On 2012-11-16 08:43:12 -0600, Merlin Moncure wrote: > On Thu, Nov 15, 2012 at 9:21 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Jeff Davis <pg...@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? Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
