Re: BTScanOpaqueData size slows down tests

On 2025-Apr-02, Tom Lane wrote:

> Andres Freund <andres(at)anarazel(dot)de> writes:
> > I'm a bit confused by the "MUST BE LAST" comment:
> > BTScanPosItem items[MaxTIDsPerBTreePage]; /* MUST BE LAST */
>
> Yeah, me too. It looks like it might've been intended to allow
> the items[] array to be only partially allocated. But as Peter
> says, we don't do that. Too long ago to be sure :=(

The mentioned commit (09cb5c0e7d6f) shows that we do partial memcpys, e.g.

+ memcpy(&so->markPos, &so->currPos,
+ offsetof(BTScanPosData, items[1]) +
+ so->currPos.lastItem * sizeof(BTScanPosItem));

That would obviously not work if this field weren't last. We still do
that, don't we? See btrestrpos().

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/
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