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, 28 insertions, 11 deletions

diff --git a/src/backend/optimizer/util/paramassign.c b/src/backend/optimizer/util/paramassign.c
index 3bd3ce37c8f..9836abf9479 100644
--- a/src/backend/optimizer/util/paramassign.c
+++ b/src/backend/optimizer/util/paramassign.c
@@ -600,7 +600,7 @@ process_subquery_nestloop_params(PlannerInfo *root, List *subplan_params)
 
 /*
  * Identify any NestLoopParams that should be supplied by a NestLoop plan
- * node with the specified lefthand rels.  Remove them from the active
+ * node with the specified lefthand input path.  Remove them from the active
  * root->curOuterParams list and return them as the result list.
  *
  * XXX Here we also hack up the returned Vars and PHVs so that they do not
@@ -626,11 +626,26 @@ process_subquery_nestloop_params(PlannerInfo *root, List *subplan_params)
  * subquery, which'd be unduly expensive.
  */
 List *
-identify_current_nestloop_params(PlannerInfo *root, Relids leftrelids)
+identify_current_nestloop_params(PlannerInfo *root, Path *leftpath)
 {
 	List	   *result;
+	Relids		leftrelids = leftpath->parent->relids;
+	Relids		outerrelids = PATH_REQ_OUTER(leftpath);
+	Relids		allleftrelids;
 	ListCell   *cell;
 
+	/*
+	 * We'll be able to evaluate a PHV in the lefthand path if it uses the
+	 * lefthand rels plus any available required-outer rels.  But don't do so
+	 * if it uses *only* required-outer rels; in that case it should be
+	 * evaluated higher in the tree.  For Vars, no such hair-splitting is
+	 * necessary since they depend on only one relid.
+	 */
+	if (outerrelids)
+		allleftrelids = bms_union(leftrelids, outerrelids);
+	else
+		allleftrelids = leftrelids;
+
 	result = NIL;
 	foreach(cell, root->curOuterParams)
 	{
@@ -653,18 +668,20 @@ identify_current_nestloop_params(PlannerInfo *root, Relids leftrelids)
 												leftrelids);
 			result = lappend(result, nlp);
 		}
-		else if (IsA(nlp->paramval, PlaceHolderVar) &&
-				 bms_is_subset(find_placeholder_info(root,
-													 (PlaceHolderVar *) nlp->paramval)->ph_eval_at,
-							   leftrelids))
+		else if (IsA(nlp->paramval, PlaceHolderVar))
 		{
 			PlaceHolderVar *phv = (PlaceHolderVar *) nlp->paramval;
+			Relids		eval_at = find_placeholder_info(root, phv)->ph_eval_at;
 
-			root->curOuterParams = foreach_delete_current(root->curOuterParams,
-														  cell);
-			phv->phnullingrels = bms_intersect(phv->phnullingrels,
-											   leftrelids);
-			result = lappend(result, nlp);
+			if (bms_is_subset(eval_at, allleftrelids) &&
+				bms_overlap(eval_at, leftrelids))
+			{
+				root->curOuterParams = foreach_delete_current(root->curOuterParams,
+															  cell);
+				phv->phnullingrels = bms_intersect(phv->phnullingrels,
+												   leftrelids);
+				result = lappend(result, nlp);
+			}
 		}
 	}
 	return result;