Re: Avoid endless futile table locks in vacuuming.

I wrote:
> Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
>> If a partially-active table develops a slug of stable all-visible,
>> non-empty pages at the end of it, then every autovacuum of that table
>> will skip the end pages on the forward scan, think they might be
>> truncatable, and take the access exclusive lock to do the truncation.
>> And then immediately fail when it sees the last page is not empty.
>> This can persist for an indefinite number of autovac rounds.
>> The simple solution is to always scan the last page of a table, so it
>> can be noticed that it is not empty and avoid the truncation attempt.

> This seems like a reasonable proposal, but I find your implementation
> unconvincing: there are two places in lazy_scan_heap() that pay attention
> to scan_all, and you touched only one of them.

After further investigation, there is another pre-existing bug: the short
circuit path for pages not requiring freezing doesn't bother to update
vacrelstats->nonempty_pages, causing the later logic to think that the
page is potentially truncatable even if we fix the second check of
scan_all! So this is pretty broken, and I almost think we should treat it
as a back-patchable bug fix.

In the attached proposed patch, I added another refinement, which is to
not bother with forcing the last page to be scanned if we already know
that we're not going to attempt truncation, because we already found a
nonempty page too close to the end of the rel. I'm not quite sure this
is worthwhile, but it takes very little added logic, and saving an I/O
is probably worth the trouble.

regards, tom lane