Re: OffsetNumber, picksplit, and GiST

> Is the GiST examples I've looked through, in the picksplit functions,
> I see code that looks roughly like this:
>
> bytea *entryvec = (bytea *) PG_GETARG_POINTER(0);
> OffsetNumber i, maxoff;
>
> maxoff = ((VARSIZE(entryvec) - VARHDRSZ) / sizeof(GISTENTRY)) - 1;
>
> for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
> {
> ... DatumGetPointer((((GISTENTRY *) (VARDATA(entryvec)))[i].key)) ...
> }
>
> I'm wondering, since FirstOffsetNumber is 1, what about the 0'th index
> in entryvec? Is it just not set?

In backend/access/gist/gist.c I see how entryvec is built:

storage = palloc(MAXALIGN(VARHDRSZ) + (*len + 1) * sizeof(GISTENTRY));
entryvec = (bytea *) (storage + MAXALIGN(VARHDRSZ) - VARHDRSZ);
decompvec = (bool *) palloc((*len + 1) * sizeof(bool));
VARATT_SIZEP(entryvec) = (*len + 1) * sizeof(GISTENTRY) + VARHDRSZ;
for (i = 1; i <= *len; i++)
{
[...] gistdentryinit(giststate, 0, &((GISTENTRY *) VARDATA(entryvec))[i], [...]
}

So it looks like the 0'th entry indeed is empty. Why?

Also, in gist.c the index "i" has type "int". No mention of
OffsetNumber anywhere. Any reason for that?

Finally, should I be asking this on pgsql-hackers?

--
Itai Zukerman <http://www.math-hat.com/~zukerman/>