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Re: Sequences - jumped after power failure

From: Steve T <steve(at)retsol(dot)co(dot)uk>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: PostGreSQL <pgsql-novice(at)postgresql(dot)org>
Subject: Re: Sequences - jumped after power failure
Date: 2008-04-15 17:04:00
Message-ID: 1208279040.27471.303.camel@retsol610 (view raw or flat)
Thread:
Lists: pgsql-novice
Cheers Tom.
I'll note that down under the section '...things to note should the
server crash...' - just under the note that says '...get a UPS...'

On Tue, 2008-04-15 at 10:58 -0400, Tom Lane wrote:

> Steve T <steve(at)retsol(dot)co(dot)uk> writes:
> > I have a set of claims tables that cover the claim itself, the customer,
> > contact points etc. Yesterday there was a power failure and the server
> > suffered an immediate power outage. When the server came back,
> > everything seemed fine, apart from the fact that the claim related
> > sequences had all jumped and left a gap of 33 (last was 52 before power
> > failure, next one allocated after power failure 85). This seems
> > consistent across all the tables related to the claim (it may be across
> > the   tables in the database - I haven't checked all of them as yet).
> 
> > Does this sound feasible and if so, what is the cause?
> 
> Yeah, this is intentional behavior designed to reduce the amount of disk
> write traffic generated by nextval()s.  From a standing start, a
> nextval() actually advances the sequence 33 times (1 + SEQ_LOG_VALS),
> so that the next 32 nextval()s won't need to generate their own WAL
> records.  I guess you must have crashed before that first nextval()
> was able to commit its result into the database ...
> 
> 			regards, tom lane



Steve Tucknott
ReTSol Ltd

DDI:         01323 488548
Mobile:     0773 671 5772

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