Re: Fixed length data types issue

Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > * Consider ways of storing rows more compactly on disk
> > o Support a smaller header for short variable-length fields?
>
> With respect to the business of having different on-disk and in-memory
> representations, we have that already today: see TOAST. It strikes me
> that it might be useful to think about solving the problem with a
> "second generation toast mechanism". The first generation only worried
> about storing large values, but the second generation would also address
> the problem of storing small values efficiently.
>
> Or you could think about it as a "second generation varlena". This
> mindset would make for a slightly different set of choices about where
> the work gets done. I'm not sure which is better.
>
> Either way, I think it would be interesting to consider
>
> (a) length word either one or two bytes, not four. You can't need more
> than 2 bytes for a datum that fits in a disk page ...

That is an interesting observation, though could compressed inline
values exceed two bytes?

> (b) alignment either one or two bytes, not four. TEXT would be
> perfectly happy with 1-byte alignment, but for NUMERIC we might want 2.
>
> I'm inclined to bag the idea of storing the length words separately from
> the data proper. Although it probably would make for some marginal gain
> in cache efficiency, I don't see any reasonable way at all to fit it
> into the current system structure, whereas either the "toast" or "next
> gen varlena" approaches seem fairly straightforward. And having to
> track an additional pointer inside the inner loops of heap_form_tuple
> and heap_deform_tuple could eat up any performance gain anyway.

Good point. How do we do it now? I assume we store just the fixed-size
toast pointer length in the heap attribute, not the toast length.

Why haven't we investigated shrinking the varlena header before?

--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +