Re: BTScanOpaqueData size slows down tests

On Wed, Apr 2, 2025 at 12:31 PM Álvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> 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().

Yes, we do -- but it's rare. It only happens in the case where we
restore a mark after leaving the page where the mark was taken. In
practice, merge joins tend to hardly ever do that (I know this because
it makes testing annoyingly difficult). In other words, the
optimization added by commit 08ae5edc5c seems to be very effective in
practice.

--
Peter Geoghegan