On 2016-05-18 18:25:39 -0400, Tom Lane wrote:
> Josh berkus <josh@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
setif * the page-level bit is clear. However, it's possible that the bit * got cleared after we checked it and
beforewe took the buffer * content lock, so we must recheck before jumping to the conclusion * that something
badhas 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
bitis 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.
Greetings,
Andres Freund