</>, and
1090 <varname>standard_conforming_strings</>.
1091 (<varname>server_encoding</>, <varname>TimeZone</>, and
1092 <varname>integer_datetimes</> were not reported by releases before 8.0;
1093 <varname>standard_conforming_strings</> was not reported by releases
1095 <varname>IntervalStyle</> was not reported by releases before 8.4;
1096 <varname>application_name</> was not reported by releases before 9.0.)
1098 <varname>server_version</>,
1099 <varname>server_encoding</> and
1100 <varname>integer_datetimes</>
1101 are pseudo-parameters that cannot change after startup.
1102 This set might change in the future, or even become configurable.
1103 Accordingly, a frontend should simply ignore ParameterStatus for
1104 parameters that it does not understand or care about.
1108 If a frontend issues a <command>LISTEN</command> command, then the
1109 backend will send a NotificationResponse message (not to be
1110 confused with NoticeResponse!) whenever a
1111 <command>NOTIFY</command> command is executed for the same
1117 At present, NotificationResponse can only be sent outside a
1118 transaction, and thus it will not occur in the middle of a
1119 command-response series, though it might occur just before ReadyForQuery.
1120 It is unwise to design frontend logic that assumes that, however.
1121 Good practice is to be able to accept NotificationResponse at any
1122 point in the protocol.
1128 <title>Canceling Requests in Progress</title>
1131 During the processing of a query, the frontend might request
1132 cancellation of the query. The cancel request is not sent
1133 directly on the open connection to the backend for reasons of
1134 implementation efficiency: we don't want to have the backend
1135 constantly checking for new input from the frontend during query
1136 processing. Cancel requests should be relatively infrequent, so
1137 we make them slightly cumbersome in order to avoid a penalty in
1142 To issue a cancel request, the frontend opens a new connection to
1143 the server and sends a CancelRequest message, rather than the
1144 StartupMessage message that would ordinarily be sent across a new
1145 connection. The server will process this request and then close
1146 the connection. For security reasons, no direct reply is made to
1147 the cancel request message.
1151 A CancelRequest message will be ignored unless it contains the
1152 same key data (PID and secret key) passed to the frontend during
1153 connection start-up. If the request matches the PID and secret
1154 key for a currently executing backend, the processing of the
1155 current query is aborted. (In the existing implementation, this is
1156 done by sending a special signal to the backend process that is
1157 processing the query.)
1161 The cancellation signal might or might not have any effect — for
1162 example, if it arrives after the backend has finished processing
1163 the query, then it will have no effect. If the cancellation is
1164 effective, it results in the current command being terminated
1165 early with an error message.
1169 The upshot of all this is that for reasons of both security and
1170 efficiency, the frontend has no direct way to tell whether a
1171 cancel request has succeeded. It must continue to wait for the
1172 backend to respond to the query. Issuing a cancel simply improves
1173 the odds that the current query will finish soon, and improves the
1174 odds that it will fail with an error message instead of
1179 Since the cancel request is sent across a new connection to the
1180 server and not across the regular frontend/backend communication
1181 link, it is possible for the cancel request to be issued by any
1182 process, not just the frontend whose query is to be canceled.
1183 This might provide additional flexibility when building
1184 multiple-process applications. It also introduces a security
1185 risk, in that unauthorized persons might try to cancel queries.
1186 The security risk is addressed by requiring a dynamically
1187 generated secret key to be supplied in cancel requests.
1192 <title>Termination</title>
1195 The normal, graceful termination procedure is that the frontend
1196 sends a Terminate message and immediately closes the connection.
1197 On receipt of this message, the backend closes the connection and
1202 In rare cases (such as an administrator-commanded database shutdown)
1203 the backend might disconnect without any frontend request to do so.
1204 In such cases the backend will attempt to send an error or notice message
1205 giving the reason for the disconnection before it closes the connection.
1209 Other termination scenarios arise from various failure cases, such as core
1210 dump at one end or the other, loss of the communications link, loss of
1211 message-boundary synchronization, etc. If either frontend or backend sees
1212 an unexpected closure of the connection, it should clean
1213 up and terminate. The frontend has the option of launching a new backend
1214 by recontacting the server if it doesn't want to terminate itself.
1215 Closing the connection is also advisable if an unrecognizable message type
1216 is received, since this probably indicates loss of message-boundary sync.
1220 For either normal or abnormal termination, any open transaction is
1221 rolled back, not committed. One should note however that if a
1222 frontend disconnects while a non-<command>SELECT</command> query
1223 is being processed, the backend will probably finish the query
1224 before noticing the disconnection. If the query is outside any
1225 transaction block (<command>BEGIN</> ... <command>COMMIT</>
1226 sequence) then its results might be committed before the
1227 disconnection is recognized.
1232 <title><acronym>SSL</acronym> Session Encryption</title>
1235 If <productname>PostgreSQL</> was built with
1236 <acronym>SSL</acronym> support, frontend/backend communications
1237 can be encrypted using <acronym>SSL</acronym>. This provides
1238 communication security in environments where attackers might be
1239 able to capture the session traffic. For more information on
1240 encrypting <productname>PostgreSQL</productname> sessions with
1241 <acronym>SSL</acronym>, see <xref linkend="ssl-tcp">.
1245 To initiate an <acronym>SSL</acronym>-encrypted connection, the
1246 frontend initially sends an SSLRequest message rather than a
1247 StartupMessage. The server then responds with a single byte
1248 containing <literal>S</> or <literal>N</>, indicating that it is
1249 willing or unwilling to perform <acronym>SSL</acronym>,
1250 respectively. The frontend might close the connection at this point
1251 if it is dissatisfied with the response. To continue after
1252 <literal>S</>, perform an <acronym>SSL</acronym> startup handshake
1253 (not described here, part of the <acronym>SSL</acronym>
1254 specification) with the server. If this is successful, continue
1255 with sending the usual StartupMessage. In this case the
1256 StartupMessage and all subsequent data will be
1257 <acronym>SSL</acronym>-encrypted. To continue after
1258 <literal>N</>, send the usual StartupMessage and proceed without
1263 The frontend should also be prepared to handle an ErrorMessage
1264 response to SSLRequest from the server. This would only occur if
1265 the server predates the addition of <acronym>SSL</acronym> support
1266 to <productname>PostgreSQL</>. (Such servers are now very ancient,
1267 and likely do not exist in the wild anymore.)
1268 In this case the connection must
1269 be closed, but the frontend might choose to open a fresh connection
1270 and proceed without requesting <acronym>SSL</acronym>.
1274 An initial SSLRequest can also be used in a connection that is being
1275 opened to send a CancelRequest message.
1279 While the protocol itself does not provide a way for the server to
1280 force <acronym>SSL</acronym> encryption, the administrator can
1281 configure the server to reject unencrypted sessions as a byproduct
1282 of authentication checking.
1287 <sect1 id="protocol-replication">
1288 <title>Streaming Replication Protocol</title>
1291 To initiate streaming replication, the frontend sends the
1292 <literal>replication</> parameter in the startup message. This tells the
1293 backend to go into walsender mode, wherein a small set of replication commands
1294 can be issued instead of SQL statements. Only the simple query protocol can be
1295 used in walsender mode.
1297 The commands accepted in walsender mode are:
1301 <term>IDENTIFY_SYSTEM</term>
1304 Requests the server to identify itself. Server replies with a result
1305 set of a single row, containing three fields:
1316 The unique system identifier identifying the cluster. This
1317 can be used to check that the base backup used to initialize the
1318 standby came from the same cluster.
1329 Current TimelineID. Also useful to check that the standby is
1330 consistent with the master.
1341 Current xlog write location. Useful to get a known location in the
1342 transaction log where streaming can start.
1353 <term>START_REPLICATION <replaceable>XXX</>/<replaceable>XXX</></term>
1356 Instructs server to start streaming WAL, starting at
1357 WAL position <replaceable>XXX</>/<replaceable>XXX</>.
1358 The server can reply with an error, e.g. if the requested section of WAL
1359 has already been recycled. On success, server responds with a
1360 CopyBothResponse message, and then starts to stream WAL to the frontend.
1361 WAL will continue to be streamed until the connection is broken;
1362 no further commands will be accepted.
1366 WAL data is sent as a series of CopyData messages. (This allows
1367 other information to be intermixed; in particular the server can send
1368 an ErrorResponse message if it encounters a failure after beginning
1369 to stream.) The payload in each CopyData message follows this format:
1387 Identifies the message as WAL data.
1397 The starting point of the WAL data in this message, given in
1408 The current end of WAL on the server, given in
1419 The server's system clock at the time of transmission,
1420 given in TimestampTz format.
1426 Byte<replaceable>n</replaceable>
1430 A section of the WAL data stream.
1441 A single WAL record is never split across two CopyData messages.
1442 When a WAL record crosses a WAL page boundary, and is therefore
1443 already split using continuation records, it can be split at the page
1444 boundary. In other words, the first main WAL record and its
1445 continuation records can be sent in different CopyData messages.
1448 Note that all fields within the WAL data and the above-described header
1449 will be in the sending server's native format. Endianness, and the
1450 format for the timestamp, are unpredictable unless the receiver has
1451 verified that the sender's system identifier matches its own
1452 <filename>pg_control</> contents.
1455 If the WAL sender process is terminated normally (during postmaster
1456 shutdown), it will send a CommandComplete message before exiting.
1457 This might not happen during an abnormal shutdown, of course.
1461 The receiving process can send replies back to the sender at any time,
1462 using one of the following message formats (also in the payload of a
1470 Primary keepalive message (B)
1481 Identifies the message as a sender keepalive.
1491 The current end of WAL on the server, given in
1502 The server's system clock at the time of transmission,
1503 given in TimestampTz format.
1518 Standby status update (F)
1529 Identifies the message as a receiver status update.
1539 The location of the last WAL byte + 1 received and written to disk
1540 in the standby, in XLogRecPtr format.
1550 The location of the last WAL byte + 1 flushed to disk in
1551 the standby, in XLogRecPtr format.
1561 The location of the last WAL byte + 1 applied in the standby, in
1572 The server's system clock at the time of transmission,
1573 given in TimestampTz format.
1588 Hot Standby feedback message (F)
1599 Identifies the message as a Hot Standby feedback message.
1609 The server's system clock at the time of transmission,
1610 given in TimestampTz format.
1620 The standby's current xmin.
1630 The standby's current epoch.
1644 <term>BASE_BACKUP [<literal>LABEL</literal> <replaceable>'label'</replaceable>] [<literal>PROGRESS</literal>] [<literal>FAST</literal>] [<literal>WAL</literal>] [<literal>NOWAIT</literal>]</term>
1647 Instructs the server to start streaming a base backup.
1648 The system will automatically be put in backup mode before the backup
1649 is started, and taken out of it when the backup is complete. The
1650 following options are accepted:
1653 <term><literal>LABEL</literal> <replaceable>'label'</replaceable></term>
1656 Sets the label of the backup. If none is specified, a backup label
1657 of <literal>base backup</literal> will be used. The quoting rules
1658 for the label are the same as a standard SQL string with
1659 <xref linkend="guc-standard-conforming-strings"> turned on.
1665 <term><literal>PROGRESS</></term>
1668 Request information required to generate a progress report. This will
1669 send back an approximate size in the header of each tablespace, which
1670 can be used to calculate how far along the stream is done. This is
1671 calculated by enumerating all the file sizes once before the transfer
1672 is even started, and may as such have a negative impact on the
1673 performance - in particular it may take longer before the first data
1674 is streamed. Since the database files can change during the backup,
1675 the size is only approximate and may both grow and shrink between
1676 the time of approximation and the sending of the actual files.
1682 <term><literal>FAST</></term>
1685 Request a fast checkpoint.
1691 <term><literal>WAL</literal></term>
1694 Include the necessary WAL segments in the backup. This will include
1695 all the files between start and stop backup in the
1696 <filename>pg_xlog</filename> directory of the base directory tar
1703 <term><literal>NOWAIT</literal></term>
1706 By default, the backup will wait until the last required xlog
1707 segment has been archived, or emit a warning if log archiving is
1708 not enabled. Specifying <literal>NOWAIT</literal> disables both
1709 the waiting and the warning, leaving the client responsible for
1710 ensuring the required log is available.
1717 When the backup is started, the server will first send two
1718 ordinary result sets, followed by one or more CopyResponse
1722 The first ordinary result set contains the starting position of the
1723 backup, given in XLogRecPtr format as a single column in a single row.
1726 The second ordinary result set has one row for each tablespace.
1727 The fields in this row are:
1733 The oid of the tablespace, or <literal>NULL</> if it's the base
1739 <term>spclocation</term>
1742 The full path of the tablespace directory, or <literal>NULL</>
1743 if it's the base directory.
1751 The approximate size of the tablespace, if progress report has
1752 been requested; otherwise it's <literal>NULL</>.
1759 After the second regular result set, one or more CopyResponse results
1760 will be sent, one for PGDATA and one for each additional tablespace other
1761 than <literal>pg_default</> and <literal>pg_global</>. The data in
1762 the CopyResponse results will be a tar format (using ustar00
1763 extensions) dump of the tablespace contents. After the tar data is
1764 complete, a final ordinary result set will be sent.
1768 The tar archive for the data directory and each tablespace will contain
1769 all files in the directories, regardless of whether they are
1770 <productname>PostgreSQL</> files or other files added to the same
1771 directory. The only excluded files are:
1772 <itemizedlist spacing="compact" mark="bullet">
1775 <filename>postmaster.pid</>
1780 <filename>postmaster.opts</>
1785 <filename>pg_xlog</>, including subdirectories. If the backup is run
1786 with WAL files included, a synthesized version of <filename>pg_xlog</filename> will be
1787 included, but it will only contain the files necessary for the
1788 backup to work, not the rest of the contents.
1792 Owner, group and file mode are set if the underlying file system on
1793 the server supports it.
1796 Once all tablespaces have been sent, a final regular result set will
1797 be sent. This result set contains the end position of the
1798 backup, given in XLogRecPtr format as a single column in a single row.
1808 <sect1 id="protocol-message-types">
1809 <title>Message Data Types</title>
1812 This section describes the base data types used in messages.
1818 Int<replaceable>n</replaceable>(<replaceable>i</replaceable>)
1822 An <replaceable>n</replaceable>-bit signed integer in network byte
1823 order (most significant byte first).
1824 If <replaceable>i</replaceable> is specified it
1825 is the exact value that will appear, otherwise the value
1826 is variable. Eg. Int16, Int32(42).
1833 Int<replaceable>n</replaceable>[<replaceable>k</replaceable>]
1837 An array of <replaceable>k</replaceable>
1838 <replaceable>n</replaceable>-bit integers, each in network
1839 byte order. The array length <replaceable>k</replaceable>
1840 is always determined by an earlier field in the message.
1848 String(<replaceable>s</replaceable>)
1852 A null-terminated string (C-style string). There is no
1853 specific length limitation on strings.
1854 If <replaceable>s</replaceable> is specified it is the exact
1855 value that will appear, otherwise the value is variable.
1856 Eg. String, String("user").
1861 <emphasis>There is no predefined limit</emphasis> on the length of a string
1862 that can be returned by the backend. Good coding strategy for a frontend
1863 is to use an expandable buffer so that anything that fits in memory can be
1864 accepted. If that's not feasible, read the full string and discard trailing
1865 characters that don't fit into your fixed-size buffer.
1873 Byte<replaceable>n</replaceable>(<replaceable>c</replaceable>)
1877 Exactly <replaceable>n</replaceable> bytes. If the field
1878 width <replaceable>n</replaceable> is not a constant, it is
1879 always determinable from an earlier field in the message.
1880 If <replaceable>c</replaceable> is specified it is the exact
1881 value. Eg. Byte2, Byte1('\n').
1890 <sect1 id="protocol-message-formats">
1891 <title>Message Formats</title>
1894 This section describes the detailed format of each message. Each is marked to
1895 indicate that it can be sent by a frontend (F), a backend (B), or both
1897 Notice that although each message includes a byte count at the beginning,
1898 the message format is defined so that the message end can be found without
1899 reference to the byte count. This aids validity checking. (The CopyData
1900 message is an exception, because it forms part of a data stream; the contents
1901 of any individual CopyData message cannot be interpretable on their own.)
1909 AuthenticationOk (B)
1921 Identifies the message as an authentication request.
1931 Length of message contents in bytes, including self.
1941 Specifies that the authentication was successful.
1954 AuthenticationKerberosV5 (B)
1966 Identifies the message as an authentication request.
1976 Length of message contents in bytes, including self.
1986 Specifies that Kerberos V5 authentication is required.
1998 AuthenticationCleartextPassword (B)
2010 Identifies the message as an authentication request.
2020 Length of message contents in bytes, including self.
2030 Specifies that a clear-text password is required.
2042 AuthenticationMD5Password (B)
2054 Identifies the message as an authentication request.
2064 Length of message contents in bytes, including self.
2074 Specifies that an MD5-encrypted password is required.
2084 The salt to use when encrypting the password.
2097 AuthenticationSCMCredential (B)
2109 Identifies the message as an authentication request.
2119 Length of message contents in bytes, including self.
2129 Specifies that an SCM credentials message is required.
2142 AuthenticationGSS (B)
2154 Identifies the message as an authentication request.
2164 Length of message contents in bytes, including self.
2174 Specifies that GSSAPI authentication is required.
2187 AuthenticationSSPI (B)
2199 Identifies the message as an authentication request.
2209 Length of message contents in bytes, including self.
2219 Specifies that SSPI authentication is required.
2230 AuthenticationGSSContinue (B)
2242 Identifies the message as an authentication request.
2252 Length of message contents in bytes, including self.
2262 Specifies that this message contains GSSAPI or SSPI data.
2268 Byte<replaceable>n</replaceable>
2272 GSSAPI or SSPI authentication data.
2297 Identifies the message as cancellation key data.
2298 The frontend must save these values if it wishes to be
2299 able to issue CancelRequest messages later.
2309 Length of message contents in bytes, including self.
2319 The process ID of this backend.
2329 The secret key of this backend.
2354 Identifies the message as a Bind command.
2364 Length of message contents in bytes, including self.
2374 The name of the destination portal
2375 (an empty string selects the unnamed portal).
2385 The name of the source prepared statement
2386 (an empty string selects the unnamed prepared statement).
2396 The number of parameter format codes that follow
2397 (denoted <replaceable>C</> below).
2398 This can be zero to indicate that there are no parameters
2399 or that the parameters all use the default format (text);
2400 or one, in which case the specified format code is applied
2401 to all parameters; or it can equal the actual number of
2408 Int16[<replaceable>C</>]
2412 The parameter format codes. Each must presently be
2413 zero (text) or one (binary).
2423 The number of parameter values that follow (possibly zero).
2424 This must match the number of parameters needed by the query.
2429 Next, the following pair of fields appear for each parameter:
2437 The length of the parameter value, in bytes (this count
2438 does not include itself). Can be zero.
2439 As a special case, -1 indicates a NULL parameter value.
2440 No value bytes follow in the NULL case.
2446 Byte<replaceable>n</replaceable>
2450 The value of the parameter, in the format indicated by the
2451 associated format code.
2452 <replaceable>n</replaceable> is the above length.
2457 After the last parameter, the following fields appear:
2465 The number of result-column format codes that follow
2466 (denoted <replaceable>R</> below).
2467 This can be zero to indicate that there are no result columns
2468 or that the result columns should all use the default format
2470 or one, in which case the specified format code is applied
2471 to all result columns (if any); or it can equal the actual
2472 number of result columns of the query.
2478 Int16[<replaceable>R</>]
2482 The result-column format codes. Each must presently be
2483 zero (text) or one (binary).
2507 Identifies the message as a Bind-complete indicator.
2517 Length of message contents in bytes, including self.
2542 Length of message contents in bytes, including self.
2552 The cancel request code. The value is chosen to contain
2553 <literal>1234</> in the most significant 16 bits, and <literal>5678</> in the
2554 least 16 significant bits. (To avoid confusion, this code
2555 must not be the same as any protocol version number.)
2565 The process ID of the target backend.
2575 The secret key for the target backend.
2600 Identifies the message as a Close command.
2610 Length of message contents in bytes, including self.
2620 '<literal>S</>' to close a prepared statement; or
2621 '<literal>P</>' to close a portal.
2631 The name of the prepared statement or portal to close
2632 (an empty string selects the unnamed prepared statement
2657 Identifies the message as a Close-complete indicator.
2667 Length of message contents in bytes, including self.
2692 Identifies the message as a command-completed response.
2702 Length of message contents in bytes, including self.
2712 The command tag. This is usually a single
2713 word that identifies which SQL command was completed.
2717 For an <command>INSERT</command> command, the tag is
2718 <literal>INSERT <replaceable>oid</replaceable>
2719 <replaceable>rows</replaceable></literal>, where
2720 <replaceable>rows</replaceable> is the number of rows
2721 inserted. <replaceable>oid</replaceable> is the object ID
2722 of the inserted row if <replaceable>rows</replaceable> is 1
2723 and the target table has OIDs;
2724 otherwise <replaceable>oid</replaceable> is 0.
2728 For a <command>DELETE</command> command, the tag is
2729 <literal>DELETE <replaceable>rows</replaceable></literal> where
2730 <replaceable>rows</replaceable> is the number of rows deleted.
2734 For an <command>UPDATE</command> command, the tag is
2735 <literal>UPDATE <replaceable>rows</replaceable></literal> where
2736 <replaceable>rows</replaceable> is the number of rows updated.
2740 For a <command>SELECT</command> or <command>CREATE TABLE AS</command>
2741 command, the tag is <literal>SELECT <replaceable>rows</replaceable></literal>
2742 where <replaceable>rows</replaceable> is the number of rows retrieved.
2746 For a <command>MOVE</command> command, the tag is
2747 <literal>MOVE <replaceable>rows</replaceable></literal> where
2748 <replaceable>rows</replaceable> is the number of rows the
2749 cursor's position has been changed by.
2753 For a <command>FETCH</command> command, the tag is
2754 <literal>FETCH <replaceable>rows</replaceable></literal> where
2755 <replaceable>rows</replaceable> is the number of rows that
2756 have been retrieved from the cursor.
2760 For a <command>COPY</command> command, the tag is
2761 <literal>COPY <replaceable>rows</replaceable></literal> where
2762 <replaceable>rows</replaceable> is the number of rows copied.
2763 (Note: the row count appears only in
2764 <productname>PostgreSQL</productname> 8.2 and later.)
2778 CopyData (F & B)
2789 Identifies the message as <command>COPY</command> data.
2799 Length of message contents in bytes, including self.
2805 Byte<replaceable>n</replaceable>
2809 Data that forms part of a <command>COPY</command> data stream. Messages sent
2810 from the backend will always correspond to single data rows,
2811 but messages sent by frontends might divide the data stream
2824 CopyDone (F & B)
2836 Identifies the message as a <command>COPY</command>-complete indicator.
2846 Length of message contents in bytes, including self.
2871 Identifies the message as a <command>COPY</command>-failure indicator.
2881 Length of message contents in bytes, including self.
2891 An error message to report as the cause of failure.
2916 Identifies the message as a Start Copy In response.
2917 The frontend must now send copy-in data (if not
2918 prepared to do so, send a CopyFail message).
2928 Length of message contents in bytes, including self.
2938 0 indicates the overall <command>COPY</command> format is textual (rows
2939 separated by newlines, columns separated by separator
2941 1 indicates the overall copy format is binary (similar
2943 See <xref linkend="sql-copy">
2944 for more information.
2954 The number of columns in the data to be copied
2955 (denoted <replaceable>N</> below).
2961 Int16[<replaceable>N</>]
2965 The format codes to be used for each column.
2966 Each must presently be zero (text) or one (binary).
2967 All must be zero if the overall copy format is textual.
2992 Identifies the message as a Start Copy Out response.
2993 This message will be followed by copy-out data.
3003 Length of message contents in bytes, including self.
3013 0 indicates the overall <command>COPY</command> format
3014 is textual (rows separated by newlines, columns
3015 separated by separator characters, etc). 1 indicates
3016 the overall copy format is binary (similar to DataRow
3017 format). See <xref linkend="sql-copy"> for more information.
3027 The number of columns in the data to be copied
3028 (denoted <replaceable>N</> below).
3034 Int16[<replaceable>N</>]
3038 The format codes to be used for each column.
3039 Each must presently be zero (text) or one (binary).
3040 All must be zero if the overall copy format is textual.
3053 CopyBothResponse (B)
3065 Identifies the message as a Start Copy Both response.
3066 This message is used only for Streaming Replication.
3076 Length of message contents in bytes, including self.
3086 0 indicates the overall <command>COPY</command> format
3087 is textual (rows separated by newlines, columns
3088 separated by separator characters, etc). 1 indicates
3089 the overall copy format is binary (similar to DataRow
3090 format). See <xref linkend="sql-copy"> for more information.
3100 The number of columns in the data to be copied
3101 (denoted <replaceable>N</> below).
3107 Int16[<replaceable>N</>]
3111 The format codes to be used for each column.
3112 Each must presently be zero (text) or one (binary).
3113 All must be zero if the overall copy format is textual.
3137 Identifies the message as a data row.
3147 Length of message contents in bytes, including self.
3157 The number of column values that follow (possibly zero).
3162 Next, the following pair of fields appear for each column:
3170 The length of the column value, in bytes (this count
3171 does not include itself). Can be zero.
3172 As a special case, -1 indicates a NULL column value.
3173 No value bytes follow in the NULL case.
3179 Byte<replaceable>n</replaceable>
3183 The value of the column, in the format indicated by the
3184 associated format code.
3185 <replaceable>n</replaceable> is the above length.
3210 Identifies the message as a Describe command.
3220 Length of message contents in bytes, including self.
3230 '<literal>S</>' to describe a prepared statement; or
3231 '<literal>P</>' to describe a portal.
3241 The name of the prepared statement or portal to describe
3242 (an empty string selects the unnamed prepared statement
3255 EmptyQueryResponse (B)
3267 Identifies the message as a response to an empty query string.
3268 (This substitutes for CommandComplete.)
3278 Length of message contents in bytes, including self.
3303 Identifies the message as an error.
3313 Length of message contents in bytes, including self.
3318 The message body consists of one or more identified fields,
3319 followed by a zero byte as a terminator. Fields can appear in
3320 any order. For each field there is the following:
3328 A code identifying the field type; if zero, this is
3329 the message terminator and no string follows.
3330 The presently defined field types are listed in
3331 <xref linkend="protocol-error-fields">.
3332 Since more field types might be added in future,
3333 frontends should silently ignore fields of unrecognized
3369 Identifies the message as an Execute command.
3379 Length of message contents in bytes, including self.
3389 The name of the portal to execute
3390 (an empty string selects the unnamed portal).
3400 Maximum number of rows to return, if portal contains
3401 a query that returns rows (ignored otherwise). Zero
3402 denotes <quote>no limit</>.
3426 Identifies the message as a Flush command.
3436 Length of message contents in bytes, including self.
3461 Identifies the message as a function call.
3471 Length of message contents in bytes, including self.
3481 Specifies the object ID of the function to call.
3491 The number of argument format codes that follow
3492 (denoted <replaceable>C</> below).
3493 This can be zero to indicate that there are no arguments
3494 or that the arguments all use the default format (text);
3495 or one, in which case the specified format code is applied
3496 to all arguments; or it can equal the actual number of
3503 Int16[<replaceable>C</>]
3507 The argument format codes. Each must presently be
3508 zero (text) or one (binary).
3518 Specifies the number of arguments being supplied to the
3524 Next, the following pair of fields appear for each argument:
3532 The length of the argument value, in bytes (this count
3533 does not include itself). Can be zero.
3534 As a special case, -1 indicates a NULL argument value.
3535 No value bytes follow in the NULL case.
3541 Byte<replaceable>n</replaceable>
3545 The value of the argument, in the format indicated by the
3546 associated format code.
3547 <replaceable>n</replaceable> is the above length.
3552 After the last argument, the following field appears:
3560 The format code for the function result. Must presently be
3561 zero (text) or one (binary).
3574 FunctionCallResponse (B)
3586 Identifies the message as a function call result.
3596 Length of message contents in bytes, including self.
3606 The length of the function result value, in bytes (this count
3607 does not include itself). Can be zero.
3608 As a special case, -1 indicates a NULL function result.
3609 No value bytes follow in the NULL case.
3615 Byte<replaceable>n</replaceable>
3619 The value of the function result, in the format indicated by
3620 the associated format code.
3621 <replaceable>n</replaceable> is the above length.
3646 Identifies the message as a no-data indicator.
3656 Length of message contents in bytes, including self.
3681 Identifies the message as a notice.
3691 Length of message contents in bytes, including self.
3696 The message body consists of one or more identified fields,
3697 followed by a zero byte as a terminator. Fields can appear in
3698 any order. For each field there is the following:
3706 A code identifying the field type; if zero, this is
3707 the message terminator and no string follows.
3708 The presently defined field types are listed in
3709 <xref linkend="protocol-error-fields">.
3710 Since more field types might be added in future,
3711 frontends should silently ignore fields of unrecognized
3735 NotificationResponse (B)
3747 Identifies the message as a notification response.
3757 Length of message contents in bytes, including self.
3767 The process ID of the notifying backend process.
3777 The name of the channel that the notify has been raised on.
3787 The <quote>payload</> string passed from the notifying process.
3800 ParameterDescription (B)
3812 Identifies the message as a parameter description.
3822 Length of message contents in bytes, including self.
3832 The number of parameters used by the statement
3838 Then, for each parameter, there is the following:
3846 Specifies the object ID of the parameter data type.
3870 Identifies the message as a run-time parameter status report.
3880 Length of message contents in bytes, including self.
3890 The name of the run-time parameter being reported.
3900 The current value of the parameter.
3924 Identifies the message as a Parse command.
3934 Length of message contents in bytes, including self.
3944 The name of the destination prepared statement
3945 (an empty string selects the unnamed prepared statement).
3955 The query string to be parsed.
3965 The number of parameter data types specified
3966 (can be zero). Note that this is not an indication of
3967 the number of parameters that might appear in the
3968 query string, only the number that the frontend wants to
3969 prespecify types for.
3974 Then, for each parameter, there is the following:
3982 Specifies the object ID of the parameter data type.
3983 Placing a zero here is equivalent to leaving the type
4008 Identifies the message as a Parse-complete indicator.
4018 Length of message contents in bytes, including self.
4043 Identifies the message as a password response. Note that
4044 this is also used for GSSAPI and SSPI response messages
4045 (which is really a design error, since the contained data
4046 is not a null-terminated string in that case, but can be
4047 arbitrary binary data).
4057 Length of message contents in bytes, including self.
4067 The password (encrypted, if requested).
4091 Identifies the message as a portal-suspended indicator.
4092 Note this only appears if an Execute message's row-count limit
4103 Length of message contents in bytes, including self.
4128 Identifies the message as a simple query.
4138 Length of message contents in bytes, including self.
4148 The query string itself.
4173 Identifies the message type. ReadyForQuery is sent
4174 whenever the backend is ready for a new query cycle.
4184 Length of message contents in bytes, including self.
4194 Current backend transaction status indicator.
4195 Possible values are '<literal>I</>' if idle (not in
4196 a transaction block); '<literal>T</>' if in a transaction
4197 block; or '<literal>E</>' if in a failed transaction
4198 block (queries will be rejected until block is ended).
4223 Identifies the message as a row description.
4233 Length of message contents in bytes, including self.
4243 Specifies the number of fields in a row (can be zero).
4248 Then, for each field, there is the following:
4266 If the field can be identified as a column of a specific
4267 table, the object ID of the table; otherwise zero.
4277 If the field can be identified as a column of a specific
4278 table, the attribute number of the column; otherwise zero.
4288 The object ID of the field's data type.
4298 The data type size (see <varname>pg_type.typlen</>).
4299 Note that negative values denote variable-width types.
4309 The type modifier (see <varname>pg_attribute.atttypmod</>).
4310 The meaning of the modifier is type-specific.
4320 The format code being used for the field. Currently will
4321 be zero (text) or one (binary). In a RowDescription
4322 returned from the statement variant of Describe, the
4323 format code is not yet known and will always be zero.
4348 Length of message contents in bytes, including self.
4358 The <acronym>SSL</acronym> request code. The value is chosen to contain
4359 <literal>1234</> in the most significant 16 bits, and <literal>5679</> in the
4360 least 16 significant bits. (To avoid confusion, this code
4361 must not be the same as any protocol version number.)
4386 Length of message contents in bytes, including self.
4396 The protocol version number. The most significant 16 bits are
4397 the major version number (3 for the protocol described here).
4398 The least significant 16 bits are the minor version number
4399 (0 for the protocol described here).
4404 The protocol version number is followed by one or more pairs of
4405 parameter name and value strings. A zero byte is required as a
4406 terminator after the last name/value pair.
4407 Parameters can appear in any
4408 order. <literal>user</> is required, others are optional.
4409 Each parameter is specified as:
4417 The parameter name. Currently recognized names are:
4426 The database user name to connect as. Required;
4427 there is no default.
4433 <literal>database</>
4437 The database to connect to. Defaults to the user name.
4447 Command-line arguments for the backend. (This is
4448 deprecated in favor of setting individual run-time
4455 In addition to the above, any run-time parameter that can be
4456 set at backend start time might be listed. Such settings
4457 will be applied during backend start (after parsing the
4458 command-line options if any). The values will act as
4469 The parameter value.
4494 Identifies the message as a Sync command.
4504 Length of message contents in bytes, including self.
4529 Identifies the message as a termination.
4539 Length of message contents in bytes, including self.
4555 <sect1 id="protocol-error-fields">
4556 <title>Error and Notice Message Fields</title>
4559 This section describes the fields that can appear in ErrorResponse and
4560 NoticeResponse messages. Each field type has a single-byte identification
4561 token. Note that any given field type should appear at most once per
4573 Severity: the field contents are
4574 <literal>ERROR</>, <literal>FATAL</>, or
4575 <literal>PANIC</> (in an error message), or
4576 <literal>WARNING</>, <literal>NOTICE</>, <literal>DEBUG</>,
4577 <literal>INFO</>, or <literal>LOG</> (in a notice message),
4578 or a localized translation of one of these. Always present.
4589 Code: the SQLSTATE code for the error (see <xref
4590 linkend="errcodes-appendix">). Not localizable. Always present.
4601 Message: the primary human-readable error message.
4602 This should be accurate but terse (typically one line).
4614 Detail: an optional secondary error message carrying more
4615 detail about the problem. Might run to multiple lines.
4626 Hint: an optional suggestion what to do about the problem.
4627 This is intended to differ from Detail in that it offers advice
4628 (potentially inappropriate) rather than hard facts.
4629 Might run to multiple lines.
4640 Position: the field value is a decimal ASCII integer, indicating
4641 an error cursor position as an index into the original query string.
4642 The first character has index 1, and positions are measured in
4643 characters not bytes.
4654 Internal position: this is defined the same as the <literal>P</>
4655 field, but it is used when the cursor position refers to an internally
4656 generated command rather than the one submitted by the client.
4657 The <literal>q</> field will always appear when this field appears.
4668 Internal query: the text of a failed internally-generated command.
4669 This could be, for example, a SQL query issued by a PL/pgSQL function.
4680 Where: an indication of the context in which the error occurred.
4681 Presently this includes a call stack traceback of active
4682 procedural language functions and internally-generated queries.
4683 The trace is one entry per line, most recent first.
4694 File: the file name of the source-code location where the error
4706 Line: the line number of the source-code location where the error
4718 Routine: the name of the source-code routine reporting the error.
4726 The client is responsible for formatting displayed information to meet its
4727 needs; in particular it should break long lines as needed. Newline characters
4728 appearing in the error message fields should be treated as paragraph breaks,
4734 <sect1 id="protocol-changes">
4735 <title>Summary of Changes since Protocol 2.0</title>
4738 This section provides a quick checklist of changes, for the benefit of
4739 developers trying to update existing client libraries to protocol 3.0.
4743 The initial startup packet uses a flexible list-of-strings format
4744 instead of a fixed format. Notice that session default values for run-time
4745 parameters can now be specified directly in the startup packet. (Actually,
4746 you could do that before using the <literal>options</> field, but given the
4747 limited width of <literal>options</> and the lack of any way to quote
4748 whitespace in the values, it wasn't a very safe technique.)
4752 All messages now have a length count immediately following the message type
4753 byte (except for startup packets, which have no type byte). Also note that
4754 PasswordMessage now has a type byte.
4758 ErrorResponse and NoticeResponse ('<literal>E</>' and '<literal>N</>')
4759 messages now contain multiple fields, from which the client code can
4760 assemble an error message of the desired level of verbosity. Note that
4761 individual fields will typically not end with a newline, whereas the single
4762 string sent in the older protocol always did.
4766 The ReadyForQuery ('<literal>Z</>') message includes a transaction status
4771 The distinction between BinaryRow and DataRow message types is gone; the
4772 single DataRow message type serves for returning data in all formats.
4773 Note that the layout of DataRow has changed to make it easier to parse.
4774 Also, the representation of binary values has changed: it is no longer
4775 directly tied to the server's internal representation.
4779 There is a new <quote>extended query</> sub-protocol, which adds the frontend
4780 message types Parse, Bind, Execute, Describe, Close, Flush, and Sync, and the
4781 backend message types ParseComplete, BindComplete, PortalSuspended,
4782 ParameterDescription, NoData, and CloseComplete. Existing clients do not
4783 have to concern themselves with this sub-protocol, but making use of it
4784 might allow improvements in performance or functionality.
4788 <command>COPY</command> data is now encapsulated into CopyData and CopyDone messages. There
4789 is a well-defined way to recover from errors during <command>COPY</command>. The special
4790 <quote><literal>\.</></quote> last line is not needed anymore, and is not sent
4791 during <command>COPY OUT</command>.
4792 (It is still recognized as a terminator during <command>COPY IN</command>, but its use is
4793 deprecated and will eventually be removed.) Binary <command>COPY</command> is supported.
4794 The CopyInResponse and CopyOutResponse messages include fields indicating
4795 the number of columns and the format of each column.
4799 The layout of FunctionCall and FunctionCallResponse messages has changed.
4800 FunctionCall can now support passing NULL arguments to functions. It also
4801 can handle passing parameters and retrieving results in either text or
4802 binary format. There is no longer any reason to consider FunctionCall a
4803 potential security hole, since it does not offer direct access to internal
4804 server data representations.
4808 The backend sends ParameterStatus ('<literal>S</>') messages during connection
4809 startup for all parameters it considers interesting to the client library.
4810 Subsequently, a ParameterStatus message is sent whenever the active value
4811 changes for any of these parameters.
4815 The RowDescription ('<literal>T</>') message carries new table OID and column
4816 number fields for each column of the described row. It also shows the format
4817 code for each column.
4821 The CursorResponse ('<literal>P</>') message is no longer generated by
4826 The NotificationResponse ('<literal>A</>') message has an additional string
4827 field, which can carry a <quote>payload</> string passed
4828 from the <command>NOTIFY</command> event sender.
4832 The EmptyQueryResponse ('<literal>I</>') message used to include an empty
4833 string parameter; this has been removed.