Just historical reasons...
Itai Zukerman wrote:
> I didn't get any responses on pgsql-sql, so I'm re-posting here...
>
>
>>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?
>
>
>
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--
Teodor Sigaev
teodor@stack.net
