On Sat, Feb 29, 2020 at 11:22 AM Jeff Janes <jeff.janes@xxxxxxxxx> wrote:
On Thu, Feb 27, 2020 at 11:33 AM Ben Snaidero <bensnaidero@xxxxxxxxxx> wrote:I have the following query that was on average running in ~2ms suddenly jump up to on average ~25ms.What are you averaging over? The plan you show us is slow enough that if you were averaging over the last 1000 executions, that one execution could skew the entire average just by itself. When individual execution times can vary over 4 powers of 10, I don't think averages are a very good way of analyzing things.This query is called millions of time per day and there were cases of the query taking 20-30 seconds. Below is the explain analyze of one such example.
When seeing this issue, the server was under some CPU pressure but even with that, I would not think it should get as slow as shown below as we are using SSDs and none of the windows disk counters (IOPS, queue length) show any value that would be of concern.What is the average and expected random read latency on your SSDs? Have you benchmarked them (outside of the database system) to see if they are performing as expected?Rows Removed by Filter: 91686
Buffers: shared hit=12102 read=13259 written=111Do the faster executions have fewer rows removed by the filter (and fewer buffers read), or are they just faster despite having about the same values?We took a perfview during the issue and below is the call stack from a process running this query, two call paths are shown.I've never used perfview. But if I try to naively interpret it similar to gdb backtrace, it doesn't make much sense to me. InitBufTable is only called by "postmaster" while starting the database, how could it be part of call paths during regular operations? Are these views of the slow-running back end itself, or of some other postgresql process which was idle at the time the snapshot was taken?Cheers,Jeff
Query statistics were averaged over ~3million calls so I don't think a single outlier would skew the results too much.
The perfview call stack is similar to gdb backtrace. I am 99% sure that this call path is from the backend running this query as we queried pg_stat_activity at the time of the perfview and cross-referenced the PIDs. That said I am going to try building on windows with debug symbols enabled and see if I can use gdb to debug and confirm.
