Re: Lots of stuck queries after upgrade to 9.4

On 07/23/2015 11:31 AM, Spiros Ioannou wrote:
> Well, so far with commit_delay=0 no problems. I will report back of couse
> if something happens, but I believe that the problem may indeed be
> solved/masked with that setting.
>
> Rough description of our setup, or how to reproduce:
> * Timeseries data in table , say, "measurements", size: 3-4TB, about 1000
> inserts/second
> * table measurements also has a trigger on insert to also insert on
> measurements_a (for daily export purposes)
> Just the above would cause a stuck query after a few days.
>
> Now for exporting we run the following CTE query (measurements_b is an
> empty table, measurements_a has about 5GB)
> * WITH d_rows AS (DELETE FROM measurement_events_a RETURNING * )  INSERT
> INTO measurement_events_b SELECT * FROM d_rows;
> The above caused the problem to appear every time, after a 10-20 minutes.

I was able to reproduce something like this with pgbench, by running a
custom little module that calls the WaitXLogInsertionsToFinish() in a
tight loop, and checks that the value it returns moves monotonically
forward. With commit_delay on, once every minute or so, it moves backwards.

I'll investigate why that happens...

- Heikki