Re: pgsql: Add a new GUC and a reloption to enable inserts in parallel-mode

Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> On 2021-Mar-23, Robert Haas wrote:
>> Likewise, the XXX comment you added to max_parallel_hazard_walker
>> claims that some of the code introduced there is to compensate for an
>> unspecified bug in the rewriter.

> I think the CTE bug is this one:
> https://www.postgresql.org/message-id/flat/CAJcOf-fAdj=nDKMsRhQzndm-O13NY4dL6xGcEvdX5Xvbbi0V7g(at)mail(dot)gmail(dot)com
> while I can't disagree with the overall conclusion that it seems safer
> to revert parallel INSERT/SELECT given the number of alleged problems,
> it is true that this bug exists, and has gone unfixed.

Yeah, because it's not totally clear whether we want to fix it by
disallowing the case, or by significantly changing the way the
rewriter works. It'd be good if some of the folks on this thread
weighed in on that choice.

(Having said that, another complaint about this particular comment
is that it doesn't mention how the planner should be changed once
the rewriter is fixed. Is it sufficient to delete the stanza below
the comment? Just looking at it, I wonder whether
max_parallel_hazard_walker is now being invoked on portions of the
Query tree that we'd not have to traverse at all given a rewriter fix.)

regards, tom lane