Re: idea: log_statement_sample_rate - bottom limit for sampling
От | Tomas Vondra |
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Тема | Re: idea: log_statement_sample_rate - bottom limit for sampling |
Дата | |
Msg-id | 20190727221948.irg6sfqh57dynoc7@development обсуждение исходный текст |
Ответ на | Re: idea: log_statement_sample_rate - bottom limit for sampling (Pavel Stehule <pavel.stehule@gmail.com>) |
Ответы |
Re: idea: log_statement_sample_rate - bottom limit for sampling
Re: idea: log_statement_sample_rate - bottom limit for sampling |
Список | pgsql-hackers |
Hi, I've started reviewing this patch, thinking that maybe I could get it committed as it's marked as RFC. In general I agree with having this fuature, but I think we need to rethink the GUC because the current approach is just confusing. The way the current patch works is that we have three GUCs: log_min_duration_statement log_statement_sample_limit log_statement_sample_rate and it essentially works like this: - If the duration exceeds log_min_duration_statement, we start sampling the commands with log_statement_sample rate. - If the duration exceeds log_statement_sample_limit, we just log the command every time (i.e. we disable sampling, using sample rate 1.0). IMO that's bound to be confusing for users, because one threshold behaves as minimum while the other behaves as maximum. What I think we should do instead is to use two minimum thresholds. 1) log_min_duration_sample - enables sampling of commands, using the existing GUC log_statement_sample_rate 2) log_min_duration_statement - logs all commands exceeding this I think this is going to be much easier for users to understand. The one difference between those approaches is in how they work with existing current settings. That is, let's say you have log_min_duration_statement = 1000 log_statement_sample_rate = 0.01 then no queries below 1000ms will be logged, and 1% of longer queries will be sampled. And with the original config (as proposed in v3 of the patch), this would still work the same way. With the new approach (two min thresholds) it'd behave differently, because we'd log *all* queries longer than 1000ms (not just 1%). And whether we'd sample any queries (using log_statement_sample_rate) would depend on how we'd pick the default value for the other threshold. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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