On 15 November 2012 22:21, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Removing those 3 hints would give us 3 more flag bits (eventually, after
>> we are sure they aren't just leftover), and it would also reduce the
>> chance that a page is dirtied for no other reason than to set them.
>
> We aren't pressed for flag bits particularly. I think the main
> attraction of this idea is precisely to reduce unnecessary page dirties,
> and so that leads me to suggest a variant: keep the four bits defined as
> now, but do not attempt to set XMIN_INVALID or XMAX_COMMITTED unless the
> page is already dirty. This would make it a straight-up trade of more
> clog consultation for fewer page dirties.
Hmm, I thought Alvaro wanted an extra flag bit for foreign key locks
but couldn't find it.
Come to think of it, why do we have XMIN_INVALID and XMAX_INVALID? We
never need both at the same time, so we can just use one...
XMIN_INVALID -> XACT_INVALID
XMAX_INVALID == XMIN_COMMITTED | XACT_INVALID
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