On 14 August 2018 at 09:23, Robert Haas <robertmhaas@gmail.com> wrote:
> On Sat, Aug 11, 2018 at 9:16 AM, David Rowley
> <david.rowley@2ndquadrant.com> wrote:
>> I started debugging this to see where things go wrong. I discovered
>> that add_paths_to_append_rel() is called yet again from
>> apply_scanjoin_target_to_paths() and this is where it's all going
>> wrong. The problem is that the gather paths have been tagged onto the
>> partial paths by this time, so accumulate_append_subpath() has no code
>> to look through those to fetch the Append/MergeAppend subpaths, so it
>> just appends the entire path to the subpaths List. This all worked
>> before 96030f9a481. This commit moved where generate_gather_paths() is
>> called.
>
> I'm baffled as to why looking through Gather to find
> Append/MergeAppend subpaths would ever be a sane thing to do.
Can you explain why it's less sane than what the current code is
doing? Below a Gather there will be partial paths, but we can also
get those in a Parallel Append, which the accumulate_append_subpath()
code already attempts to handle. If the Gather Path is there already
then I guess one difference would be that the caller would need to
ensure that another Gather path is placed below the Parallel Append
again.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
