Re: [PATCH] Unremovable tuple monitoring

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Nov 15, 2011 at 10:29 AM, Alvaro Herrera
> <alvherre(at)commandprompt(dot)com> wrote:
>> Excerpts from Robert Haas's message of mar nov 15 12:16:54 -0300 2011:
>>> I guess this is a dumb question, but why don't we remove all the dead
>>> tuples?

>> They were deleted but there are transactions with older snapshots.

> Oh. I was thinking "dead" meant "no longer visible to anyone". But
> it sounds what we call "unremovable" here is what we elsewhere call
> "recently dead".

Would have to look at the code to be sure, but I think that
"nonremovable" is meant to count both live tuples and
dead-but-still-visible-to-somebody tuples.

The question that I think needs to be asked is why it would be useful
to track this using the pgstats mechanisms. By definition, the
difference between this and the live-tuple count is going to be
extremely unstable --- I don't say small, necessarily, but short-lived.
So it's debatable whether it's worth memorializing the count obtained
by the last VACUUM at all. And doing it through pgstats is an expensive
thing. We've already had push-back about the size of the stats table
on large (lots-o-tables) databases. Adding another counter will impose
a performance overhead on everybody, whether they care about this number
or not.

What's more, to the extent that I can think of use-cases for knowing
this number, I think I would want a historical trace of it --- that is,
not only the last VACUUM's result but those of previous VACUUM cycles.
So pgstats seems like it's both expensive and useless for the purpose.

Right now the only good solution is trawling the postmaster log.
Possibly something like pgfouine could track the numbers in a more
useful fashion.

regards, tom lane