Make PlaceHolderInfo lookup O(1).

Up to now we've just searched the placeholder_list when we want to
find the PlaceHolderInfo with a given ID.  While there's no evidence
of that being a problem in the field, an upcoming patch will add
find_placeholder_info() calls in build_joinrel_tlist(), which seems
likely to make it more of an issue: a joinrel emitting lots of
PlaceHolderVars would incur O(N^2) cost, and we might be building
a lot of joinrels in complex queries.  Hence, add an array that
can be indexed directly by phid to make the lookups constant-time.

Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us

src/backend/optimizer/plan/analyzejoins.c

@@ -388,8 +388,11 @@ remove_rel_from_query(PlannerInfo *root, int relid, Relids joinrelids)
 		Assert(!bms_is_member(relid, phinfo->ph_lateral));
 		if (bms_is_subset(phinfo->ph_needed, joinrelids) &&
 			bms_is_member(relid, phinfo->ph_eval_at))
+		{
 			root->placeholder_list = foreach_delete_current(root->placeholder_list,
 															l);
+			root->placeholder_array[phinfo->phid] = NULL;
+		}
 		else
 		{
 			phinfo->ph_eval_at = bms_del_member(phinfo->ph_eval_at, relid);

src/backend/optimizer/plan/planmain.c

@@ -75,6 +75,8 @@ query_planner(PlannerInfo *root,
 	root->full_join_clauses = NIL;
 	root->join_info_list = NIL;
 	root->placeholder_list = NIL;
+	root->placeholder_array = NULL;
+	root->placeholder_array_size = 0;
 	root->fkey_list = NIL;
 	root->initial_rels = NIL;
 

src/backend/optimizer/util/placeholder.c

@@ -71,16 +71,19 @@ find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv,
 {
 	PlaceHolderInfo *phinfo;
 	Relids		rels_used;
-	ListCell   *lc;
 
 	/* if this ever isn't true, we'd need to be able to look in parent lists */
 	Assert(phv->phlevelsup == 0);
 
-	foreach(lc, root->placeholder_list)
+	/* Use placeholder_array to look up existing PlaceHolderInfo quickly */
+	if (phv->phid < root->placeholder_array_size)
+		phinfo = root->placeholder_array[phv->phid];
+	else
+		phinfo = NULL;
+	if (phinfo != NULL)
 	{
-		phinfo = (PlaceHolderInfo *) lfirst(lc);
-		if (phinfo->phid == phv->phid)
-			return phinfo;
+		Assert(phinfo->phid == phv->phid);
+		return phinfo;
 	}
 
 	/* Not found, so create it */
@@ -115,8 +118,38 @@ find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv,
 	phinfo->ph_width = get_typavgwidth(exprType((Node *) phv->phexpr),
 									   exprTypmod((Node *) phv->phexpr));
 
+	/*
+	 * Add to both placeholder_list and placeholder_array.  Note: because we
+	 * store pointers to the PlaceHolderInfos in two data structures, it'd be
+	 * unsafe to pass the whole placeholder_list structure through
+	 * expression_tree_mutator or the like --- or at least, you'd have to
+	 * rebuild the placeholder_array afterwards.
+	 */
 	root->placeholder_list = lappend(root->placeholder_list, phinfo);
 
+	if (phinfo->phid >= root->placeholder_array_size)
+	{
+		/* Must allocate or enlarge placeholder_array */
+		int			new_size;
+
+		new_size = root->placeholder_array_size ? root->placeholder_array_size * 2 : 8;
+		while (phinfo->phid >= new_size)
+			new_size *= 2;
+		if (root->placeholder_array)
+		{
+			root->placeholder_array = (PlaceHolderInfo **)
+				repalloc(root->placeholder_array,
+						 sizeof(PlaceHolderInfo *) * new_size);
+			MemSet(root->placeholder_array + root->placeholder_array_size, 0,
+				   sizeof(PlaceHolderInfo *) * (new_size - root->placeholder_array_size));
+		}
+		else
+			root->placeholder_array = (PlaceHolderInfo **)
+				palloc0(new_size * sizeof(PlaceHolderInfo *));
+		root->placeholder_array_size = new_size;
+	}
+	root->placeholder_array[phinfo->phid] = phinfo;
+
 	/*
 	 * The PHV's contained expression may contain other, lower-level PHVs.  We
 	 * now know we need to get those into the PlaceHolderInfo list, too, so we

src/backend/optimizer/util/var.c

@@ -210,15 +210,8 @@ pull_varnos_walker(Node *node, pull_varnos_context *context)
 
 			if (phv->phlevelsup == 0)
 			{
-				ListCell   *lc;
-
-				foreach(lc, context->root->placeholder_list)
-				{
-					phinfo = (PlaceHolderInfo *) lfirst(lc);
-					if (phinfo->phid == phv->phid)
-						break;
-					phinfo = NULL;
-				}
+				if (phv->phid < context->root->placeholder_array_size)
+					phinfo = context->root->placeholder_array[phv->phid];
 			}
 			if (phinfo == NULL)
 			{

src/include/nodes/pathnodes.h

@@ -357,6 +357,11 @@ struct PlannerInfo
 	/* list of PlaceHolderInfos */
 	List	   *placeholder_list;
 
+	/* array of PlaceHolderInfos indexed by phid */
+	struct PlaceHolderInfo **placeholder_array pg_node_attr(read_write_ignore, array_size(placeholder_array_size));
+	/* allocated size of array */
+	int			placeholder_array_size pg_node_attr(read_write_ignore);
+
 	/* list of ForeignKeyOptInfos */
 	List	   *fkey_list;