Re: Do we need so many hint bits?

On Thu, Nov 15, 2012 at 9:21 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jeff Davis <pgsql@j-davis.com> writes:
>> It occurred to me recently that many of the hint bits aren't terribly
>> important (at least it's not obvious to me). HEAP_XMIN_COMMITTED clearly
>> has a purpose, and we'd expect it to be used many times following the
>> initial CLOG lookup.
>
> Right.
>
>> But the other tuple hint bits seem to be there just for symmetry,
>> because they shouldn't last long. If HEAP_XMIN_INVALID or
>> HEAP_XMAX_COMMITTED is set, then it's (hopefully) going to be vacuumed
>> soon, and gone completely. And if HEAP_XMAX_INVALID is set, then it
>> should just be changed to InvalidTransactionId.
>
> Hm.  It is not cheaper to change xmax to 0 than it is to set the hint
> bit --- you still need a write, and there are also added locking and
> atomicity worries --- so I'm not convinced by your argument there.
> But you might be right that the expected number of wins from the other
> two bits is a lot less.

Is that true in a post checksum world though? Given that we are
logging changes can we relax atomicity expectations?  IIRC xmin/xmax
are aligned, how come you can't just set InvalidTransactionId for
INVALID and 'FrozenTransactionId' for COMMITTED?   Why can't you do
this now?

merlin