tcn 模块提供了一个触发器函数，用于通知在该触发器函数上附加的任何表内容更改的监听器。它必须用作 AFTER 触发器 FOR EACH ROW。

此模块被认为是“受信任的”，这意味着非超级用户也可以在其拥有的数据库上安装它，前提是他们具有 CREATE 权限。

在 CREATE TRIGGER 语句中，该函数最多只能提供一个参数，并且该参数是可选的。如果提供了参数，则它将用于通知的频道名称。如果省略，则 tcn 将用作频道名称。

通知的有效载荷包含表名、指示执行何种操作的字母以及主键列的列名/值对。每个部分之间用逗号分隔。为了方便使用正则表达式进行解析，表名和列名始终用双引号括起来，数据值始终用单引号括起来。嵌入的引号会加倍。

下面是一个使用该扩展的简短示例。

test=# create table tcndata
test-#   (
test(#     a int not null,
test(#     b date not null,
test(#     c text,
test(#     primary key (a, b)
test(#   );
CREATE TABLE
test=# create trigger tcndata_tcn_trigger
test-#   after insert or update or delete on tcndata
test-#   for each row execute function triggered_change_notification();
CREATE TRIGGER
test=# listen tcn;
LISTEN
test=# insert into tcndata values (1, date '2012-12-22', 'one'),
test-#                            (1, date '2012-12-23', 'another'),
test-#                            (2, date '2012-12-23', 'two');
INSERT 0 3
Asynchronous notification "tcn" with payload ""tcndata",I,"a"='1',"b"='2012-12-22'" received from server process with PID 22770.
Asynchronous notification "tcn" with payload ""tcndata",I,"a"='1',"b"='2012-12-23'" received from server process with PID 22770.
Asynchronous notification "tcn" with payload ""tcndata",I,"a"='2',"b"='2012-12-23'" received from server process with PID 22770.
test=# update tcndata set c = 'uno' where a = 1;
UPDATE 2
Asynchronous notification "tcn" with payload ""tcndata",U,"a"='1',"b"='2012-12-22'" received from server process with PID 22770.
Asynchronous notification "tcn" with payload ""tcndata",U,"a"='1',"b"='2012-12-23'" received from server process with PID 22770.
test=# delete from tcndata where a = 1 and b = date '2012-12-22';
DELETE 1
Asynchronous notification "tcn" with payload ""tcndata",D,"a"='1',"b"='2012-12-22'" received from server process with PID 22770.