Re: interval / interval -> double operator

On 5/18/07, Andrew Hammond <andrew(dot)george(dot)hammond(at)gmail(dot)com> wrote:
>
> On 5/17/07, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >
> > "Andrew Hammond" <andrew(dot)george(dot)hammond(at)gmail(dot)com> writes:
> > > On 5/17/07, Tom Lane < tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > >> What are the grounds for defining it that way rather than some other
> > >> way?
> >
> > > The only alternative that came to mind when I wrote it was using a
> > numeric
> > > instead of float.
> >
> > No, I'm wondering what's the justification for smashing it to a single
> > number at all, when the inputs are three-field values. Interval divided
> > by float doesn't produce just a float, for example.
> >
>
>
> I think I see what you're getting at here. '1 month' / '1 day' could
> return a number of reasonable values depending on how many days are in the
> month (28 to 31) and on how many hours are in a day (generally 24, but can
> be 23 or 25 for DST adjustments). The definition above simply assumes that
> EXTRACT(epoch...) does the Right Thing. Hmmm. I'm at a loss for the right
> way to solve this. It seems very reasonable to want to divide intervals by
> intervals (how many nanocenturies in a fortnight?), but I'm at a loss for
> how to do that correctly. I'll read the code from EXTRACT(epoch...) and see
> what happening there.
>

Ok, I've been hunting through src/backend to try and find the code for
EXTRACT(epoch ...). I found EXTRACT in src/backend/parser/gram.y, which
seems like a reasonable place to start.

| EXTRACT '(' extract_list ')'
{
FuncCall *n = makeNode(FuncCall);
n->funcname = SystemFuncName("date_part");
n->args = $3;
n->agg_star = FALSE;
n->agg_distinct = FALSE;
n->location = @1;
$$ = (Node *)n;
}

Which got me looking for "date_part". But that only seems to be in the
gram.y file, include/catalog/pg_proc.h and the test suite. The pg_proc.h
stuff looks pretty interesting, but to decipher it, I figured I need to read
up on SystemFuncName(). So I grepped for SystemFuncName(). It turns out to
be a wrapper around list_make2(), which is part of the linked list package.
Then I checked out makeNode(), which is a wrapper around newNode(), which in
turn is memory allocation stuff. At this point I'm kind of lost. I'm pretty
sure that the next thing I need to hunt up is in the parser, but I don't
know where to look.

Can anyone please tell me what is the right way to chase down the actual
code that implements EXTRACT(epoch ...)? (please note that I'm not asking
where that code is, but how to find it.) Or even better, is there a web page
or other document someone can give me a pointer to?

Andrew