pg_listen dbHandle notifyName callbackCommand
pg_listen creates, changes, or
cancels a request to listen for asynchronous NOTIFY messages from
the PostgreSQL backend. With a
callbackCommand parameter, the
request is established, or the command string of an already
existing request is replaced. With no callbackCommand parameter, a prior request is
canceled.
After a pg_listen request is
established, the specified command string is executed whenever a
NOTIFY message bearing the given name arrives from the backend.
This occurs when any PostgreSQL
client application issues a NOTIFY command referencing that name.
(Note that the name can be, but does not have to be, that of an
existing relation in the database.) The command string is
executed from the Tcl idle loop. That is the normal idle state of
an application written with Tk. In non-Tk Tcl shells, you can
execute update or vwait to cause the idle loop to be entered.
You should not invoke the SQL statements LISTEN or UNLISTEN directly
when using pg_listen. Pgtcl takes care of issuing those statements
for you. But if you want to send a NOTIFY message yourself,
invoke the SQL NOTIFY statement using pg_exec.