Re: [HACKERS] Removing [Merge]Append nodes which contain a single subpath - Mailing list pgsql-hackers

On Thu, 3 Jan 2019 at 08:01, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
> AFAICS the patch essentially does two things: (a) identifies Append
> paths with a single member and (b) ensures the Vars are properly mapped
> when the Append node is skipped when creating the plan.

Yes, but traditional Append and MergeAppend paths with a single member
are never actually generated.  I say "traditional" as we do happen to
use a single-subpath Append as the "proxy" path to represent a path
that needs to not have a plan node generated during createplan.
Originally I had a new Path type named "ProxyPath" to do this, but per
recommendation from Tom, I overloaded Append to mean that. (Append
already has its meaning overloaded in regards to dummy paths.)

> I agree doing (a) in setrefs.c is too late, as mentioned earlier. But
> perhaps we could do at least (b) to setrefs.c?

The problem I found with doing var translations in setrefs.c was that
the plan tree is traversed there breadth-first and in createplan.c we
build the plan depth-first.  The problem I didn't manage to overcome
with that is that when we replace a "ProxyPath" (actually an Append
path marked as is_proxy=true) it only alter targetlists, etc of nodes
above that in the plan tree. The nodes below it and their targetlists
don't care, as these don't fetch Vars from nodes above them.  If we do
the Var translation in setrefs then we've not yet looked at the nodes
that don't care and we've already done the nodes that do care. So the
tree traversal is backwards.  I sort of borrowed the traversal in
createplan.c since it was in the correct depth-first order.   Equally,
I could have invented an entirely new stage that traverses the path
tree depth-first, but I imagined that would have added more overhead
and raised even more questions. Piggy-backing on createplan seemed
like the best option at the time.

> I'd say mapping the Vars is the most fragile and error-prone piece of
> this patch. It's a bit annoying that it's inventing a new infrastructure
> to do that, translates expressions in various places, establishes new
> rules for tlists (having to call finalize_plan_tlist when calling
> build_path_tlist before create_plan_recurse), etc.

I entirely agree. It's by far the worst part of the patch.  Getting
the code to a working state to do this was hard. I kept finding places
that I'd missed the translation.  I'd rather it worked some other way.
I just don't yet know what that way is.  I changed the core regression
table "tenk" to become a partitioned table with a single partition in
the hope to catch all of the places that need the translations to be
performed.  I'm not entirely confident that I've caught them all by
doing this.

I've attached an updated patch that's really just a rebased version of
v8.  The old version conflicted with changes made in 1db5667b. The
only real change was to the commit message. I'd previously managed to
miss out the part about not generating needless Append/MergeAppend
paths can allow plan shapes that were previously not possible.  I'd
only mentioned the executor not having to pull tuples through these
nodes, which increases performance. Not having this perhaps caused
some of the confusion earlier in this thread.

One drawback to this patch, which I'm unsure if I previously mentioned
is that run-time pruning only works for Append and MergeAppend.  If we
remove the Append/MergeAppend node then the single Append/MergeAppend
sub-plan has no hope of being pruned.  Going from 1 to 0 sub-plans may
not be that common, but no longer supporting that is something that
needs to be considered.   I had imagined that gained flexibility of
plan shapes plus less executor overhead was better than that, but
there likely is a small use-case for keeping that ability.  It seems
impossible to have both advantages of this patch without that
disadvantage.

-- 
 David Rowley                   http://www.2ndQuadrant.com/
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