Get rid of the planner's LateralJoinInfo data structure.

I originally modeled this data structure on SpecialJoinInfo, but after
commit acfcd45cac that looks like a pretty poor decision.
All we really need is relid sets identifying laterally-referenced rels;
and most of the time, what we want to know about includes indirect lateral
references, a case the LateralJoinInfo data was unsuited to compute with
any efficiency.  The previous commit redefined RelOptInfo.lateral_relids
as the transitive closure of lateral references, so that it easily supports
checking indirect references.  For the places where we really do want just
direct references, add a new RelOptInfo field direct_lateral_relids, which
is easily set up as a copy of lateral_relids before we perform the
transitive closure calculation.  Then we can just drop lateral_info_list
and LateralJoinInfo and the supporting code.  This makes the planner's
handling of lateral references noticeably more efficient, and shorter too.

Such a change can't be back-patched into stable branches for fear of
breaking extensions that might be looking at the planner's data structures;
but it seems not too late to push it into 9.5, so I've done so.

src/backend/optimizer/util/placeholder.c

@@ -363,12 +363,10 @@ fix_placeholder_input_needed_levels(PlannerInfo *root)
 
 /*
  * add_placeholders_to_base_rels
- *		Add any required PlaceHolderVars to base rels' targetlists, and
- *		update lateral_vars lists for lateral references contained in them.
+ *		Add any required PlaceHolderVars to base rels' targetlists.
  *
  * If any placeholder can be computed at a base rel and is needed above it,
- * add it to that rel's targetlist, and add any lateral references it requires
- * to the rel's lateral_vars list.  This might look like it could be merged
+ * add it to that rel's targetlist.  This might look like it could be merged
  * with fix_placeholder_input_needed_levels, but it must be separate because
  * join removal happens in between, and can change the ph_eval_at sets.  There
  * is essentially the same logic in add_placeholders_to_joinrel, but we can't
@@ -385,58 +383,22 @@ add_placeholders_to_base_rels(PlannerInfo *root)
 		Relids		eval_at = phinfo->ph_eval_at;
 		int			varno;
 
-		if (bms_get_singleton_member(eval_at, &varno))
+		if (bms_get_singleton_member(eval_at, &varno) &&
+			bms_nonempty_difference(phinfo->ph_needed, eval_at))
 		{
 			RelOptInfo *rel = find_base_rel(root, varno);
 
-			/* add it to reltargetlist if needed above the rel scan level */
-			if (bms_nonempty_difference(phinfo->ph_needed, eval_at))
-				rel->reltargetlist = lappend(rel->reltargetlist,
-											 copyObject(phinfo->ph_var));
-			/* if there are lateral refs in it, add them to lateral_vars */
-			if (phinfo->ph_lateral != NULL)
-			{
-				List	   *vars = pull_var_clause((Node *) phinfo->ph_var->phexpr,
-												   PVC_RECURSE_AGGREGATES,
-												   PVC_INCLUDE_PLACEHOLDERS);
-				ListCell   *lc2;
-
-				foreach(lc2, vars)
-				{
-					Node	   *node = (Node *) lfirst(lc2);
-
-					if (IsA(node, Var))
-					{
-						Var		   *var = (Var *) node;
-
-						if (var->varno != varno)
-							rel->lateral_vars = lappend(rel->lateral_vars,
-														var);
-					}
-					else if (IsA(node, PlaceHolderVar))
-					{
-						PlaceHolderVar *other_phv = (PlaceHolderVar *) node;
-						PlaceHolderInfo *other_phi;
-
-						other_phi = find_placeholder_info(root, other_phv,
-														  false);
-						if (!bms_is_subset(other_phi->ph_eval_at, eval_at))
-							rel->lateral_vars = lappend(rel->lateral_vars,
-														other_phv);
-					}
-					else
-						Assert(false);
-				}
-
-				list_free(vars);
-			}
+			rel->reltargetlist = lappend(rel->reltargetlist,
+										 copyObject(phinfo->ph_var));
 		}
 	}
 }
 
 /*
  * add_placeholders_to_joinrel
- *		Add any required PlaceHolderVars to a join rel's targetlist.
+ *		Add any required PlaceHolderVars to a join rel's targetlist;
+ *		and if they contain lateral references, add those references to the
+ *		joinrel's direct_lateral_relids.
  *
  * A join rel should emit a PlaceHolderVar if (a) the PHV is needed above
  * this join level and (b) the PHV can be computed at or below this level.
@@ -463,6 +425,10 @@ add_placeholders_to_joinrel(PlannerInfo *root, RelOptInfo *joinrel)
 				joinrel->reltargetlist = lappend(joinrel->reltargetlist,
 												 phinfo->ph_var);
 				joinrel->width += phinfo->ph_width;
+				/* Adjust joinrel's direct_lateral_relids as needed */
+				joinrel->direct_lateral_relids =
+					bms_add_members(joinrel->direct_lateral_relids,
+									phinfo->ph_lateral);
 			}
 		}
 	}

src/backend/optimizer/util/relnode.c

@@ -111,6 +111,7 @@ build_simple_rel(PlannerInfo *root, int relid, RelOptKind reloptkind)
 	rel->cheapest_total_path = NULL;
 	rel->cheapest_unique_path = NULL;
 	rel->cheapest_parameterized_paths = NIL;
+	rel->direct_lateral_relids = NULL;
 	rel->lateral_relids = NULL;
 	rel->relid = relid;
 	rel->rtekind = rte->rtekind;
@@ -373,6 +374,10 @@ build_join_rel(PlannerInfo *root,
 	joinrel->cheapest_total_path = NULL;
 	joinrel->cheapest_unique_path = NULL;
 	joinrel->cheapest_parameterized_paths = NIL;
+	/* init direct_lateral_relids from children; we'll finish it up below */
+	joinrel->direct_lateral_relids =
+		bms_union(outer_rel->direct_lateral_relids,
+				  inner_rel->direct_lateral_relids);
 	joinrel->lateral_relids = min_join_parameterization(root, joinrel->relids,
 														outer_rel, inner_rel);
 	joinrel->relid = 0;			/* indicates not a baserel */
@@ -422,6 +427,18 @@ build_join_rel(PlannerInfo *root,
 	build_joinrel_tlist(root, joinrel, inner_rel);
 	add_placeholders_to_joinrel(root, joinrel);
 
+	/*
+	 * add_placeholders_to_joinrel also took care of adding the ph_lateral
+	 * sets of any PlaceHolderVars computed here to direct_lateral_relids, so
+	 * now we can finish computing that.  This is much like the computation of
+	 * the transitively-closed lateral_relids in min_join_parameterization,
+	 * except that here we *do* have to consider the added PHVs.
+	 */
+	joinrel->direct_lateral_relids =
+		bms_del_members(joinrel->direct_lateral_relids, joinrel->relids);
+	if (bms_is_empty(joinrel->direct_lateral_relids))
+		joinrel->direct_lateral_relids = NULL;
+
 	/*
 	 * Construct restrict and join clause lists for the new joinrel. (The
 	 * caller might or might not need the restrictlist, but I need it anyway

src/backend/optimizer/util/var.c

@@ -782,7 +782,6 @@ flatten_join_alias_vars_mutator(Node *node,
 	Assert(!IsA(node, SubPlan));
 	/* Shouldn't need to handle these planner auxiliary nodes here */
 	Assert(!IsA(node, SpecialJoinInfo));
-	Assert(!IsA(node, LateralJoinInfo));
 	Assert(!IsA(node, PlaceHolderInfo));
 	Assert(!IsA(node, MinMaxAggInfo));