On 29 January 2014 09:16, Magnus Hagander <mag...@hagander.net> wrote:
> On Wed, Jan 29, 2014 at 8:58 AM, Peter Geoghegan <p...@heroku.com> wrote:
>> I am not opposed in principle to adding new things to the counters
>> struct in pg_stat_statements. I just think that the fact that the
>> overhead of installing the module on a busy production system is
>> currently so low is of *major* value, and therefore any person that
>> proposes to expand that struct should be required to very conclusively
>> demonstrate that there is no appreciably increase in overhead. Having
>> a standard deviation column would be nice, but it's still not that
>> important. Maybe when we have portable atomic addition we can afford
>> to be much more inclusive of that kind of thing.
> Not (at this point) commenting on the details, but a big +1 on the fact that
> the overhead is low is a *very* important property. If the overhead starts
> growing it will be uninstalled - and that will make things even harder to
> diagnose.

+1 also. I think almost everybody agrees.

Having said that, I'm not certain that moves us forwards with how to
handle the patch at hand. Here is my attempt at an objective view of
whether or not to apply the patch:

The purpose of the patch is to provide additional help to diagnose
performance issues. Everybody seems to want this as an additional
feature, given the above caveat. The author has addressed the original
performance concerns there and provided some evidence to show that
appears effective.

It is possible that adding this extra straw breaks the camel's back,
but it seems unlikely to be that simple. A new feature to help
performance problems will be of a great use to many people; complaints
about the performance of pg_stat_statements are unlikely to increase
greatly from this change, since such changes would only affect those
right on the threshold of impact. The needs of the many...

Further changes to improve scalability of pg_stat_statements seem
warranted in the future, but I see no reason to block the patch for
that reason.

If somebody has some specific evidence that the impact outweighs the
benefit, please produce it or I am inclined to proceed to commit.

 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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