Re: Do we need so many hint bits?

On Thu, Nov 15, 2012 at 10:37 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> 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

Hm, I wonder if you could squeeze two bits out. ISTM here are the
interesting cases enumerated:

0: xmin unknown
1: xmin invalid
2: xmin valid, xmax unknown
3: xmin valid, xmax invalid
4: xmin valid, xmax valid

Did I miss any? If not, I think case #3 could be covered by utilizing
xmax == InvalidTransactionId or simply ignored.  That makes the check
a little dirtier than a bit test though, but you could be sneaky and
map both xmin=valid cases to a bit.

merlin