From 90d1109aa3397c6beb50b12cd8e5364861037424 Mon Sep 17 00:00:00 2001
From: Tomas Vondra
Date: Sat, 10 Jun 2017 19:40:50 +0200
Subject: Restrict subplan nodes even for equal distributions

Many regression tests were failing because the expected plan contains

    Remote Subquery Scan on all (datanode_1)

but we were producing

    Remote Subquery Scan on any (datanode_1,datanode_2)

Both those plans are in fact valid (at least on replicated tables).
The difference is that in the first case the restriction was computed
in adjust_subplan_distribution() while in the second case this did not
happen (and instead will happen at execution time).

The restriction is not applied because adjust_subplan_distribution()
contains this condition

    if (subd && !equal(subd, pathd))
    {
        ... restrict execution nodes ...
    }

In Postgres-XL 9.6 the two distributions happen to be equal in some
cases where where that was not the case in Postgres-XL 9.5. It's not
entirely clear why this happens (it seems to be another consequence
of the upper-planner pathification), but the consequence is that the
restriction code is skipped.

Removing the equal() call from the condition fixes all the regression
failures caused by plans switching between any/all restrictions. In
fact, this fixes all remaining regressions failures in five regression
suites: create_view, subselect, aggregates, rangefuncs, xc_having.

In the future, we will probably pathify adjust_subplan_distribution(),
i.e. we'll probably get rid of it entirely and compute the restriction
either when constructing the path or possibly defer it until execution
time. Before the upper-planner pathification this was not possible.
---
 src/backend/optimizer/plan/createplan.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

(limited to 'src')

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 637926ff3a..2a0aac6cba 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -2355,13 +2355,9 @@ adjust_subplan_distribution(PlannerInfo *root, Distribution *pathd,
 	root->curOuterRestrict = NULL;
 
 	/*
-	 * Set new restriction for the subpath
-	 * Do not restrict if distributions are equal, they are going to be merged
-	 * and subplan will be executed on caller nodes.
-	 * However if there are upper query levels caller's distribution may be
-	 * adjusted.
+	 * Set new restriction for the subpath.
 	 */
-	if (subd && !equal(subd, pathd))
+	if (subd)
 	{
 		/*
 		 * If subpath is replicated without restriction choose one execution
-- 
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