[pgpool-general: 2058] Re: trouble with recovery of a downed node

Sean Hogan sean at compusult.net
Wed Aug 21 23:43:47 JST 2013

On 13-08-21 12:14 AM, Tatsuo Ishii wrote:
>> Tatsuo, thanks very much for your response.  I've tried online
>> recovery with the database clients disconnected, but it didn't have
>> any effect.  The database recovers correctly, works for a while and
>> then when a certain update takes place one of the nodes blows up. It's
>> not always the same statement but one of them is:
>> 2013-08-20 17:04:02 ERROR: pid 30688: pgpool detected difference of
>> the number of inserted, updated or deleted tuples. Possible last query
>> was: "UPDATE ws_cached_searches SET search_data = $1, cache_time =
>> current_timestamp WHERE cached_search_id = $2"
>> 2013-08-20 17:04:02 LOG: pid 30688: CommandComplete: Number of
>> affected tuples are: 1 0 1
> I think there's no instance way to find out the cause of the
> problem. I recommend following steps:
> 1) right after recovery, make sure that the contents of the DBs are
>     identical. For this you can use pg_dump or some existing tool to
>     compare database (e.g. export as text file and compare).
> 2) enable log_per_node_statement and wait until the error happens.
> 3) examine the log of SQL command issued to each DB node to look for
>     cause of the problem.
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese: http://www.sraoss.co.jp

Ah, I believe I understand the problem now: the sequence values are 
different (off by one) between the original and recovered databases.  
The sample pgpool_sample_pitr script has:

     SELECT setval(oid, nextval(oid)) FROM pg_class WHERE relkind = 'S'

When I change that to

     SELECT setval(oid, 1*currval(oid)) FROM pg_class WHERE relkind = 'S'

I get identical databases and the problem has not reoccurred!  (The "1*" 
is to defeat optimization; not sure if it is needed.)

Thanks for your excellent advice.

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