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📄 ABORT.7(2.17 KB)
📄 ALTER_AGGREGATE.7(4.97 KB)
📄 ALTER_COLLATION.7(2.8 KB)
📄 ALTER_CONVERSION.7(2.89 KB)
📄 ALTER_DATABASE.7(5.71 KB)
📄 ALTER_DEFAULT_PRIVILEGES.7(6.34 KB)
📄 ALTER_DOMAIN.7(6.96 KB)
📄 ALTER_EVENT_TRIGGER.7(2.42 KB)
📄 ALTER_EXTENSION.7(6.89 KB)
📄 ALTER_FOREIGN_DATA_WRAPPER.7(4.56 KB)
📄 ALTER_FOREIGN_TABLE.7(12.8 KB)
📄 ALTER_FUNCTION.7(8.26 KB)
📄 ALTER_GROUP.7(3.08 KB)
📄 ALTER_INDEX.7(5.59 KB)
📄 ALTER_LANGUAGE.7(2.1 KB)
📄 ALTER_LARGE_OBJECT.7(1.97 KB)
📄 ALTER_MATERIALIZED_VIEW.7(4.66 KB)
📄 ALTER_OPERATOR.7(3.67 KB)
📄 ALTER_OPERATOR_CLASS.7(2.86 KB)
📄 ALTER_OPERATOR_FAMILY.7(9.12 KB)
📄 ALTER_POLICY.7(2.88 KB)
📄 ALTER_ROLE.7(9.04 KB)
📄 ALTER_RULE.7(2.21 KB)
📄 ALTER_SCHEMA.7(2.31 KB)
📄 ALTER_SEQUENCE.7(7.53 KB)
📄 ALTER_SERVER.7(3.41 KB)
📄 ALTER_SYSTEM.7(3.8 KB)
📄 ALTER_TABLE.7(35.2 KB)
📄 ALTER_TABLESPACE.7(3.35 KB)
📄 ALTER_TEXT_SEARCH_CONFIGURATION.7(4.79 KB)
📄 ALTER_TEXT_SEARCH_DICTIONARY.7(4.02 KB)
📄 ALTER_TEXT_SEARCH_PARSER.7(2.23 KB)
📄 ALTER_TEXT_SEARCH_TEMPLATE.7(2.26 KB)
📄 ALTER_TRIGGER.7(2.98 KB)
📄 ALTER_TYPE.7(7.15 KB)
📄 ALTER_USER.7(2.66 KB)
📄 ALTER_USER_MAPPING.7(3.26 KB)
📄 ALTER_VIEW.7(4.67 KB)
📄 ANALYZE.7(7.67 KB)
📄 BEGIN.7(4.06 KB)
📄 CHECKPOINT.7(2.29 KB)
📄 CLOSE.7(2.72 KB)
📄 CLUSTER.7(6.23 KB)
📄 COMMENT.7(10.2 KB)
📄 COMMIT.7(2.1 KB)
📄 COMMIT_PREPARED.7(2.58 KB)
📄 COPY.7(29.88 KB)
📄 CREATE_ACCESS_METHOD.7(2.9 KB)
📄 CREATE_AGGREGATE.7(23.35 KB)
📄 CREATE_CAST.7(13.55 KB)
📄 CREATE_COLLATION.7(3.97 KB)
📄 CREATE_CONVERSION.7(3.78 KB)
📄 CREATE_DATABASE.7(8.48 KB)
📄 CREATE_DOMAIN.7(6.52 KB)
📄 CREATE_EVENT_TRIGGER.7(3.74 KB)
📄 CREATE_EXTENSION.7(6.57 KB)
📄 CREATE_FOREIGN_DATA_WRAPPER.7(4.96 KB)
📄 CREATE_FOREIGN_TABLE.7(9 KB)
📄 CREATE_FUNCTION.7(24.73 KB)
📄 CREATE_GROUP.7(2.11 KB)
📄 CREATE_INDEX.7(19.56 KB)
📄 CREATE_LANGUAGE.7(9.83 KB)
📄 CREATE_MATERIALIZED_VIEW.7(4.21 KB)
📄 CREATE_OPERATOR.7(6.88 KB)
📄 CREATE_OPERATOR_CLASS.7(8.52 KB)
📄 CREATE_OPERATOR_FAMILY.7(3.6 KB)
📄 CREATE_POLICY.7(15.92 KB)
📄 CREATE_ROLE.7(12.69 KB)
📄 CREATE_RULE.7(8.63 KB)
📄 CREATE_SCHEMA.7(6.15 KB)
📄 CREATE_SEQUENCE.7(9.79 KB)
📄 CREATE_SERVER.7(3.63 KB)
📄 CREATE_TABLE.7(42.1 KB)
📄 CREATE_TABLESPACE.7(4.47 KB)
📄 CREATE_TABLE_AS.7(7.8 KB)
📄 CREATE_TEXT_SEARCH_CONFIGURATION.7(3.19 KB)
📄 CREATE_TEXT_SEARCH_DICTIONARY.7(3.34 KB)
📄 CREATE_TEXT_SEARCH_PARSER.7(3.56 KB)
📄 CREATE_TEXT_SEARCH_TEMPLATE.7(3.38 KB)
📄 CREATE_TRANSFORM.7(5.79 KB)
📄 CREATE_TRIGGER.7(16.3 KB)
📄 CREATE_TYPE.7(28.96 KB)
📄 CREATE_USER.7(2.45 KB)
📄 CREATE_USER_MAPPING.7(3.27 KB)
📄 CREATE_VIEW.7(14.63 KB)
📄 DEALLOCATE.7(1.98 KB)
📄 DECLARE.7(10.22 KB)
📄 DELETE.7(7.36 KB)
📄 DISCARD.7(2.81 KB)
📄 DO.7(3.08 KB)
📄 DROP_ACCESS_METHOD.7(2.35 KB)
📄 DROP_AGGREGATE.7(3.8 KB)
📄 DROP_CAST.7(2.26 KB)
📄 DROP_COLLATION.7(2.42 KB)
📄 DROP_CONVERSION.7(2.36 KB)
📄 DROP_DATABASE.7(2.38 KB)
📄 DROP_DOMAIN.7(2.34 KB)
📄 DROP_EVENT_TRIGGER.7(2.38 KB)
📄 DROP_EXTENSION.7(2.73 KB)
📄 DROP_FOREIGN_DATA_WRAPPER.7(2.61 KB)
📄 DROP_FOREIGN_TABLE.7(2.59 KB)
📄 DROP_FUNCTION.7(3.35 KB)
📄 DROP_GROUP.7(1.57 KB)
📄 DROP_INDEX.7(3.08 KB)
📄 DROP_LANGUAGE.7(2.86 KB)
📄 DROP_MATERIALIZED_VIEW.7(2.64 KB)
📄 DROP_OPERATOR.7(3.01 KB)
📄 DROP_OPERATOR_CLASS.7(3.48 KB)
📄 DROP_OPERATOR_FAMILY.7(3.24 KB)
📄 DROP_OWNED.7(3.14 KB)
📄 DROP_POLICY.7(2.53 KB)
📄 DROP_ROLE.7(3.05 KB)
📄 DROP_RULE.7(2.35 KB)
📄 DROP_SCHEMA.7(2.76 KB)
📄 DROP_SEQUENCE.7(2.46 KB)
📄 DROP_SERVER.7(2.39 KB)
📄 DROP_TABLE.7(2.97 KB)
📄 DROP_TABLESPACE.7(2.48 KB)
📄 DROP_TEXT_SEARCH_CONFIGURATION.7(2.83 KB)
📄 DROP_TEXT_SEARCH_DICTIONARY.7(2.75 KB)
📄 DROP_TEXT_SEARCH_PARSER.7(2.66 KB)
📄 DROP_TEXT_SEARCH_TEMPLATE.7(2.69 KB)
📄 DROP_TRANSFORM.7(2.59 KB)
📄 DROP_TRIGGER.7(2.6 KB)
📄 DROP_TYPE.7(2.53 KB)
📄 DROP_USER.7(1.65 KB)
📄 DROP_USER_MAPPING.7(2.56 KB)
📄 DROP_VIEW.7(2.43 KB)
📄 END.7(2.19 KB)
📄 EXECUTE.7(2.9 KB)
📄 EXPLAIN.7(13.03 KB)
📄 FETCH.7(8.48 KB)
📄 GRANT.7(21.29 KB)
📄 IMPORT_FOREIGN_SCHEMA.7(3.98 KB)
📄 INSERT.7(19.61 KB)
📄 LISTEN.7(3.51 KB)
📄 LOAD.7(2.54 KB)
📄 LOCK.7(8.01 KB)
📄 MOVE.7(2.92 KB)
📄 NOTIFY.7(8.21 KB)
📄 PREPARE.7(8.2 KB)
📄 PREPARE_TRANSACTION.7(5.74 KB)
📄 REASSIGN_OWNED.7(3.01 KB)
📄 REFRESH_MATERIALIZED_VIEW.7(3.92 KB)
📄 REINDEX.7(7.69 KB)
📄 RELEASE_SAVEPOINT.7(3 KB)
📄 RESET.7(2.93 KB)
📄 REVOKE.7(10.44 KB)
📄 ROLLBACK.7(2.09 KB)
📄 ROLLBACK_PREPARED.7(2.6 KB)
📄 ROLLBACK_TO_SAVEPOINT.7(3.94 KB)
📄 SAVEPOINT.7(3.5 KB)
📄 SECURITY_LABEL.7(5.39 KB)
📄 SELECT.7(63.33 KB)
📄 SELECT_INTO.7(4.37 KB)
📄 SET.7(7.57 KB)
📄 SET_CONSTRAINTS.7(4.48 KB)
📄 SET_ROLE.7(4.36 KB)
📄 SET_SESSION_AUTHORIZATION.7(3.88 KB)
📄 SET_TRANSACTION.7(8.27 KB)
📄 SHOW.7(4.22 KB)
📄 START_TRANSACTION.7(2.9 KB)
📄 TABLE.7(18 B)
📄 TRUNCATE.7(6.39 KB)
📄 UNLISTEN.7(2.75 KB)
📄 UPDATE.7(12.26 KB)
📄 VACUUM.7(7.92 KB)
📄 VALUES.7(6.94 KB)
📄 WITH.7(18 B)

Editing: NOTIFY.7

'\" t
.\"     Title: NOTIFY
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: 2018
.\"    Manual: PostgreSQL 9.6.9 Documentation
.\"    Source: PostgreSQL 9.6.9
.\"  Language: English
.\"
.TH "NOTIFY" "7" "2018" "PostgreSQL 9.6.9" "PostgreSQL 9.6.9 Documentation"
.\" -----------------------------------------------------------------
.\" * Define some portability stuff
.\" -----------------------------------------------------------------
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.\" http://bugs.debian.org/507673
.\" http://lists.gnu.org/archive/html/groff/2009-02/msg00013.html
.\" ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.ie \n(.g .ds Aq \(aq
.el       .ds Aq '
.\" -----------------------------------------------------------------
.\" * set default formatting
.\" -----------------------------------------------------------------
.\" disable hyphenation
.nh
.\" disable justification (adjust text to left margin only)
.ad l
.\" -----------------------------------------------------------------
.\" * MAIN CONTENT STARTS HERE *
.\" -----------------------------------------------------------------
.SH "NAME"
NOTIFY \- generate a notification
.SH "SYNOPSIS"
.sp
.nf
NOTIFY \fIchannel\fR [ , \fIpayload\fR ]
.fi
.SH "DESCRIPTION"
.PP
The
\fBNOTIFY\fR
command sends a notification event together with an optional
\(lqpayload\(rq
string to each client application that has previously executed
\fBLISTEN \fR\fB\fIchannel\fR\fR
for the specified channel name in the current database\&. Notifications are visible to all users\&.
.PP
\fBNOTIFY\fR
provides a simple interprocess communication mechanism for a collection of processes accessing the same
PostgreSQL
database\&. A payload string can be sent along with the notification, and higher\-level mechanisms for passing structured data can be built by using tables in the database to pass additional data from notifier to listener(s)\&.
.PP
The information passed to the client for a notification event includes the notification channel name, the notifying session\*(Aqs server process
PID, and the payload string, which is an empty string if it has not been specified\&.
.PP
It is up to the database designer to define the channel names that will be used in a given database and what each one means\&. Commonly, the channel name is the same as the name of some table in the database, and the notify event essentially means,
\(lqI changed this table, take a look at it to see what\*(Aqs new\(rq\&. But no such association is enforced by the
\fBNOTIFY\fR
and
\fBLISTEN\fR
commands\&. For example, a database designer could use several different channel names to signal different sorts of changes to a single table\&. Alternatively, the payload string could be used to differentiate various cases\&.
.PP
When
\fBNOTIFY\fR
is used to signal the occurrence of changes to a particular table, a useful programming technique is to put the
\fBNOTIFY\fR
in a statement trigger that is triggered by table updates\&. In this way, notification happens automatically when the table is changed, and the application programmer cannot accidentally forget to do it\&.
.PP
\fBNOTIFY\fR
interacts with SQL transactions in some important ways\&. Firstly, if a
\fBNOTIFY\fR
is executed inside a transaction, the notify events are not delivered until and unless the transaction is committed\&. This is appropriate, since if the transaction is aborted, all the commands within it have had no effect, including
\fBNOTIFY\fR\&. But it can be disconcerting if one is expecting the notification events to be delivered immediately\&. Secondly, if a listening session receives a notification signal while it is within a transaction, the notification event will not be delivered to its connected client until just after the transaction is completed (either committed or aborted)\&. Again, the reasoning is that if a notification were delivered within a transaction that was later aborted, one would want the notification to be undone somehow \(em but the server cannot
\(lqtake back\(rq
a notification once it has sent it to the client\&. So notification events are only delivered between transactions\&. The upshot of this is that applications using
\fBNOTIFY\fR
for real\-time signaling should try to keep their transactions short\&.
.PP
If the same channel name is signaled multiple times from the same transaction with identical payload strings, the database server can decide to deliver a single notification only\&. On the other hand, notifications with distinct payload strings will always be delivered as distinct notifications\&. Similarly, notifications from different transactions will never get folded into one notification\&. Except for dropping later instances of duplicate notifications,
\fBNOTIFY\fR
guarantees that notifications from the same transaction get delivered in the order they were sent\&. It is also guaranteed that messages from different transactions are delivered in the order in which the transactions committed\&.
.PP
It is common for a client that executes
\fBNOTIFY\fR
to be listening on the same notification channel itself\&. In that case it will get back a notification event, just like all the other listening sessions\&. Depending on the application logic, this could result in useless work, for example, reading a database table to find the same updates that that session just wrote out\&. It is possible to avoid such extra work by noticing whether the notifying session\*(Aqs server process
PID
(supplied in the notification event message) is the same as one\*(Aqs own session\*(Aqs
PID
(available from
libpq)\&. When they are the same, the notification event is one\*(Aqs own work bouncing back, and can be ignored\&.
.SH "PARAMETERS"
.PP
\fIchannel\fR
.RS 4
Name of the notification channel to be signaled (any identifier)\&.
.RE
.PP
\fIpayload\fR
.RS 4
The
\(lqpayload\(rq
string to be communicated along with the notification\&. This must be specified as a simple string literal\&. In the default configuration it must be shorter than 8000 bytes\&. (If binary data or large amounts of information need to be communicated, it\*(Aqs best to put it in a database table and send the key of the record\&.)
.RE
.SH "NOTES"
.PP
There is a queue that holds notifications that have been sent but not yet processed by all listening sessions\&. If this queue becomes full, transactions calling
\fBNOTIFY\fR
will fail at commit\&. The queue is quite large (8GB in a standard installation) and should be sufficiently sized for almost every use case\&. However, no cleanup can take place if a session executes
\fBLISTEN\fR
and then enters a transaction for a very long time\&. Once the queue is half full you will see warnings in the log file pointing you to the session that is preventing cleanup\&. In this case you should make sure that this session ends its current transaction so that cleanup can proceed\&.
.PP
The function
\fBpg_notification_queue_usage\fR
returns the fraction of the queue that is currently occupied by pending notifications\&. See
Section 9.25, \(lqSystem Information Functions\(rq, in the documentation
for more information\&.
.PP
A transaction that has executed
\fBNOTIFY\fR
cannot be prepared for two\-phase commit\&.
.SS "pg_notify"
.PP
To send a notification you can also use the function
\fBpg_notify\fR(text, text)\&. The function takes the channel name as the first argument and the payload as the second\&. The function is much easier to use than the
\fBNOTIFY\fR
command if you need to work with non\-constant channel names and payloads\&.
.SH "EXAMPLES"
.PP
Configure and execute a listen/notify sequence from
psql:
.sp
.if n \{\
.RS 4
.\}
.nf
LISTEN virtual;
NOTIFY virtual;
Asynchronous notification "virtual" received from server process with PID 8448\&.
NOTIFY virtual, \*(AqThis is the payload\*(Aq;
Asynchronous notification "virtual" with payload "This is the payload" received from server process with PID 8448\&.

LISTEN foo;
SELECT pg_notify(\*(Aqfo\*(Aq || \*(Aqo\*(Aq, \*(Aqpay\*(Aq || \*(Aqload\*(Aq);
Asynchronous notification "foo" with payload "payload" received from server process with PID 14728\&.
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
There is no
\fBNOTIFY\fR
statement in the SQL standard\&.
.SH "SEE ALSO"
\fBLISTEN\fR(7), \fBUNLISTEN\fR(7)

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