From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Setting visibility map in VACUUM's second phase |
Date: | 2012-12-06 18:57:39 |
Message-ID: | 7343.1354820259@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> One other thought: I'm wondering if we shouldn't try to push the work
> of setting the all-visible bit into heap_page_prune().
Hm, maybe ...
> But it seems to me that a page can't be all-visible unless there are
> no dead line pointers and no HOT chains of length != 1, and
> heap_prune_chain() does manage to call HeapTupleSatisfiesVacuum() for
> every tuple, so the raw information seems like it is available without
> any additional CLOG lookups.
HeapTupleSatisfiesVacuum is interested in whether a dead tuple is dead
to everybody, but I don't think it figures out whether a live tuple is
live to everybody. On the assumption that most tuples are live, adding
the latter calculation might represent significant expense.
regards, tom lane
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