Thread

Re: COMMUTATOR doesn't seem to work

D'Arcy Cain <darcy@druid.net> — 2012-06-22T16:28:19Z

On 12-06-22 11:36 AM, Tom Lane wrote:
> "D'Arcy Cain"<darcy@druid.net>  writes:
> The thing is that either of those approaches is hugely more expensive
> than just providing a second C function.  It costs probably thousands
> of cycles to inline that SQL function, each time it's used in a query.

I assumed itwould be more expensive but didn't know it would be that
much more.

> I doubt that an "auto reverse the arguments" facility would be very
> much cheaper.  You could maybe argue that the aggregated maintenance
> and space costs of all the commutator-pair functions are enough to
> justify having some such solution instead, but I'm doubtful --- and
> even if true, getting from here to there would be painful.

And it would only apply to a very specific type of function.

The other idea I had was to just have the second C function call the
first but that didn't work.  Here is what I tried.

PG_FUNCTION_INFO_V1(chkpass_eq);
Datum
chkpass_eq(PG_FUNCTION_ARGS)
{
     chkpass    *a1 = (chkpass *) PG_GETARG_POINTER(0);
     text       *a2 = (text *) PG_GETARG_TEXT_P(1);
     char        str[9];

     strlcpy(str, a2->vl_dat, sizeof(str));
     PG_RETURN_BOOL(strcmp(a1->password, crypt(str, a1->password)) == 0);
}

PG_FUNCTION_INFO_V1(chkpass_eq2);
Datum
chkpass_eq2(PG_FUNCTION_ARGS)
{
     return chkpass_eq(PG_GETARG_POINTER(1), PG_GETARG_TEXT_P(0));
}

Now in this specific case the function is trivial and writing it twice
is no big deal but in general I hate writing the same code twice.  I
suppose I could extract the actual operation out to a third function
and call it from the others.  I may do that anyway just for the value
of the example.  Or is there a way to do what I tried above?

-- 
D'Arcy J.M. Cain <darcy@druid.net>         |  Democracy is three wolves
http://www.druid.net/darcy/                |  and a sheep voting on
+1 416 425 1212     (DoD#0082)    (eNTP)   |  what's for dinner.
IM: darcy@Vex.Net