Re: equal() perf tweak

On Mon, 2003-11-03 at 10:04, Tom Lane wrote:
> You have effectively reverted the code to its previous slow state.

Erm, the original version of this code in CVS (from ~7 years ago) is the
following:

case T_List:
{
List *la = (List*)a;
List *lb = (List*)b;
List *l;

if (a==NULL && b==NULL)
return (true);
if (length(a)!=length(b))
return (false);
foreach(l, la) {
if (!equal(lfirst(l), lfirst(lb)))
return (false);
lb = lnext(lb);
}
retval = true;
}
break;

i.e. it is effectively the same as the code in CVS HEAD. To what
"previous state" does this patch revert the code?

> The problem with what you've done is that it recursively applies equal()
> to the list contents, which may take a very large number of cycles, only
> to eventually fail because one list is a prefix of the other. The point
> of the current coding is to detect that condition before we recurse.

I don't understand: granted, this applies equal() to each element of the
list, but why would that be particularly slow?

equal() applied to the lfirst() of a list doesn't invoke equal() on a
T_List node (the tag of the lfirst() of the list is the data type of the
elements of the list), so there should only be a single level of
recursion.

I was going to benchmark this, but pulling the list code out of the rest
of the source is a bit hairy. Let me know if you think this would be
helpful.

-Neil