Re: Mat view sometimes taking 10x the time to refresh concurrently

Wells Oliver <wells(dot)oliver(at)gmail(dot)com> writes:
> Hey, we have a mat view that normally takes ~18m to re-materialize
> concurrently, which we do daily. Maybe once a week, it takes ~180m to
> refresh, and we're at a loss as to why.
> We are running backs during the same general time window, where we do
> backup the schema where this mat view is located, could this be an issue?
> We have not noticed any lock errors in the backup or log, though.
> Any tips on tracing this down would be appreciated.

My own mindset would be to wonder if a different/worse plan is being
chosen. You could investigate that perhaps by running an EXPLAIN
on the matview's query just before each refresh, to see if it changes.

Checking pg_locks for ungranted locks while the REFRESH is running
would be good to do too, although I'd think that a backup vs. a
concurrent refresh shouldn't have that sort of problem.

It seems possible also that you're just maxing out the machine's
I/O capacity between these two tasks.

regards, tom lane