On Thu, 4 Jul 2024 at 21:46, Sergey Solovev
<sergey.soloviev@tantorlabs.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

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?