Remove planner's have_dangerous_phv() join-order restriction.

Commit 85e5e222b, which added (a forerunner of) this logic,
argued that

    Adding the necessary complexity to make this work doesn't seem like
    it would be repaid in significantly better plans, because in cases
    where such a PHV exists, there is probably a corresponding join order
    constraint that would allow a good plan to be found without using the
    star-schema exception.

The flaw in this claim is that there may be other join-order
restrictions that prevent us from finding a join order that doesn't
involve a "dangerous" PHV.  In particular we now recognize that
small join_collapse_limit or from_collapse_limit could prevent it.
Therefore, let's bite the bullet and make the case work.

We don't have to extend the executor's support for nestloop parameters
as I thought at the time, because we can instead push the evaluation
of the placeholder's expression into the left-hand input of the
NestLoop node.  So there's not really a lot of downside to this
solution, and giving the planner more join-order flexibility should
have value beyond just avoiding failure.

Having said that, there surely is a nonzero risk of introducing
new bugs.  Since this failure mode escaped detection for ten years,
such cases don't seem common enough to justify a lot of risk.
Therefore, let's put this fix into master but leave the back branches
alone (for now anyway).

Bug: #18953
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Diagnosed-by: Richard Guo <guofenglinux@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18953-1c9883a9d4afeb30@postgresql.org

1 files changed, 43 insertions, 3 deletions

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 4ad30b7627e..8baf36ba4b7 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -4348,9 +4348,11 @@ create_nestloop_plan(PlannerInfo *root,
 	List	   *joinrestrictclauses = best_path->jpath.joinrestrictinfo;
 	List	   *joinclauses;
 	List	   *otherclauses;
-	Relids		outerrelids;
 	List	   *nestParams;
+	List	   *outer_tlist;
+	bool		outer_parallel_safe;
 	Relids		saveOuterRels = root->curOuterRels;
+	ListCell   *lc;
 
 	/*
 	 * If the inner path is parameterized by the topmost parent of the outer
@@ -4412,9 +4414,47 @@ create_nestloop_plan(PlannerInfo *root,
 	 * Identify any nestloop parameters that should be supplied by this join
 	 * node, and remove them from root->curOuterParams.
 	 */
-	outerrelids = best_path->jpath.outerjoinpath->parent->relids;
-	nestParams = identify_current_nestloop_params(root, outerrelids);
+	nestParams = identify_current_nestloop_params(root,
+												  best_path->jpath.outerjoinpath);
+
+	/*
+	 * While nestloop parameters that are Vars had better be available from
+	 * the outer_plan already, there are edge cases where nestloop parameters
+	 * that are PHVs won't be.  In such cases we must add them to the
+	 * outer_plan's tlist, since the executor's NestLoopParam machinery
+	 * requires the params to be simple outer-Var references to that tlist.
+	 */
+	outer_tlist = outer_plan->targetlist;
+	outer_parallel_safe = outer_plan->parallel_safe;
+	foreach(lc, nestParams)
+	{
+		NestLoopParam *nlp = (NestLoopParam *) lfirst(lc);
+		TargetEntry *tle;
+
+		if (IsA(nlp->paramval, Var))
+			continue;			/* nothing to do for simple Vars */
+		if (tlist_member((Expr *) nlp->paramval, outer_tlist))
+			continue;			/* already available */
+
+		/* Make a shallow copy of outer_tlist, if we didn't already */
+		if (outer_tlist == outer_plan->targetlist)
+			outer_tlist = list_copy(outer_tlist);
+		/* ... and add the needed expression */
+		tle = makeTargetEntry((Expr *) copyObject(nlp->paramval),
+							  list_length(outer_tlist) + 1,
+							  NULL,
+							  true);
+		outer_tlist = lappend(outer_tlist, tle);
+		/* ... and track whether tlist is (still) parallel-safe */
+		if (outer_parallel_safe)
+			outer_parallel_safe = is_parallel_safe(root,
+												   (Node *) nlp->paramval);
+	}
+	if (outer_tlist != outer_plan->targetlist)
+		outer_plan = change_plan_targetlist(outer_plan, outer_tlist,
+											outer_parallel_safe);
 
+	/* And finally, we can build the join plan node */
 	join_plan = make_nestloop(tlist,
 							  joinclauses,
 							  otherclauses,