convert_EXISTS_sublink_to_lateral_join.patch

diff --git a/src/backend/optimizer/plan/subselect.c b/src/backend/optimizer/plan/subselect.c
index d71ed958e31..6dbbc3d9b72 100644
--- a/src/backend/optimizer/plan/subselect.c
+++ b/src/backend/optimizer/plan/subselect.c
@@ -104,6 +104,96 @@ static Bitmapset *finalize_plan(PlannerInfo *root,
 static bool finalize_primnode(Node *node, finalize_primnode_context *context);
 static bool finalize_agg_primnode(Node *node, finalize_primnode_context *context);
 
+/*
+ * convert_EXISTS_sublink_to_lateral_join: 
+ * 		try to convert an EXISTS SubLink to a lateral join
+ *
+ * The API of this function is identical to convert_ANY_sublink_to_join's,
+ * except that we also support the case where the caller has found NOT EXISTS,
+ * so we need an additional input parameter "under_not".
+ */
+JoinExpr *
+convert_EXISTS_sublink_to_lateral_join(PlannerInfo *root, SubLink *sublink,
+							   bool under_not, Relids available_rels)
+{
+	JoinExpr   *result;
+	Query	   *parse = root->parse;
+	Query	   *subselect = (Query *) sublink->subselect;
+	int			rtindex;
+	ParseNamespaceItem *nsitem;
+	RangeTblEntry *rte;
+	RangeTblRef *rtr;
+	ParseState *pstate;
+	Relids		sub_ref_outer_relids;
+	bool		use_lateral;
+
+	Assert(sublink->subLinkType == EXISTS_SUBLINK);
+
+	/*
+	 * See if the subquery can be simplified based on the knowledge that it's
+	 * being used in EXISTS().  If we aren't able to get rid of its
+	 * targetlist, we have to fail, because the pullup operation leaves us
+	 * with noplace to evaluate the targetlist.
+	 */
+	if (!simplify_EXISTS_query(root, subselect))
+		return NULL;
+
+	/*
+	 * If the sub-select contains any Vars of the parent query, we treat it as
+	 * LATERAL.  (Vars from higher levels don't matter here.)
+	 */
+	sub_ref_outer_relids = pull_varnos_of_level(NULL, (Node *) subselect, 1);
+	use_lateral = !bms_is_empty(sub_ref_outer_relids);
+
+	/*
+	 * Can't convert if the sub-select contains parent-level Vars of relations
+	 * not in available_rels.
+	 */
+	if (!bms_is_subset(sub_ref_outer_relids, available_rels))
+		return NULL;
+
+	/* Create a dummy ParseState for addRangeTableEntryForSubquery */
+	pstate = make_parsestate(NULL);
+
+	/*
+	 * Okay, pull up the sub-select into upper range table.
+	 *
+	 * We rely here on the assumption that the outer query has no references
+	 * to the inner (necessarily true, other than the Vars that we build
+	 * below). Therefore this is a lot easier than what pull_up_subqueries has
+	 * to go through.
+	 */
+	nsitem = addRangeTableEntryForSubquery(pstate,
+										   subselect,
+										   makeAlias("EXISTS_subquery", NIL),
+										   use_lateral,
+										   false);
+	rte = nsitem->p_rte;
+	parse->rtable = lappend(parse->rtable, rte);
+	rtindex = list_length(parse->rtable);
+
+	/*
+	 * Form a RangeTblRef for the pulled-up sub-select.
+	 */
+	rtr = makeNode(RangeTblRef);
+	rtr->rtindex = rtindex;
+
+	/*
+	 * And finally, build the JoinExpr node.
+	 */
+	result = makeNode(JoinExpr);
+	result->jointype = under_not ? JOIN_ANTI : JOIN_SEMI;;
+	result->isNatural = false;
+	result->larg = NULL;		/* caller must fill this in */
+	result->rarg = (Node *) rtr;
+	result->usingClause = NIL;
+	result->join_using_alias = NULL;
+	result->quals = NULL;
+	result->alias = NULL;
+	result->rtindex = 0;		/* we don't need an RTE for it */
+
+	return result;
+}
 
 /*
  * Get the datatype/typmod/collation of the first column of the plan's output.
@@ -1462,6 +1552,9 @@ convert_EXISTS_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 
 	Assert(sublink->subLinkType == EXISTS_SUBLINK);
 
+	return convert_EXISTS_sublink_to_lateral_join(root, sublink, 
+										under_not, available_rels);
+
 	/*
 	 * Can't flatten if it contains WITH.  (We could arrange to pull up the
 	 * WITH into the parent query's cteList, but that risks changing the