"As fast as possible" and "PL/PgSQL function" don't go that well together. PL/PgSQL is well and good for a great many jobs, but I doubt this is one of them.
Yes, I know. It was just example to demostrate functionality I need.
If you're willing to spend the time to do it, consider writing a simple C extension function to do this job. It'll be a heck of a lot faster, though you'd need to be pretty careful about handing subtransactions.
I don't know much about writing C extensions. Are there any good resources explaining this topic in deep? I also need some tips on how to ensure that variable will be cleared at the start/end of transaction.
Alternately, you might be able to use a custom GUC from a rather smaller PL/PgSQL function. At transaction start, issue:
set_config('myapp.trigger_time', '', 't');
This is problem with using custom GUC - clearing variable at transaction start. Without clearing it's not sufficient solution (see my response to Pavel's mail). I don't want to do clearing from application and as far as i know there is not "transaction start" trigger.
to define the var and make sure that subsequent current_setting() calls will not report an error. Then in your trigger, check the value and set it if it's empty:
if it's empty. I haven't tested this approach. You could avoid the need for the initial set_config by using a BEGIN ... EXCEPTION block to trap the error, but this uses subtransactions and would affect performance quite significantly.
Custom GUCs don't seem to appear in the pg_settings view or be output by the pg_show_all_settings() function the view is based on, so I don't think you can use an EXISTS test on pg_settings as an alternative. Run the set_config on transaction start, or consider implementing a C function to do the job.
Thanks for advices. Maybe with some help I will be able to write C function that can handle my problem.
