On Thu, Jun 25, 2020 at 4:56 PM David Steele <[email protected]> wrote:
> On 6/25/20 8:43 AM, Magnus Hagander wrote: > > On Thu, Jun 25, 2020 at 2:11 PM David Steele <[email protected] > > <mailto:[email protected]>> wrote: > > On 6/24/20 6:27 PM, Tom Lane wrote: > > > > > > I was able to force it like this: > > > > > > regression=# set force_parallel_mode TO 'on'; > > > SET > > > > Ah, yes, that works. Now at least I can test it. > > > > > It doesn't seem terribly likely that anybody would be using > > > force_parallel_mode = on in production, but perhaps there's some > > > reasonable combination of the other parallelization planning GUCs > > > that can make this plan look attractive. > > > > I'll also ask the user if they have this GUC enabled. > > The user confirmed they are running with force_parallel_mode=on so that > probably explains why we have never seen this in the field before. > That's good. > Maybe have pgbackrest issue an explicit SET force_parallel_mode=off when > > it runs against a 9.6? > > According to the documentation the way to disable parallelism is: > > set max_parallel_workers_per_gather = 0 > Interesting. I had somehow gotten into my head that force_parallel_mode would override that. But it doesn't. So we added that to session initialization in pgBackRest: > > > https://github.com/pgbackrest/pgbackrest/commit/ea04ec7b3f4c6cf25c1b827c25c6a3c5896159a8 Personally I would've done it *just* for 9.6 and not for 10+, but that's mostly semantic :) But if you do it for 9.6 then *eventually* you will be able to retire it. I'm worried that (as Tom said) the planner might find another reason to > choose a parallel query. > > I'm looking at this as more than a fix for 9.6. I can't see any reason > for the backup control session to run queries in parallel and possibly > use more resources, so parallelism is disabled for all versions that > support it. > Right. But since the parameters are flagged as parallel restricted in 10+... Or are you saying you're worried about other things, completely unrelated to start/stop backup, that the session might run? -- Magnus Hagander Me: https://www.hagander.net/ <http://www.hagander.net/> Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>
