Re: Add min and max execute statement time in pg_stat_statement

On Fri, Jan 31, 2014 at 5:07 AM, Mitsumasa KONDO
<kondo(dot)mitsumasa(at)gmail(dot)com> wrote:
> And past result shows that your patch's most weak point is that deleting
> most old statement
> and inserting new old statement cost is very high, as you know.

No, there is no reason to imagine that entry_dealloc() is any slower,
really. There will perhaps be some additional contention with shared
lockers, but that isn't likely to be a major aspect. When the hash
table is full, in reality at that point it's very unlikely that there
will be two simultaneous sessions that need to create a new entry. As
I said, on many of the systems I work with it takes weeks to see a new
entry. This will be especially true now that the *.max default is
5,000.

> It accelerate to affect
> update(delete and insert) cost in pg_stat_statements table. So you proposed
> new setting
> 10k in default max value. But it is not essential solution, because it is
> also good perfomance
> for old pg_stat_statements.

I was merely pointing out that that would totally change the outcome
of your very artificial test-case. Decreasing the number of statements
to 5,000 would too. I don't think we should assign much weight to any
test case where the large majority or all statistics are wrong
afterwards, due to there being so much churn.

--
Peter Geoghegan