Re: Sample rate added to pg_stat_statements

On 22.11.2024 09:08, Alexander Korotkov wrote:
> On Wed, Nov 20, 2024 at 12:07 AM Michael Paquier<michael(at)paquier(dot)xyz> wrote:
>> On Tue, Nov 19, 2024 at 09:39:21AM -0500, Greg Sabino Mullane wrote:
>>> Oh, and a +1 in general to the patch, OP, although it would also be nice to
>>> start finding the bottlenecks that cause such performance issues.
>> FWIW, I'm not eager to integrate this proposal without looking at this
>> exact argument in depth.
>>
>> One piece of it would be to see how much of such "bottlenecks" we
>> would be able to get rid of by integrating pg_stat_statements into
>> the central pgstats with the custom APIs, without pushing the module
>> into core. This means that we would combine the existing hash of pgss
>> to shrink to 8 bytes for objid rather than 13 bytes now as the current
>> code relies on (toplevel, userid, queryid) for the entry lookup (entry
>> removal is sniped with these three values as well, or dshash seq
>> scans). The odds of conflicts still still play in our favor even if
>> we have a few million entries, or even ten times that.
> If you run "pgbench -S -M prepared" on a pretty large machine with
> high concurrency, then spin lock in pgss_store() could become pretty
> much of a bottleneck. And I'm not sure switching all counters to
> atomics could somehow improve the situation given we already have
> pretty many counters.
>
> I'm generally +1 for the approach taken in this thread. But I would
> suggest introducing a threshold value for a query execution time, and
> sample just everything below that threshold. The slower query
> shouldn't be sampled, because it can't be too frequent, and also it
> could be more valuable to be counter individually (while very fast
> queries probably only matter "in average").
>
> ------
> Regards,
> Alexander Korotkov
> Supabase

I really liked your idea, and I’d like to propose an enhancement that I
believe improves it further.

Yes, if a query’s execution time exceeds the threshold, it should always
be tracked without sampling. However, for queries with execution times
below the threshold, the sampling logic should prioritize shorter
queries over those closer to the threshold. In my view, the ideal
approach is for shorter queries to have the highest probability of being
sampled, while queries closer to the threshold are less likely to be
sampled.

This behavior can be achieved with the following logic:

pg_stat_statements.sample_exectime_threshold * random(0, 1) < total_time

Here’s how it works:

* As a query’s execution time approaches zero, the probability of it
being sampled approaches one.
* Conversely, as a query’s execution time approaches the threshold,
the probability of it being sampled approaches zero.

In other words, the sampling probability decreases linearly from 1 to 0
as the execution time gets closer to the threshold.

I believe this approach offers an ideal user experience. I have attached
a new patch implementing this logic. Please let me know if you have any
feedback regarding the comments in the code, the naming of variables or
documentation. I’m always open to discussion.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.