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Vacuuming leaked temp tables (once again)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Vacuuming leaked temp tables (once again)
Date: 2008-06-27 15:45:13
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
This thread
kind of wandered off into the weeds after identifying a semi-related
bug in CLUSTER, but the original problem still remains: if a backend
crashes after creating some temp tables, the tables remain present.
Such tables will get recycled next time someone reuses the same
pg_temp_NNN schema.  But if the failed backend had been occupying an
unusually high-numbered BackendId slot, then its pg_temp_NNN schema
might go unused for a long time --- long enough for the temp tables to
pose an xid-wraparound problem.  There's another report of this issue
today in pgsql-general.

The only solution proposed in that thread was to auto-delete temp
tables at postmaster restart; which I opposed on the grounds that
throwing away data right after a crash was a terrible idea from a
forensic standpoint.  I still think that, but I had another idea
about how to cope with the situation.  It's reasonably easy to
tell (by looking into the sinval state) whether a given BackendId
slot is actually in use, so we could detect whether a temp table
actually belongs to a live backend or not.  What I'm thinking is
we should adjust autovacuum so that it will apply anti-wraparound 
vacuuming operations even to temp tables, if they belong to pg_temp
schemas that belong to inactive BackendId slots.  This'd fix the
wraparound issue without any risk of discarding data that someone
might want back.

Note that this should be safe even if someone claims the pg_temp_NNN
schema and tries to drop the old temp table while we're vacuuming it.
Operations on temp tables take the normal types of locks, so that
will get interlocked properly.

A small hole in this idea is that the BackendId slot might be occupied
by some new backend that actually hasn't created any temp tables yet
(hence not "taken possession" of the pg_temp_NNN schema).  We could fix
that by making each backend's has-temp-tables state globally visible.
However, I'm inclined to think it's not really an issue, because you
wouldn't get into trouble unless this was always the case over many
repeated autovacuum visits to the table, which seems pretty improbable.

Another issue is that leftover temp tables would be significantly more
likely to be self-inconsistent than normal tables, since operations on
them are not WAL-logged and it's entirely likely that the owning backend
crashed with some dirty pages not written out from its local buffers.
AFAICS this shouldn't be any big problem for vacuuming the table proper,
since heap pages are pretty independent, at least at the level
understood by plain vacuum.  There is a risk that indexes would be
corrupt enough to make vacuum error out, thus preventing the xid
wraparound cleanup from completing.  But that leaves us no worse off
than we are now, and at least there would be signs of distress in the
postmaster log for the DBA to see.

Or we could have autovacuum just drop orphaned temp tables, *if*
they have gotten old enough to need anti-wraparound vacuuming.
While I'm still uncomfortable with having autovac drop anything,
at least this would avoid the worst cases of "gee I really needed
that data to investigate the crash".  The main attractions of this
idea are avoiding the corrupt-index issue and not doing vacuuming
work that's 99.99% sure to be useless.


			regards, tom lane


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