On Mon, Oct 12, 2020 at 12:38 PM Greg Nancarrow <gregn4...@gmail.com> wrote: > > On Mon, Oct 12, 2020 at 5:36 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > > > > > I have not thought about this yet but I don't understand your > > > proposal. How will you set it only for the scope of Gather (Merge)? > > > The execution of the Gather node will be interleaved with the Insert > > > node, basically, you fetch a tuple from Gather, and then you need to > > > Insert it. Can you be a bit more specific on what you have in mind for > > > this? > > > > > > > One more thing I would like to add here is that we can't exit > > parallel-mode till the workers are running (performing the scan or > > other operation it is assigned with) and shared memory is not > > destroyed. Otherwise, the leader can perform un-safe things like > > assigning new commandsids or probably workers can send some error for > > which the leader should still be in parallel-mode. So, considering > > this I think we need quite similar checks (similar to parallel > > inserts) to make even the Select part parallel for Inserts. If we do > > that then you won't face many of the problems you mentioned above like > > executing triggers that contain parallel-unsafe stuff. I feel still it > > will be beneficial to do this as it will cover cases like Insert with > > GatherMerge underneath it which would otherwise not possible. > > > > Yes, I see what you mean, exiting parallel-mode can't be done safely > where I had hoped it could, so looks like, even for making just the > Select part of Insert parallel, I need to add checks (along the same > lines as for Parallel Insert) to avoid the parallel Select in certain > potentially-unsafe cases. >
Right, after we take care of that, we can think of assigning xid or things like that before we enter parallel mode. Say we have a function like PrepareParallelMode (or PrepareEnterParallelMode) or something like that where we can check whether we need to perform parallel-safe-write operation (as of now Insert) and then do the required preparation like assign xid, etc. I think this might not be idle because it is possible that we don't fetch even a single row (say due to filter condition) which needs to be inserted and then we will waste xid but such cases might not occur often enough to worry. -- With Regards, Amit Kapila.
