On 2023-10-23 16:31:30 -0700, Adrian Klaver wrote:
> On 10/23/23 14:55, pf@xxxxxxxxxxx wrote:
> > Next, I made a query list for 30 tables, turned on timing and pasted the
> > queries. Pasting results inline:
> > ncsbe=# select count(*) from ncvhis_2012_12_06;
> > select count(*) from ncvhis_2016_12_03;
> > select count(*) from ncvhis_2020_08_01;
> > select count(*) from ncvhis_2020_10_31;
[...]
> > Time: 72404.786 ms (01:12.405)
> > Time: 301.246 ms
> > Time: 409.974 ms
> > Time: 363.836 ms
> > Time: 351.167 ms
> > Time: 348.378 ms
> > Time: 348.712 ms
> > Time: 351.901 ms
> > Time: 487.837 ms
> > Time: 40987.826 ms (00:40.988)
> > Time: 76964.281 ms (01:16.964)
> > Time: 483.329 ms
> > Time: 18919.267 ms (00:18.919)
> > Time: 50896.978 ms (00:50.897)
> > Time: 25784.000 ms (00:25.784)
> > Time: 45407.599 ms (00:45.408)
> > Time: 75171.609 ms (01:15.172)
> > Time: 88871.004 ms (01:28.871)
> > Time: 128565.127 ms (02:08.565)
> > Time: 210248.222 ms (03:30.248)
> > Time: 246609.561 ms (04:06.610) 4 minutes! ;p
> > Time: 166406.283 ms (02:46.406)
> > Time: 291483.538 ms (04:51.484) nearly 5 minutes ;p ;p
> > Time: 2269.961 ms (00:02.270)
> > Time: 413.219 ms
> > Time: 433.817 ms
> > Time: 395.844 ms
> > Time: 7251.637 ms (00:07.252)
> > Time: 384.328 ms
> > Time: 384.887 ms
> >
> > I don't see a pattern in the above; matches the randomness I saw using
> > WB...
It would be interesting to see whether there's a difference in plan
between the slow and the fast queries.
If your experiment above is repeatable, you can just prefix every
query with
explain (analyze, buffers)
explain (analyze, buffers) select count(*) from ncvhis_2016_12_03;
explain (analyze, buffers) select count(*) from ncvhis_2020_08_01;
explain (analyze, buffers) select count(*) from ncvhis_2020_10_31;
...
If it is hard to reproduce (happens only sometimes), the best way to
catch is probably to enable auto_explain
<https://www.postgresql.org/docs/current/auto-explain.html>, possibly
with the auto_explain.log_analyze option and check the logs after it
happened. (auto_explain is generally a good idea, IMHO).
> > Before the above finished, I issued this command on another konsole...
> >
> > $ while true; do ls -l > /tmp/ll; date; done
This is unlikely to generate noticeable disk waits. The current
directory will be in the cache after the first ls and the writes happen
asynchroneously.
hp
--
_ | Peter J. Holzer | Story must make more sense than reality.
|_|_) | |
| | | hjp@xxxxxx | -- Charles Stross, "Creative writing
__/ | http://www.hjp.at/ | challenge!"
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