Re: Frequent Update Project: Design Overview of HOT Updates

On 11/10/06, Heikki Linnakangas <heikki(at)enterprisedb(dot)com> wrote:
>
> Tom Lane wrote:
>
> > (Actually, the assumption that you can throw an additional back-pointer
> > into overflow tuple headers is the worst feature of this proposal in
> > that regard --- it's really not that easy to support multiple header
> > formats.)
>
> Well, we already have a variable length null bitmap in the header. It
> seems quite straightforward to me to add the new field before the null
> bitmap. It certainly requires some changes, in particular to places that
> access the null bitmap, but it's not an insurmountable effort. Or am I
> missing some less obvious consequences?
>
>
We have added the overflow header (which right now contains a single entry
i.e.
the back pointer) on the very similar lines to optional Oid field in the
tuple header.
A flag (the last free in the t_infomask) is used to check if there is an
additional
overflow header and if so t_hoff is adjusted appropriately.

So in the current prototype, the overflow header is after the null bitmap
and before the Oid, if it exists.

Regards,
Pavan