Re: Do we need so many hint bits? - Mailing list pgsql-hackers

From Merlin Moncure
Subject Re: Do we need so many hint bits?
Date
Msg-id CAHyXU0yXRTGBssqXXL-PPTyik+ZgtmhPYe8jrVKmYQq0nzVVag@mail.gmail.com
Whole thread Raw
In response to Re: Do we need so many hint bits?  (Andres Freund <andres@2ndquadrant.com>)
List pgsql-hackers
On Fri, Nov 16, 2012 at 8:58 AM, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2012-11-16 08:43:12 -0600, Merlin Moncure wrote:
>> 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?
>
> Uhm. The latter doesn't really work if you have any transactions that
> might not see that row or am I missing something?

nope. you're right.

merlin



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