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Re: [HACKERS] Yipes, I'm getting bit by duplicate tuples

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: tgl(at)sss(dot)pgh(dot)pa(dot)us (Tom Lane)
Cc: pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] Yipes, I'm getting bit by duplicate tuples
Date: 1998-09-23 22:21:44
Message-ID: 199809232221.SAA22315@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
> I've got a production application (currently running on 6.3.2 + Btree
> patch) in which several clients concurrently update the same set of
> tables.  The client requests are pretty simple --- SELECTs using
> btree-indexed fields, INSERTs, and UPDATEs.  Updates are always of the
> form "UPDATE table SET some-fields WHERE id = constant", where id
> is a unique identifier field with a btree index.  (Some of the tables
> use OID for this, some have an ID field generated off a SEQUENCE
> object.)  We never delete any tuples.  We use BEGIN TRANSACTION / LOCK /
> END TRANSACTION to protect groups of related updates.
> 
> What we're seeing is that under load, the tables occasionally get
> corrupted by duplication of tuples.  It's clearly a backend bug, because
> the duplicates are duplicate right down to the OID.  There's no way that
> a client could request creation of two tuples with the same OID, right?
> 
> I speculate that the backend is updating a tuple (or rearranging a page
> to update another one) and neglecting to mark the old copy dead.
> I don't know whether having multiple backends running in parallel is
> necessary to cause the bug, but it seems likely --- if this kind of
> thing happened with only one backend, surely it'd have been fixed long
> ago.
> 
> The interesting thing is that both tuples are getting inserted into
> the table's indexes, because the way we generally find out about it
> is that a client fails with "Cannot insert a duplicate key into a unique
> index" when it tries to UPDATE one of the duplicated tuples.  Since the
> UPDATE is "... WHERE id = constant", presumably this indicates that both
> tuples are getting found by the index search.  (But there are duplicate
> tuples, not duplicate index records pointing at the same tuple, because
> I see the duplicates if I just dump out the table with COPY.  Also,
> once or twice I have seen near-duplicates in which the OID is the same
> but one or two of the other fields disagree.  Possibly these were formed
> at the instant of updating the original tuple to modify those fields,
> with the original tuple not getting cleared?)
> 
> I rooted through the archives and found mention of this sort of thing
> from last year sometime, as well as a current thread on pgsql-admin
> that looks like it might be related.
> 
> Can anyone give me guidance on dealing with this?  Any chance that a
> fix is already present in 6.4-beta?  I'm going to start out by trying
> to develop a repeatable test case, but I don't really know enough about
> the backend innards to debug it competently...

I fixed some very strange indexing code that could have been the cause. 
It was one of those, "Gee, wonder why things are working with this
code."

It is part of 6.4, so it may already be fixed.  Also, the locking of
buffer pages in the pre-6.4 code was kind of messed up, so it is
possible locks were not being kept.  I believe to have fixed that too,
so running it though a test with 6.4 may show it is fixed.

-- 
  Bruce Momjian                        |  maillist(at)candle(dot)pha(dot)pa(dot)us
  830 Blythe Avenue                    |  http://www.op.net/~candle
  Drexel Hill, Pennsylvania 19026      |  (610) 353-9879(w)
  +  If your life is a hard drive,     |  (610) 853-3000(h)
  +  Christ can be your backup.        |  

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