Re: Add min and max execute statement time in pg_stat_statement

Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> I could live with just stddev. Not sure others would be so happy.

FWIW, I'd vote for just stddev, on the basis that min/max appear to add
more to the counter update time than stddev does; you've got
this:

+ e->counters.total_sqtime += total_time * total_time;

versus this:

+ if (e->counters.min_time > total_time || e->counters.min_time == EXEC_TIME_INIT)
+ e->counters.min_time = total_time;
+ if (e->counters.max_time < total_time)
+ e->counters.max_time = total_time;

I think on most modern machines, a float multiply-and-add is pretty
darn cheap; a branch that might or might not be taken, OTOH, is a
performance bottleneck.

Similarly, the shared memory footprint hit is more: two new doubles
for min/max versus one for total_sqtime (assuming we're happy with
the naive stddev calculation).

If we felt that min/max were of similar value to stddev then this
would be mere nitpicking. But since people seem to agree they're
worth less, I'm thinking the cost/benefit ratio isn't there.

regards, tom lane