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VACUUM/ANALYZE counting of in-doubt tuples

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: VACUUM/ANALYZE counting of in-doubt tuples
Date: 2007-11-17 17:27:48
Message-ID: 18064.1195320468@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
There was some discussion in pgsql-performance about the problem that
the live-and-dead-tuple counts that ANALYZE reports to the stats
collector don't reflect insert-in-progress tuples properly:
http://archives.postgresql.org/pgsql-performance/2007-11/msg00225.php
I proposed a patch here:
http://archives.postgresql.org/pgsql-patches/2007-11/msg00169.php
to switch ANALYZE over to using HeapTupleSatisfiesVacuum() to determine
tuple status, which would at least give it awareness of tuples that are
in-doubt because of concurrent transactions.

On further thought though, that's not the whole story, and in fact
VACUUM itself isn't doing very well at accounting for in-doubt tuples.
The current implementation is that whatever live and dead tuple totals
are arrived at by a VACUUM or ANALYZE are sent to the stats collector
and simply overwrite its counters on arrival.  Meanwhile, the
transaction that is responsible for an in-doubt tuple will send a
stats message to increment either the live-tuple or dead-tuple count
as appropriate when it commits or aborts.  If that happens before
the VACUUM or ANALYZE completes, the increment will get overwritten
by VACUUM/ANALYZE's total; if afterwards, the increment will get
added onto the total.  So ideally we'd count the state change as already
done if we knew the other transaction would commit first, otherwise
not.

Since we don't know whether the other transaction will commit or abort,
much less when, it seems hopeless to have any really accurate accounting
for in-doubt tuples.  We could avoid the whole problem if we gave up on
having VACUUM/ANALYZE reset the stats counters (instead, VACUUM would
just send an incremental dead-tuple-count reduction, and ANALYZE would
do nothing); as indeed was already proposed by Itagaki-san.  But I am
still convinced that that cure would be worse than the disease --- if
the counters are driven entirely by incremental updates then there is
no way to correct errors that will inevitably creep in, and the
accumulation of errors will allow the counts to diverge arbitrarily
far from reality.  I'm happier with the idea that the error in the
counts can be bounded by the number of in-doubt tuples seen by the
last VACUUM/ANALYZE.

So having said all that, we need to devise heuristic rules that minimize
the error.  In that light, I suggest that the accounting rules assume
that the transactions responsible for in-doubt tuples will finish after
the VACUUM/ANALYZE finishes.  That probably sounds wrong, because in
most of the sorts of applications we think about, updating transactions
are short and are likely to finish before the VACUUM/ANALYZE does.  But
what I'm thinking is that a short transaction by definition can't change
very many rows.  The cases that can seriously hurt the accuracy of our
counts are long-running bulk inserts or updates, and for those it seems
entirely likely that ANALYZE in particular could finish first.

Given that assumption, then the rule should be to not count
INSERT_IN_PROGRESS tuples at all --- they will be added to the 
appropriate count when the inserting transaction commits or aborts.
And DELETE_IN_PROGRESS tuples should be counted as live --- if the
deleting transaction commits, it will increment the dead-tuple count
at that time, and if it aborts then the tuple remains live.

I feel fairly comfortable with this analysis for ANALYZE, and the
patch I posted yesterday can easily be adjusted to accommodate it.
However, what of VACUUM?  As that code stands, every non-removable
tuple (including RECENTLY_DEAD ones) is counted as live, and the
dead-tuples count gets reset to zero.  That seems clearly bogus.
But the other-transaction-commits-second hypothesis seems a good bit
more dubious for VACUUM than it is for ANALYZE.

Should we attempt to adjust VACUUM's accounting as well, or leave it
for 8.4?  For that matter, should adjusting ANALYZE be left for 8.4?
Thoughts?

			regards, tom lane

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