Re: XLog changes for 9.3

On Thursday, June 07, 2012 06:53:58 PM Tom Lane wrote:
> Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> > dance. If the record can be smeared over two pages there is no point in
> > storing it aligned.
>
> I think this is not true. The value of requiring alignment is that you
> can read the record-length field without first having to copy it somewhere.
> In particular, it will get really ugly if the record length field itself
> could cross a page boundary. I think we want to be able to determine
> the record length before we do any data copying, so that we can malloc
> the record buffer and then just do one copy step.
Hm, I had assumed the record would get copied into a temp/static buffer first
and only get reassembled together with the data afterwards.
But if thats not the way to go, sure, storing it aligned so that the length
can always be read aligned within a page is sensible.

Andres