Re: [PATCH] Add log_transaction setting

On Thu, 4 Jul 2024 at 21:46, Sergey Solovev
<sergey(dot)soloviev(at)tantorlabs(dot)ru> wrote:
>
> Hi.
>
> We encountered a problem with excessive logging when transaction is
> sampled.
> When it is getting sampled every statement is logged, event SELECT. This
> can
> lead to performance degradation and log polluting.
> I have added new setting to filter statements when transaction is
> sampled - log_transaction.
>
> Overview
> ===================
> This parameter works very similar to log_statement, but it filters out
> statements only
> in sampled transaction. It has same values as log_statement, access
> rights (only superuser),
> setup in postgresql.conf or by superuser with SET.
> But by default it has value "all" in compliance with existing behaviour
> (to log every statement).
>
> Example usage
> ==================
> Log every DDL, but only subset of other statements.
>
> postgresql.conf
> > log_transaction = ddl
> > log_statement = all
> > log_transaction_sample_rate = 1
> > log_statement_sample_rate = 0.3
>
> backend:
> > BEGIN;
> > CREATE TABLE t1(value text);
> > INSERT INTO t1(value) VALUES ('hello'), ('world');
> > SELECT * FROM t1;
> > DROP TABLE t1;
> > COMMIT;
>
> logfile:
> > LOG: duration: 6.465 ms statement: create table t1(value text);
> > LOG: statement: insert into t1(value) values ('hello'), ('world');
> > LOG: duration: 6.457 ms statement: drop table t1;
>
> As you can see CREATE and DROP were logged with duration (because it is
> DDL), but
> only INSERT was logged (without duration) - not SELECT.
>
> Testing
> ===================
> All tests are passed - default configuration is compatible with existing
> behaviour.
> Honestly, I could not find any TAP/regress tests that would test logging
> - some
> tests only use logs to ensure test results.
> I did not find suitable place for such tests, so no tests provided
>
> Implementation details
> ===================
> I have modified signature of check_log_duration function - this accepts
> List of
> statements that were executed. This is to decide whether we should log
> current statement if transaction is sampled.
> But this list can be empty, in that case we decide to log. NIL is passed
> only
> in fast path, PARSE and BIND functions - by default these are logged
> if transaction is sampled, so we are compliant with existing behaviour
> again.
> In first implementation version, there was boolean flag (instead of
> List), but
> it was replaced by List to defer determining (function call) whether it
> is worth logging.
>
> -------------
> Best regards, Sergey Solovev

Hi!

As I understand, the need of this GUC variable comes from fact, that
is the transaction is sampled for logging, all statements within this
tx are logged and this is not configurable now, correct?
Well, if this is the case, why should we add a new GUC? Maybe we
should use `log_statement` in this case too, so there is a bug, that
log_statement is honored not during tx sampling?

Also, tests are failing[1]

[1] https://cirrus-ci.com/task/5645711230894080

--
Best regards,
Kirill Reshke