Date: Sun, 4 Aug 2019 22:48:48 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Adrien Nayrat <adrien.nayrat@anayrat.info>,
	pgsql-hackers@lists.postgresql.org
Subject: Re: idea: log_statement_sample_rate - bottom limit for sampling
Message-ID: <20190804204848.7f7ypsbijpgxojxx@development>
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On Sun, Aug 04, 2019 at 04:25:12PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> On Sun, Aug 04, 2019 at 03:16:12PM -0400, Tom Lane wrote:
>>> Isn't the issue here the interaction between log_transaction_sample_rate
>>> and log_min_duration_statement?
>
>> No, that interaction only affects statement-level sampling.
>
>OK, I was confusing the features.
>
>> For transaction-level sampling we do the sampling independently of the
>> statement duration, i.e. we when starting a transaction we determine
>> whether the whole transaction will be sampled. It has nothing to do with
>> the proposed log_statement_sample_limit.
>
>So, to clarify: our plan is that a given statement will be logged
>if any of these various partial-logging features says to do so?
>

Yes, I think that's the expected behavior.

- did it exceed log_min_duration_statement? -> log it
- is it part of sampled xact? -> log it
- maybe sample the statement (to be reverted / reimplemented)

>(And the knock on HEAD's behavior is exactly that it breaks that
>independence for log_min_duration_statement.)
>

Yeah. There's no way to use sampling, while ensure logging of all
queries longer than some limit.


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services