# F.37\. tcn The `tcn` module provides a trigger function that notifies listeners of changes to any table on which it is attached. It must be used as an `AFTER` trigger `FOR EACH ROW`. Only one parameter may be supplied to the function in a `CREATE TRIGGER` statement, and that is optional. If supplied it will be used for the channel name for the notifications. If omitted `tcn` will be used for the channel name. The payload of the notifications consists of the table name, a letter to indicate which type of operation was performed, and column name/value pairs for primary key columns. Each part is separated from the next by a comma. For ease of parsing using regular expressions, table and column names are always wrapped in double quotes, and data values are always wrapped in single quotes. Embedded quotes are doubled. A brief example of using the extension follows. ``` 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 procedure 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. ```