Thread: Trigger function cost

Hi Chaps,

Can anyone point me to docs for trigger function estimated cost?

I see that when I create a volatile plpgsql trigger function it gets given a cost of 100 and a c function gets given a
costof 1. 

Is there any reason to mess with this?

Thanks
Glyn

Glyn Astill <glynastill@yahoo.co.uk> writes:
> I see that when I create a volatile plpgsql trigger function it gets given a cost of 100 and a c function gets given
acost of 1. 

> Is there any reason to mess with this?

No.  The planner doesn't actually bother to figure the cost of triggers
anyway, since presumably every correct plan will fire the same set of
triggers.  So even if you had a more accurate cost estimate than that
one, it wouldn't get used for anything.

Now, for ordinary non-trigger functions, it might be worth paying
some attention to the cost estimate.  "1" is intended to denote the
cost of a reasonably simple C function, so PL functions should pretty
much always have costs that are large multiples of that.  100 is a
reasonable default, but if you know better you can put something else.

            regards, tom lane

> From: Tom Lane <tgl@sss.pgh.pa.us>
>
> > Is there any reason to mess with this?
>
> No.  The planner doesn't actually bother to figure the
> cost of triggers
> anyway, since presumably every correct plan will fire the
> same set of
> triggers.  So even if you had a more accurate cost estimate
> than that
> one, it wouldn't get used for anything.
>

Excellent, that's good with me.

> Now, for ordinary non-trigger functions, it might be worth
> paying
> some attention to the cost estimate.  "1" is
> intended to denote the
> cost of a reasonably simple C function, so PL functions
> should pretty
> much always have costs that are large multiples of that.
> 100 is a
> reasonable default, but if you know better you can put
> something else.
>

Cool, I'll leave it alone for now then, interesting stuff, thanks Tom.