Tuesday, April 7, 2015

Dynamically disabling triggers without locks

Recently Simon Riggs committed a patch by himself and Andreas Karlsson to reduce the lock strength required by certain ALTER TABLE commands, including those to enable or disable triggers. Now the lock level required is SHARE ROW EXCLUSIVE instead of ACCESS EXCLUSIVE. That means it doesn't block SELECT commands any more, and isn't blocked by them, although it will still block and be blocked by INSERT, UPDATE and DELETE operations. Very nice.

However, without formally disabling a trigger you can tell it dynamically not to do anything in the current session without taking any locks at all. Here's a little bit of PLpgsql code I wrote recently for this sort of operation in an INSERT trigger:
        disabled := current_setting('mypackage.foo_trigger_disabled');
        when others then disabled := 'false';
    if disabled = 'true' then
       return NEW;
    end if;
Note that this will only block the trigger from doing anything in sessions where this variable is set. But that's often exactly what you want. In the case this was written for, the trigger is redundant (and expensive) for certain bulk operations, but required for normal operations.  So in a session where we are performing the bulk operation, we can simply set this and avoid taking out a heavy lock on the table, and do this instead, before running our bulk operation:
    set mypackage.foo_trigger_disabled = 'true';
The code above is a bit ugly because of the requirement for the exception handler. There's a cure for that coming, too. David Christensen has submitted a patch to provide a form of current_setting() which will return NULL for unset variables instead of raising an exception.

Note, too, that you could use a value in a one-row one-column table if you wanted something that could apply in all sessions, not just the current session. It would be a bit less efficient, though. This mechanism is pretty light-weight.

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