Help on triggers

Hello,
I have an application already running, accessing the database (postgresql 8.1). I need a solution, that prints (on a file, or somehow) the queries being executed every time the app accesses the database.
Is there any ON DATABASE ACCESS trigger ?!
Any idea how that can be done?

Thanks in advance.

First: update to a supported version now. 8.1 is really, really old and should not be used any longer.

You can achieve this without a trigger. Just enable statement logging for Postgres in the file postgres.conf using the properties "log_duration"

More details in the manual: http://www.postgresql.org/docs/8.1/static/runtime-config-logging.html#RUNTIME-CONFIG-LOGGING-WHAT

with a modern version of Postgres you would have a more fined grained control over the actual statements being logged.