Re: [PATCH] Push limit to sort through a subquery

Robert Haas <robertmhaas@gmail.com> writes:
> Buildfarm members with force_parallel_mode=regress are failing now.  I
> haven't had a chance to investigate exactly what's going on here, but
> I think there are probably several issues:

> 1. It's definitely the case that the details about a sort operation
> aren't propagated from a worker back to the leader.  This has elicited
> complaint previously.

Check; this must be the explanation for the buildfarm failures.  That'd
be worth fixing but it seems like material for an independent patch.

> 2. pass_down_bound() probably gets the Gather node at the top of the
> tree and stops, since it doesn't know how to pass down a bound through
> a Gather.  We could probably teach it to pass down the bound through
> Gather or Gather Merge on the same theory as Append/MergeAppend.

While that might be worth doing, it's not the issue here, because
the forced Gather is on top of the Limit node.

> In the short run, I'm not sure we have a better alternative than
> removing this test - unless somebody has a better idea? - but it would
> be good to work on all of the above problems.

To get the buildfarm back to green, I agree with the suggestion of
setting force_parallel_mode=off for this test, at least for now.

I don't greatly like the way that the regression test case filters
the output; it hides too much IMO.  I'd be inclined to try to return
the EXPLAIN output with as much detail preserved as possible.  Maybe
we could use regex substitution on lines of the output to get rid of
stuff that won't be invariant.

Unless you're already on it, I can go do that.

			regards, tom lane