Re: log_duration_sample config option patch

Поиск
Список
Период
Сортировка
От Timo Savola
Тема Re: log_duration_sample config option patch
Дата
Msg-id 49801C33.4010409@dynamoid.com
обсуждение исходный текст
Ответ на Re: log_duration_sample config option patch  (Euler Taveira de Oliveira <euler@timbira.com>)
Список pgsql-hackers
Euler Taveira de Oliveira wrote:

> IIRC pgFouine shows exact percentage of query by type; this new GUC implies we
> can not rely on it anymore. Also, you could skip us from logging long (and
> bad) running queries. Another statistics data in pgFouine will suffer from the
> same problem. I see your point in reducing amount of log data but I think it
> will limit the tool usability (you can always start/stop statement collection
> at runtime).

The accuracy of numbers suffers - yes.  But this option basically gives 
you the choice of logging all queries for a short time (leaving out 
queries before and after that time period) or some queries for a long 
time (leaving out randomly selected queries in between).  If you're 
interested in a bigger picture, the numbers will be more meaningful. 
(Also, if the number of queries is high enough for someone to use this 
option, it means that there will be a lot of samples anyway.)

For example, it might be more useful to log 1% of queries during the 
peak hours of each day of the month, than all queries during the peak 
hours of one day.  Or you could log 0.01% of all queries always... 
Especially queries which are executed many times can be found just as 
well with this approach, and chronically slow queries will eventually 
pop up also.  But if someone wants to specifically track any and all 
long queries, then there's the option to not enable log_duration_sample 
but to set log_min_duration_statement to something appropriate instead.

Another point is that during the time of the biggest load (= the most 
interesting time) full logging would increase I/O load and bias the 
results (and degrade the service quality).  By spreading out the load, 
the bias is smaller and the logging can actually be enabled at all 
during peak hours.

Thanks for the feedback.

timo


В списке pgsql-hackers по дате отправления:

Предыдущее
От: Richard Huxton
Дата:
Сообщение: Re: 8.4 release planning
Следующее
От: Peter Eisentraut
Дата:
Сообщение: Re: 8.4 release planning