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Re: Maximum number of sequences that can be created

From: Andres Freund <andres(at)anarazel(dot)de>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Maximum number of sequences that can be created
Date: 2012-05-15 10:57:53
Message-ID: 201205151257.54141.andres@anarazel.de (view raw or flat)
Thread:
Lists: pgsql-performance
On Tuesday, May 15, 2012 08:29:11 AM Віталій Тимчишин wrote:
> 2012/5/13 Robert Klemme <shortcutter(at)googlemail(dot)com>
> 
> > On Sun, May 13, 2012 at 10:12 AM, Віталій Тимчишин <tivv00(at)gmail(dot)com>
> > 
> > wrote:
> > > 2012/5/11 Robert Klemme <shortcutter(at)googlemail(dot)com>
> > > 
> > >> On the contrary: what would be the /advantage/ of being able to create
> > >> millions of sequences?  What's the use case?
> > > 
> > > We are using sequences as statistics counters - they produce almost no
> > > performance impact and we can tolerate it's non-transactional nature. I
> > 
> > can
> > 
> > > imaging someone who wants to have a  sequence per user or other
> > > relation row.
> > 
> > I can almost see the point. But my natural choice in that case would
> > be a table with two columns.  Would that actually be so much less
> > efficient? Of course you'd have fully transactional behavior and thus
> > locking.
> 
> We've had concurrency problems with table solution (a counter that is
> updated by many concurrent queries), so we traded transactionality for
> speed. We are actually using this data to graph pretty graphs in nagios, so
> it's quite OK. But we have only ~10 sequences, not millions :)
I would rather suggest going with a suming table if you need to do something 
like that:

sequence_id | value
1 | 3434334
1 | 1
1 | -1
1 | 1
1 | 1
...

You then can get the current value with SELECT SUM(value) WHERE sequence_id = 
1. For garbage collection you can delete those values and insert the newly 
summed up value again.
That solution won't ever block if done right.

Andres

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