2011/12/10 Tomas Vondra <t...@fuzzy.cz> > On 10.12.2011 23:40, Daniel Cristian Cruz wrote: > > At work we have a 24 cores server, with a load average around 2.5. > > A single query is processes by a single CPU, so even if the system is > not busy a single query may hit CPU bottleneck. The real issue is the > instrumentation overhead - timing etc. On some systems (with slow > gettimeofday) this may be a significant problem as the query hits the > CPU boundary sooner. >
Yes, I forgot it will run on the same PID. Since analyze will cause all queries to slow down, maybe the 24 cores could became overloaded. > > > I don't know yet if a system which use some unused CPU to minimize the > > load of a bad query identified early is bad or worse. > > Not really, due to the "single query / single CPU" rule. I guess it will be a nice tool to run in the validation server. > > Indeed, I don't know if my boss would let me test this at production > > too, but it could be good to know how things work in "auto-pilot" mode. > > What I was pointing out is that you probably should not enable loggin > "explain analyze" output by "auto_explain.log_analyze = true". There are > three levels of detail: > > 1) basic, just log_min_duration_statement > > 2) auto_explain, without 'analyze' - just explain plain > > 3) auto_explain, with 'analyze' - explain plan with actual values > > Levels (1) and (2) are quite safe (unless the minimum execution time is > too low). > I would start with 5 seconds. Reading the manual again and I saw that enabling analyze, it analyze all queries, even the ones that wasn't 5 second slower. And understood that there is no way to disable for slower queries, since there is no way to know it before it ends... I read Bruce blog about some features going to multi-core. Could explain analyze go multi-core too? Another thing I saw is that I almost never look at times in explain analyze. I always look for rows divergence and methods used for scan and joins when looking for something to get better performance. I had the nasty idea of putting a // before de gettimeofday in the code for explain analyze (I guess it could be very more complicated than this). Sure, its ugly, but I think it could be an option for an explain analyze "with no time", and in concept, it's what I'm looking for. -- Daniel Cristian Cruz クルズ クリスチアン ダニエル
