Re: [PoC] Improve dead tuple storage for lazy vacuum

On Fri, Dec 8, 2023 at 3:06 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> BTW Given that the actual value size can be calculated only by the
> caller, how does the tree know if the value is embedded or not? It's
> probably related to how to store combined pointer/value slots.

Right, this is future work. At first, variable-length types will have
to be single-value leaves. In fact, the idea for storing up to 3
offsets in the bitmap header could be done this way -- it would just
be a (small) single-value leaf.

(Reminder: Currently, fixed-length values are compile-time embeddable
if the platform pointer size is big enough.)

> If leaf
> nodes have a bitmap array that indicates the corresponding slot is an
> embedded value or a pointer to a value, it would be easy.

That's the most general way to do it. We could do it much more easily
with a pointer tag, although for the above idea it may require some
endian-aware coding. Both were mentioned in the paper, I recall.

> But since
> the bitmap array is needed only in the leaf nodes, internal nodes and
> leaf nodes will no longer be identical structure, which is not a bad
> thing to me, though.

Absolutely no way we are going back to double everything: double
types, double functions, double memory contexts. Plus, that bitmap in
inner nodes could indicate a pointer to a leaf that got there by "lazy
expansion".