On 2016-05-18 18:25:39 -0400, Tom Lane wrote:
> Josh berkus <j...@agliodbs.com> writes:
> > Maybe this is the wrong perspective.  I mean, is there a reason we even
> > need this option, other than a lack of any other way to do a full table
> > scan to check for corruption, etc.?  If we're only doing this for
> > integrity checking, then maybe it's better if it becomes a function,
> > which could be later extended with additional forensic features?
> Yes, I've been wondering that too.  VACUUM is not meant as a corruption
> checker, and should not be made into one, so what is the point of this
> flag exactly?

Well, so far a VACUUM FREEZE (or just setting vacuum_freeze_table_age =
0) verified the correctness of the visibility map; and that found a
number of bugs.  Now visibilitymap grew additional responsibilities,
with a noticeable risk of data eating bugs, and there's no way to verify
whether visibilitymap's frozen bits are set correctly.

> (AFAIK, "select count(*) from table" would offer a similar amount of
> sanity checking as a full-table VACUUM scan does, so it's not like
> we've removed functionality with no near-term replacement.)

I don't think that'd do anything comparable to
                 * As of PostgreSQL 9.2, the visibility map bit should never be 
set if
                 * the page-level bit is clear.  However, it's possible that 
the bit
                 * got cleared after we checked it and before we took the buffer
                 * content lock, so we must recheck before jumping to the 
                 * that something bad has happened.
                else if (all_visible_according_to_vm && !PageIsAllVisible(page)
                                 && VM_ALL_VISIBLE(onerel, blkno, &vmbuffer))
                        elog(WARNING, "page is not marked all-visible but 
visibility map bit is set in relation \"%s\" page %u",
                                 relname, blkno);
                        visibilitymap_clear(onerel, blkno, vmbuffer);

If we had a checking module for all this it'd possibly be sufficient,
but we don't.


Andres Freund

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