Andres Freund wrote:

> > (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 
> conclusion
>                * 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.

Here's an idea.  We need core-blessed extensions (src/extensions/, you
know I've proposed this before), so why not take this opportunity to
create our first such and make it carry a function to scan a table
completely to do this task.

Since we were considering a new VACUUM option, surely this is serious
enough to warrant more than just contrib.

Álvaro Herrera      
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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