Re: Add min and max execute statement time in pg_stat_statement

Andrew Dunstan <andrew@dunslane.net> writes:
> On 12/21/2014 01:23 PM, Alvaro Herrera wrote:
>> The point, I think, is that without atomic instructions you have to hold
>> a lock while incrementing the counters.

> Hmm, do we do that now?

We already have a spinlock mutex around the counter adjustment code, so
I'm not sure why this discussion is being held.

> I would like someone more versed in numerical analysis than me to 
> tell me how safe using sum of squares actually is in our case.

That, on the other hand, might be a real issue.  I'm afraid that
accumulating across a very long series of statements could lead
to severe roundoff error in the reported values, unless we use
a method chosen for numerical stability.
        regards, tom lane