On 9 August 2018 at 15:33, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Nonetheless, it's a damfool query plan, because we'll be going through
> parallel worker startup/shutdown overhead 4 times for no benefit.
> We really should put in some kind of logic to force those sibling
> Gathers to be aggregated into one, or else disallow them altogether.

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

I'm having a hard time understanding why we need to call
add_paths_to_append_rel() from apply_scanjoin_target_to_paths().  The
name of the function does not seem to imply that we'd do this. The
comment just explains "what" rather than "why" making it a bit

/* Build new paths for this relation by appending child paths. */
if (live_children != NIL)
add_paths_to_append_rel(root, rel, live_children);

I also think that accumulate_append_subpath() is a bit of a fragile
way of flattening the append rel's paths, but I feel it's probably
apply_scanjoin_target_to_paths() that's at fault here.

 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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