Re: Slow concurrent update of same row in a given table

On Thu, 29 Sep 2005, Rajesh Kumar Mallah wrote:

> On 9/29/05, Gavin Sherry <swm@alcove.com.au> wrote:
> > On Wed, 28 Sep 2005, Rajesh Kumar Mallah wrote:
> >
> > > > > Number of Copies | Update perl Sec
> > > > >
> > > > > 1  --> 119
> > > > > 2  ---> 59
> > > > > 3  --->  38
> > > > > 4  ---> 28
> > > > > 5 --> 22
> > > > > 6 --> 19
> > > > > 7 --> 16
> > > > > 8 --> 14
> > > > > 9 --> 11
> > > > > 10 --> 11
> > > > > 11 --> 10
> > > >
> > > > So, 11 instances result in 10 updated rows per second, database wide or
> > > > per instance? If it is per instance, then 11 * 10 is close to the
> > > > performance for one connection.
> > >
> > >
> > > Sorry do not understand the difference between "database wide"
> > > and "per instance"
> >
> > Per instance.
> >
> > >
> > > >
> > > > That being said, when you've got 10 connections fighting over one row, I
> > > > wouldn't be surprised if you had bad performance.
> > > >
> > > > Also, at 119 updates a second, you're more than doubling the table's
> > > > initial size (dead tuples) each second. How often are you vacuuming and
> > > > are you using vacuum or vacuum full?
> > >
> > >
> > > Yes I realize the obvious phenomenon now, (and the uselessness of the script)
> > >  , we should not consider it a performance degradation.
> > >
> > > I am having performance issue in my live database thats why i tried to
> > > simulate the situation(may the the script was overstresser).
> > >
> > > My original problem is  that i send 100 000s of emails carrying a
> > > beacon for tracking readership every tuesday and on wednesday i see
> > > lot of the said query in pg_stat_activity each of these query update
> > > the SAME row that corresponds to the dispatch of last day and it is
> > > then i face the performance problem.
> > >
> > > I think i can only post further details next wednesday , please lemme
> > > know how should i be dealing with the situation if each the updates takes
> > > 100times more time that normal update duration.
> >
> > I see. These problems regularly come up in database design. The best thing
> > you can do is modify your database design/application such that instead of
> > incrementing a count in a single row, you insert a row into a table,
> > recording the 'dispatch_id'. Counting the number of rows for a given
> > dispatch id will give you your count.
> >
>
> sorry i will be accumulating huge amount of rows in seperate table
> with no extra info when i really want just the count. Do you have
> a better database design in mind?
>
> Also i encounter same problem in implementing read count of
> articles in sites and in counting banner impressions where same
> row get updated by multiple processes frequently.

As I said in private email, accumulating large numbers of rows is not a
problem. In your current application, you are write bound, not read bound.
I've designed many similar systems which have hundred of millions of rows.
It takes a while to generate the count, but you just do it periodically in
non-busy periods.

With 8.1, constraint exclusion will give you significantly better
performance with this system, as well.

Thanks,

Gavin