diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index 5d06fa4..304a473 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -1179,9 +1179,17 @@ ExplainNode(PlanState *planstate, List *ancestors,
 						appendStringInfo(es->str, " %s Join", jointype);
 					else if (!IsA(plan, NestLoop))
 						appendStringInfoString(es->str, " Join");
+
+					if (((Join *) plan)->unique_inner)
+						appendStringInfoString(es->str, " (inner unique)");
+
 				}
 				else
+				{
 					ExplainPropertyText("Join Type", jointype, es);
+					ExplainPropertyText("Inner unique",
+							((Join *)plan)->unique_inner ? "true" : "false", es);
+				}
 			}
 			break;
 		case T_SetOp:
diff --git a/src/backend/executor/nodeHashjoin.c b/src/backend/executor/nodeHashjoin.c
index 1d78cdf..f2471aa 100644
--- a/src/backend/executor/nodeHashjoin.c
+++ b/src/backend/executor/nodeHashjoin.c
@@ -306,10 +306,12 @@ ExecHashJoin(HashJoinState *node)
 					}
 
 					/*
-					 * In a semijoin, we'll consider returning the first
-					 * match, but after that we're done with this outer tuple.
+					 * We'll consider returning the first match if the inner
+					 * is unique, but after that we're done with this outer
+					 * tuple. For the case of SEMI joins, we want to skip to
+					 * the next outer row after having matched 1 inner row.
 					 */
-					if (node->js.jointype == JOIN_SEMI)
+					if (node->js.unique_inner)
 						node->hj_JoinState = HJ_NEED_NEW_OUTER;
 
 					if (otherqual == NIL ||
@@ -451,6 +453,7 @@ ExecInitHashJoin(HashJoin *node, EState *estate, int eflags)
 	hjstate = makeNode(HashJoinState);
 	hjstate->js.ps.plan = (Plan *) node;
 	hjstate->js.ps.state = estate;
+	hjstate->js.unique_inner = node->join.unique_inner;
 
 	/*
 	 * Miscellaneous initialization
@@ -498,8 +501,10 @@ ExecInitHashJoin(HashJoin *node, EState *estate, int eflags)
 	/* set up null tuples for outer joins, if needed */
 	switch (node->join.jointype)
 	{
-		case JOIN_INNER:
 		case JOIN_SEMI:
+			hjstate->js.unique_inner = true;
+			/* fall through */
+		case JOIN_INNER:
 			break;
 		case JOIN_LEFT:
 		case JOIN_ANTI:
diff --git a/src/backend/executor/nodeMergejoin.c b/src/backend/executor/nodeMergejoin.c
index 34b6cf6..100f085 100644
--- a/src/backend/executor/nodeMergejoin.c
+++ b/src/backend/executor/nodeMergejoin.c
@@ -840,10 +840,12 @@ ExecMergeJoin(MergeJoinState *node)
 					}
 
 					/*
-					 * In a semijoin, we'll consider returning the first
-					 * match, but after that we're done with this outer tuple.
+					 * We'll consider returning the first match if the inner
+					 * is unique, but after that we're done with this outer
+					 * tuple. For the case of SEMI joins, we want to skip to
+					 * the next outer row after having matched 1 inner row.
 					 */
-					if (node->js.jointype == JOIN_SEMI)
+					if (node->js.unique_inner)
 						node->mj_JoinState = EXEC_MJ_NEXTOUTER;
 
 					qualResult = (otherqual == NIL ||
@@ -1486,6 +1488,8 @@ ExecInitMergeJoin(MergeJoin *node, EState *estate, int eflags)
 	mergestate->js.ps.plan = (Plan *) node;
 	mergestate->js.ps.state = estate;
 
+	mergestate->js.unique_inner = node->join.unique_inner;
+
 	/*
 	 * Miscellaneous initialization
 	 *
@@ -1553,8 +1557,10 @@ ExecInitMergeJoin(MergeJoin *node, EState *estate, int eflags)
 
 	switch (node->join.jointype)
 	{
-		case JOIN_INNER:
 		case JOIN_SEMI:
+			mergestate->js.unique_inner = true;
+			/* fall through */
+		case JOIN_INNER:
 			mergestate->mj_FillOuter = false;
 			mergestate->mj_FillInner = false;
 			break;
diff --git a/src/backend/executor/nodeNestloop.c b/src/backend/executor/nodeNestloop.c
index e66bcda..b5e6de1 100644
--- a/src/backend/executor/nodeNestloop.c
+++ b/src/backend/executor/nodeNestloop.c
@@ -247,10 +247,12 @@ ExecNestLoop(NestLoopState *node)
 			}
 
 			/*
-			 * In a semijoin, we'll consider returning the first match, but
-			 * after that we're done with this outer tuple.
+			 * We'll consider returning the first match if the inner is
+			 * unique, but after that we're done with this outer tuple.
+			 * For the case of SEMI joins, we want to skip to the next outer
+			 * row after having matched 1 inner row.
 			 */
-			if (node->js.jointype == JOIN_SEMI)
+			if (node->js.unique_inner)
 				node->nl_NeedNewOuter = true;
 
 			if (otherqual == NIL || ExecQual(otherqual, econtext, false))
@@ -310,6 +312,8 @@ ExecInitNestLoop(NestLoop *node, EState *estate, int eflags)
 	nlstate->js.ps.plan = (Plan *) node;
 	nlstate->js.ps.state = estate;
 
+	nlstate->js.unique_inner = node->join.unique_inner;
+
 	/*
 	 * Miscellaneous initialization
 	 *
@@ -354,8 +358,10 @@ ExecInitNestLoop(NestLoop *node, EState *estate, int eflags)
 
 	switch (node->join.jointype)
 	{
-		case JOIN_INNER:
 		case JOIN_SEMI:
+			nlstate->js.unique_inner = true;
+			/* fall through */
+		case JOIN_INNER:
 			break;
 		case JOIN_LEFT:
 		case JOIN_ANTI:
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index bd2e80e..c72ed39 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -2032,6 +2032,7 @@ _copySpecialJoinInfo(const SpecialJoinInfo *from)
 	COPY_SCALAR_FIELD(jointype);
 	COPY_SCALAR_FIELD(lhs_strict);
 	COPY_SCALAR_FIELD(delay_upper_joins);
+	COPY_SCALAR_FIELD(is_unique_join);
 	COPY_SCALAR_FIELD(semi_can_btree);
 	COPY_SCALAR_FIELD(semi_can_hash);
 	COPY_NODE_FIELD(semi_operators);
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index 19412fe..12f7787 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -837,6 +837,7 @@ _equalSpecialJoinInfo(const SpecialJoinInfo *a, const SpecialJoinInfo *b)
 	COMPARE_SCALAR_FIELD(jointype);
 	COMPARE_SCALAR_FIELD(lhs_strict);
 	COMPARE_SCALAR_FIELD(delay_upper_joins);
+	COMPARE_SCALAR_FIELD(is_unique_join);
 	COMPARE_SCALAR_FIELD(semi_can_btree);
 	COMPARE_SCALAR_FIELD(semi_can_hash);
 	COMPARE_NODE_FIELD(semi_operators);
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index a878498..13c83b3 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -2015,6 +2015,7 @@ _outSpecialJoinInfo(StringInfo str, const SpecialJoinInfo *node)
 	WRITE_ENUM_FIELD(jointype, JoinType);
 	WRITE_BOOL_FIELD(lhs_strict);
 	WRITE_BOOL_FIELD(delay_upper_joins);
+	WRITE_BOOL_FIELD(is_unique_join);
 	WRITE_BOOL_FIELD(semi_can_btree);
 	WRITE_BOOL_FIELD(semi_can_hash);
 	WRITE_NODE_FIELD(semi_operators);
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 7069f60..2636bdb 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1783,7 +1783,7 @@ cost_group(Path *path, PlannerInfo *root,
  */
 void
 initial_cost_nestloop(PlannerInfo *root, JoinCostWorkspace *workspace,
-					  JoinType jointype,
+					  JoinType jointype, bool unique_inner,
 					  Path *outer_path, Path *inner_path,
 					  SpecialJoinInfo *sjinfo,
 					  SemiAntiJoinFactors *semifactors)
@@ -1817,7 +1817,9 @@ initial_cost_nestloop(PlannerInfo *root, JoinCostWorkspace *workspace,
 	inner_run_cost = inner_path->total_cost - inner_path->startup_cost;
 	inner_rescan_run_cost = inner_rescan_total_cost - inner_rescan_start_cost;
 
-	if (jointype == JOIN_SEMI || jointype == JOIN_ANTI)
+	if (jointype == JOIN_SEMI ||
+		jointype == JOIN_ANTI ||
+		unique_inner)
 	{
 		/*
 		 * SEMI or ANTI join: executor will stop after first match.
@@ -1888,7 +1890,9 @@ final_cost_nestloop(PlannerInfo *root, NestPath *path,
 
 	/* cost of inner-relation source data (we already dealt with outer rel) */
 
-	if (path->jointype == JOIN_SEMI || path->jointype == JOIN_ANTI)
+	if (path->jointype == JOIN_SEMI ||
+		path->jointype == JOIN_ANTI ||
+		path->unique_inner)
 	{
 		/*
 		 * SEMI or ANTI join: executor will stop after first match.
@@ -2758,7 +2762,9 @@ final_cost_hashjoin(PlannerInfo *root, HashPath *path,
 
 	/* CPU costs */
 
-	if (path->jpath.jointype == JOIN_SEMI || path->jpath.jointype == JOIN_ANTI)
+	if (path->jpath.jointype == JOIN_SEMI ||
+		path->jpath.jointype == JOIN_ANTI ||
+		path->jpath.unique_inner)
 	{
 		double		outer_matched_rows;
 		Selectivity inner_scan_frac;
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
index a35c881..0cf15d8 100644
--- a/src/backend/optimizer/path/joinpath.c
+++ b/src/backend/optimizer/path/joinpath.c
@@ -19,6 +19,7 @@
 #include "executor/executor.h"
 #include "foreign/fdwapi.h"
 #include "optimizer/cost.h"
+#include "optimizer/planmain.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 
@@ -44,7 +45,8 @@ static List *select_mergejoin_clauses(PlannerInfo *root,
 						 List *restrictlist,
 						 JoinType jointype,
 						 bool *mergejoin_allowed);
-
+static inline bool clause_sides_match_join(RestrictInfo *rinfo, RelOptInfo *outerrel,
+										   RelOptInfo *innerrel);
 
 /*
  * add_paths_to_joinrel
@@ -81,12 +83,45 @@ add_paths_to_joinrel(PlannerInfo *root,
 	JoinPathExtraData extra;
 	bool		mergejoin_allowed = true;
 	ListCell   *lc;
+	bool		unique_inner;
+
+	/* left joins were already been analyzed for uniqueness in mark_unique_joins() */
+	if (jointype == JOIN_LEFT)
+		unique_inner = sjinfo->is_unique_join;
+	else if (jointype == JOIN_INNER &&
+		restrictlist != NIL &&
+		rel_supports_distinctness(root, innerrel))
+	{
+		/*
+		 * remember the number of items that were in the restrictlist as
+		 * the call to relation_has_unique_index_for may add more items
+		 * which we'll need to remove later.
+		 */
+		int org_len = list_length(restrictlist);
+
+		/*
+		 * rel_is_distinct_for requires restrict infos to have the
+		 * correct clause direction info
+		 */
+		foreach(lc, restrictlist)
+		{
+			clause_sides_match_join((RestrictInfo *)lfirst(lc),
+				outerrel, innerrel);
+		}
+		unique_inner = rel_is_distinct_for(root, innerrel, restrictlist);
+
+		/* Remove any list items added by rel_is_distinct_for */
+		list_truncate(restrictlist, org_len);
+	}
+	else
+		unique_inner = false;	/* we can't prove uniqueness */
 
 	extra.restrictlist = restrictlist;
 	extra.mergeclause_list = NIL;
 	extra.sjinfo = sjinfo;
 	extra.param_source_rels = NULL;
 	extra.extra_lateral_rels = NULL;
+	extra.unique_inner = unique_inner;
 
 	/*
 	 * Find potential mergejoin clauses.  We can skip this if we are not
@@ -399,7 +434,7 @@ try_nestloop_path(PlannerInfo *root,
 	 * The latter two steps are expensive enough to make this two-phase
 	 * methodology worthwhile.
 	 */
-	initial_cost_nestloop(root, &workspace, jointype,
+	initial_cost_nestloop(root, &workspace, jointype, extra->unique_inner,
 						  outer_path, inner_path,
 						  extra->sjinfo, &extra->semifactors);
 
@@ -411,6 +446,7 @@ try_nestloop_path(PlannerInfo *root,
 				 create_nestloop_path(root,
 									  joinrel,
 									  jointype,
+									  extra->unique_inner,
 									  &workspace,
 									  extra->sjinfo,
 									  &extra->semifactors,
@@ -494,6 +530,7 @@ try_mergejoin_path(PlannerInfo *root,
 				 create_mergejoin_path(root,
 									   joinrel,
 									   jointype,
+									   extra->unique_inner,
 									   &workspace,
 									   extra->sjinfo,
 									   outer_path,
@@ -565,6 +602,7 @@ try_hashjoin_path(PlannerInfo *root,
 				 create_hashjoin_path(root,
 									  joinrel,
 									  jointype,
+									  extra->unique_inner,
 									  &workspace,
 									  extra->sjinfo,
 									  &extra->semifactors,
diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index b2cc9f0..51caeab 100644
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -639,6 +639,7 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2)
 		/* we don't bother trying to make the remaining fields valid */
 		sjinfo->lhs_strict = false;
 		sjinfo->delay_upper_joins = false;
+		sjinfo->is_unique_join = false;
 		sjinfo->semi_can_btree = false;
 		sjinfo->semi_can_hash = false;
 		sjinfo->semi_operators = NIL;
diff --git a/src/backend/optimizer/plan/analyzejoins.c b/src/backend/optimizer/plan/analyzejoins.c
index 7912b15..7f7c674 100644
--- a/src/backend/optimizer/plan/analyzejoins.c
+++ b/src/backend/optimizer/plan/analyzejoins.c
@@ -33,11 +33,37 @@
 
 /* local functions */
 static bool join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo);
+static bool specialjoin_is_unique_join(PlannerInfo *root,
+					  SpecialJoinInfo *sjinfo);
 static void remove_rel_from_query(PlannerInfo *root, int relid,
 					  Relids joinrelids);
 static List *remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved);
 static Oid	distinct_col_search(int colno, List *colnos, List *opids);
 
+/*
+ * mark_unique_joins
+		Analyze joins in order to determine if their inner side is unique based
+		on the join condition.
+ */
+void
+mark_unique_joins(PlannerInfo *root, List *joinlist)
+{
+	ListCell   *lc;
+
+	foreach(lc, root->join_info_list)
+	{
+		SpecialJoinInfo *sjinfo = (SpecialJoinInfo *) lfirst(lc);
+
+		/*
+		 * Currently we're only interested in LEFT JOINs that have not already
+		 * been marked as unique by a previous call.
+		 */
+		if (sjinfo->jointype == JOIN_LEFT &&
+			!sjinfo->is_unique_join &&
+			specialjoin_is_unique_join(root, sjinfo))
+			sjinfo->is_unique_join = true;
+	}
+}
 
 /*
  * remove_useless_joins
@@ -91,6 +117,12 @@ restart:
 		root->join_info_list = list_delete_ptr(root->join_info_list, sjinfo);
 
 		/*
+		 * We may now be able to mark some joins as unique which we could
+		 * not do before
+		 */
+		mark_unique_joins(root, joinlist);
+
+		/*
 		 * Restart the scan.  This is necessary to ensure we find all
 		 * removable joins independently of ordering of the join_info_list
 		 * (note that removal of attr_needed bits may make a join appear
@@ -151,17 +183,17 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 {
 	int			innerrelid;
 	RelOptInfo *innerrel;
-	Query	   *subquery = NULL;
 	Relids		joinrelids;
-	List	   *clause_list = NIL;
-	ListCell   *l;
 	int			attroff;
+	ListCell   *l;
 
 	/*
-	 * Must be a non-delaying left join to a single baserel, else we aren't
-	 * going to be able to do anything with it.
+	 * Join must not duplicate its outer side and must be a non-delaying left
+	 * join to a single baserel, else we aren't going to be able to do anything
+	 * with it.
 	 */
-	if (sjinfo->jointype != JOIN_LEFT ||
+	if (!sjinfo->is_unique_join ||
+		sjinfo->jointype != JOIN_LEFT ||
 		sjinfo->delay_upper_joins)
 		return false;
 
@@ -170,38 +202,7 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 
 	innerrel = find_base_rel(root, innerrelid);
 
-	if (innerrel->reloptkind != RELOPT_BASEREL)
-		return false;
-
-	/*
-	 * Before we go to the effort of checking whether any innerrel variables
-	 * are needed above the join, make a quick check to eliminate cases in
-	 * which we will surely be unable to prove uniqueness of the innerrel.
-	 */
-	if (innerrel->rtekind == RTE_RELATION)
-	{
-		/*
-		 * For a plain-relation innerrel, we only know how to prove uniqueness
-		 * by reference to unique indexes.  If there are no indexes then
-		 * there's certainly no unique indexes so there's no point in going
-		 * further.
-		 */
-		if (innerrel->indexlist == NIL)
-			return false;
-	}
-	else if (innerrel->rtekind == RTE_SUBQUERY)
-	{
-		subquery = root->simple_rte_array[innerrelid]->subquery;
-
-		/*
-		 * If the subquery has no qualities that support distinctness proofs
-		 * then there's no point in going further.
-		 */
-		if (!query_supports_distinctness(subquery))
-			return false;
-	}
-	else
-		return false;			/* unsupported rtekind */
+	Assert(innerrel->reloptkind == RELOPT_BASEREL);
 
 	/* Compute the relid set for the join we are considering */
 	joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
@@ -212,7 +213,8 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 	 *
 	 * Note that this test only detects use of inner-rel attributes in higher
 	 * join conditions and the target list.  There might be such attributes in
-	 * pushed-down conditions at this join, too.  We check that case below.
+	 * pushed-down conditions at this join, too, but in this case the join
+	 * would not have been marked as unique.
 	 *
 	 * As a micro-optimization, it seems better to start with max_attr and
 	 * count down rather than starting with min_attr and counting up, on the
@@ -253,6 +255,44 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 			return false;		/* it does reference innerrel */
 	}
 
+	return true;
+}
+
+/*
+ * specialjoin_is_unique_join
+ *		True if it can be proved that this special join can only ever match at
+ *		most 1 inner row for any single outer row. False is returned if there's
+ *		insufficient evidence to prove the join is unique.
+ */
+static bool
+specialjoin_is_unique_join(PlannerInfo *root, SpecialJoinInfo *sjinfo)
+{
+	int			innerrelid;
+	RelOptInfo *innerrel;
+	Relids		joinrelids;
+	ListCell   *l;
+	List	   *clause_list = NIL;
+
+	/* if there's more than 1 relation involved then punt */
+	if (!bms_get_singleton_member(sjinfo->min_righthand, &innerrelid))
+		return false;
+
+	innerrel = find_base_rel(root, innerrelid);
+
+	if (innerrel->reloptkind != RELOPT_BASEREL)
+		return false;
+
+	/*
+	 * Before we go to the effort of pulling out the join condition's columns,
+	 * make a quick check to eliminate cases in which we will surely be unable
+	 * to prove uniqueness of the innerrel.
+	 */
+	if (!rel_supports_distinctness(root, innerrel))
+		return false;
+
+	/* Compute the relid set for the join we are considering */
+	joinrelids = bms_union(sjinfo->min_lefthand, sjinfo->min_righthand);
+
 	/*
 	 * Search for mergejoinable clauses that constrain the inner rel against
 	 * either the outer rel or a pseudoconstant.  If an operator is
@@ -274,10 +314,8 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 			!bms_equal(restrictinfo->required_relids, joinrelids))
 		{
 			/*
-			 * If such a clause actually references the inner rel then join
-			 * removal has to be disallowed.  We have to check this despite
-			 * the previous attr_needed checks because of the possibility of
-			 * pushed-down clauses referencing the rel.
+			 * If such a clause actually references the inner rel then we can't
+			 * mark the join as unique.
 			 */
 			if (bms_is_member(innerrelid, restrictinfo->clause_relids))
 				return false;
@@ -300,71 +338,9 @@ join_is_removable(PlannerInfo *root, SpecialJoinInfo *sjinfo)
 		clause_list = lappend(clause_list, restrictinfo);
 	}
 
-	/*
-	 * relation_has_unique_index_for automatically adds any usable restriction
-	 * clauses for the innerrel, so we needn't do that here.  (XXX we are not
-	 * considering restriction clauses for subqueries; is that worth doing?)
-	 */
-
-	if (innerrel->rtekind == RTE_RELATION)
-	{
-		/* Now examine the indexes to see if we have a matching unique index */
-		if (relation_has_unique_index_for(root, innerrel, clause_list, NIL, NIL))
-			return true;
-	}
-	else	/* innerrel->rtekind == RTE_SUBQUERY */
-	{
-		List	   *colnos = NIL;
-		List	   *opids = NIL;
-
-		/*
-		 * Build the argument lists for query_is_distinct_for: a list of
-		 * output column numbers that the query needs to be distinct over, and
-		 * a list of equality operators that the output columns need to be
-		 * distinct according to.
-		 */
-		foreach(l, clause_list)
-		{
-			RestrictInfo *rinfo = (RestrictInfo *) lfirst(l);
-			Oid			op;
-			Var		   *var;
-
-			/*
-			 * Get the equality operator we need uniqueness according to.
-			 * (This might be a cross-type operator and thus not exactly the
-			 * same operator the subquery would consider; that's all right
-			 * since query_is_distinct_for can resolve such cases.)  The
-			 * mergejoinability test above should have selected only OpExprs.
-			 */
-			Assert(IsA(rinfo->clause, OpExpr));
-			op = ((OpExpr *) rinfo->clause)->opno;
-
-			/* clause_sides_match_join identified the inner side for us */
-			if (rinfo->outer_is_left)
-				var = (Var *) get_rightop(rinfo->clause);
-			else
-				var = (Var *) get_leftop(rinfo->clause);
-
-			/*
-			 * If inner side isn't a Var referencing a subquery output column,
-			 * this clause doesn't help us.
-			 */
-			if (!var || !IsA(var, Var) ||
-				var->varno != innerrelid || var->varlevelsup != 0)
-				continue;
-
-			colnos = lappend_int(colnos, var->varattno);
-			opids = lappend_oid(opids, op);
-		}
-
-		if (query_is_distinct_for(subquery, colnos, opids))
-			return true;
-	}
+	if (rel_is_distinct_for(root, innerrel, clause_list))
+		return true;
 
-	/*
-	 * Some day it would be nice to check for other methods of establishing
-	 * distinctness.
-	 */
 	return false;
 }
 
@@ -562,6 +538,125 @@ remove_rel_from_joinlist(List *joinlist, int relid, int *nremoved)
 	return result;
 }
 
+/*
+ * rel_is_distinct_for
+ *		Returns True if rel can be proved to be distinct over clause_list
+ *
+ * Note: We expect clause_list to be already processed to check if the
+ * RestrictInfos are in the form "outerrel_expr op innerrel_expr" or
+ * "innerrel_expr op outerrel_expr".
+ *
+ * Note: this method may add items to clause_list, callers should either
+ * make a copy of the list or trim it back to it's original length after
+ * calling this function.
+ */
+bool
+rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel, List *clause_list)
+{
+	int relid = rel->relid;
+
+	/*
+	 * relation_has_unique_index_for automatically adds any usable restriction
+	 * clauses for the rel, so we needn't do that here.  (XXX we are not
+	 * considering restriction clauses for subqueries; is that worth doing?)
+	 */
+	if (rel->rtekind == RTE_RELATION)
+	{
+		/* Now examine the indexes to see if we have a matching unique index */
+		if (relation_has_unique_index_for(root, rel, clause_list, NIL, NIL))
+			return true;
+	}
+	else if (rel->rtekind == RTE_SUBQUERY)
+	{
+		List	   *colnos = NIL;
+		List	   *opids = NIL;
+		ListCell   *l;
+		Query	   *subquery = root->simple_rte_array[relid]->subquery;
+
+		/*
+		 * Build the argument lists for query_is_distinct_for: a list of
+		 * output column numbers that the query needs to be distinct over, and
+		 * a list of equality operators that the output columns need to be
+		 * distinct according to.
+		 */
+		foreach(l, clause_list)
+		{
+			RestrictInfo *rinfo = (RestrictInfo *)lfirst(l);
+			Oid			op;
+			Var		   *var;
+
+			if (!IsA(rinfo->clause, OpExpr))
+				continue;
+
+			/*
+			 * Get the equality operator we need uniqueness according to.
+			 * (This might be a cross-type operator and thus not exactly the
+			 * same operator the subquery would consider; that's all right
+			 * since query_is_distinct_for can resolve such cases.)  The
+			 * mergejoinability test above should have selected only OpExprs.
+			 */
+			op = ((OpExpr *)rinfo->clause)->opno;
+
+			/* clause_sides_match_join identified the inner side for us */
+			if (rinfo->outer_is_left)
+				var = (Var *)get_rightop(rinfo->clause);
+			else
+				var = (Var *)get_leftop(rinfo->clause);
+
+			/*
+			 * If inner side isn't a Var referencing a subquery output column,
+			 * this clause doesn't help us.
+			 */
+			if (!var || !IsA(var, Var) ||
+				var->varno != relid || var->varlevelsup != 0)
+				continue;
+
+			colnos = lappend_int(colnos, var->varattno);
+			opids = lappend_oid(opids, op);
+		}
+
+		if (query_is_distinct_for(subquery, colnos, opids))
+			return true;
+	}
+	return false; /* can't prove rel to be distinct over clause_list */
+}
+/*
+ * rel_supports_distinctness
+ *		Returns true if rel has some properties which can prove the relation
+ *		to be unique over some set of columns.
+ *
+ * This is effectively a pre-checking function for rel_is_distinct_for().
+ * It must return TRUE if rel_is_distinct_for() could possibly return TRUE
+ */
+bool
+rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel)
+{
+	if (rel->rtekind == RTE_RELATION)
+	{
+		/*
+		 * For a plain-relation, we only know how to prove uniqueness
+		 * by reference to unique indexes.  If there are no indexes then
+		 * there's certainly no unique indexes so there's nothing to prove
+		 * uniqueness on the relation.
+		 */
+		if (rel->indexlist != NIL)
+			return true;
+	}
+	else if (rel->rtekind == RTE_SUBQUERY)
+	{
+		Query *subquery = root->simple_rte_array[rel->relid]->subquery;
+
+		/* Check if the subquery has any qualities that support distinctness */
+		if (query_supports_distinctness(subquery))
+			return true;
+	}
+
+	/*
+	 * Some day it would be nice to check for other methods of establishing
+	 * distinctness.
+	 */
+	return false;
+}
 
 /*
  * query_supports_distinctness - could the query possibly be proven distinct
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 404c6f5..fbe76b2 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -136,13 +136,12 @@ static BitmapAnd *make_bitmap_and(List *bitmapplans);
 static BitmapOr *make_bitmap_or(List *bitmapplans);
 static NestLoop *make_nestloop(List *tlist,
 			  List *joinclauses, List *otherclauses, List *nestParams,
-			  Plan *lefttree, Plan *righttree,
-			  JoinType jointype);
+			  Plan *lefttree, Plan *righttree, JoinPath *jpath);
 static HashJoin *make_hashjoin(List *tlist,
 			  List *joinclauses, List *otherclauses,
 			  List *hashclauses,
 			  Plan *lefttree, Plan *righttree,
-			  JoinType jointype);
+			  JoinPath *jpath);
 static Hash *make_hash(Plan *lefttree,
 		  Oid skewTable,
 		  AttrNumber skewColumn,
@@ -157,7 +156,7 @@ static MergeJoin *make_mergejoin(List *tlist,
 			   int *mergestrategies,
 			   bool *mergenullsfirst,
 			   Plan *lefttree, Plan *righttree,
-			   JoinType jointype);
+			   JoinPath *jpath);
 static Sort *make_sort(PlannerInfo *root, Plan *lefttree, int numCols,
 		  AttrNumber *sortColIdx, Oid *sortOperators,
 		  Oid *collations, bool *nullsFirst,
@@ -2313,7 +2312,7 @@ create_nestloop_plan(PlannerInfo *root,
 							  nestParams,
 							  outer_plan,
 							  inner_plan,
-							  best_path->jointype);
+							  best_path);
 
 	copy_path_costsize(&join_plan->join.plan, &best_path->path);
 
@@ -2607,7 +2606,7 @@ create_mergejoin_plan(PlannerInfo *root,
 							   mergenullsfirst,
 							   outer_plan,
 							   inner_plan,
-							   best_path->jpath.jointype);
+							   &best_path->jpath);
 
 	/* Costs of sort and material steps are included in path cost already */
 	copy_path_costsize(&join_plan->join.plan, &best_path->jpath.path);
@@ -2733,7 +2732,7 @@ create_hashjoin_plan(PlannerInfo *root,
 							  hashclauses,
 							  outer_plan,
 							  (Plan *) hash_plan,
-							  best_path->jpath.jointype);
+							  &best_path->jpath);
 
 	copy_path_costsize(&join_plan->join.plan, &best_path->jpath.path);
 
@@ -3866,7 +3865,7 @@ make_nestloop(List *tlist,
 			  List *nestParams,
 			  Plan *lefttree,
 			  Plan *righttree,
-			  JoinType jointype)
+			  JoinPath *jpath)
 {
 	NestLoop   *node = makeNode(NestLoop);
 	Plan	   *plan = &node->join.plan;
@@ -3876,8 +3875,9 @@ make_nestloop(List *tlist,
 	plan->qual = otherclauses;
 	plan->lefttree = lefttree;
 	plan->righttree = righttree;
-	node->join.jointype = jointype;
+	node->join.jointype = jpath->jointype;
 	node->join.joinqual = joinclauses;
+	node->join.unique_inner = jpath->unique_inner;
 	node->nestParams = nestParams;
 
 	return node;
@@ -3890,7 +3890,7 @@ make_hashjoin(List *tlist,
 			  List *hashclauses,
 			  Plan *lefttree,
 			  Plan *righttree,
-			  JoinType jointype)
+			  JoinPath *jpath)
 {
 	HashJoin   *node = makeNode(HashJoin);
 	Plan	   *plan = &node->join.plan;
@@ -3901,8 +3901,9 @@ make_hashjoin(List *tlist,
 	plan->lefttree = lefttree;
 	plan->righttree = righttree;
 	node->hashclauses = hashclauses;
-	node->join.jointype = jointype;
+	node->join.jointype = jpath->jointype;
 	node->join.joinqual = joinclauses;
+	node->join.unique_inner = jpath->unique_inner;
 
 	return node;
 }
@@ -3950,7 +3951,7 @@ make_mergejoin(List *tlist,
 			   bool *mergenullsfirst,
 			   Plan *lefttree,
 			   Plan *righttree,
-			   JoinType jointype)
+			   JoinPath *jpath)
 {
 	MergeJoin  *node = makeNode(MergeJoin);
 	Plan	   *plan = &node->join.plan;
@@ -3965,8 +3966,9 @@ make_mergejoin(List *tlist,
 	node->mergeCollations = mergecollations;
 	node->mergeStrategies = mergestrategies;
 	node->mergeNullsFirst = mergenullsfirst;
-	node->join.jointype = jointype;
+	node->join.jointype = jpath->jointype;
 	node->join.joinqual = joinclauses;
+	node->join.unique_inner = jpath->unique_inner;
 
 	return node;
 }
diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index 2f4e818..c5a5677 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -1089,6 +1089,7 @@ make_outerjoininfo(PlannerInfo *root,
 	sjinfo->jointype = jointype;
 	/* this always starts out false */
 	sjinfo->delay_upper_joins = false;
+	sjinfo->is_unique_join = false;
 
 	compute_semijoin_info(sjinfo, clause);
 
diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index 848df97..55310d8 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -174,6 +174,9 @@ query_planner(PlannerInfo *root, List *tlist,
 	 */
 	fix_placeholder_input_needed_levels(root);
 
+	/* Analyze joins to find out which ones have a unique inner side */
+	mark_unique_joins(root, joinlist);
+
 	/*
 	 * Remove any useless outer joins.  Ideally this would be done during
 	 * jointree preprocessing, but the necessary information isn't available
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 935bc2b..ab4389a 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1541,6 +1541,7 @@ calc_non_nestloop_required_outer(Path *outer_path, Path *inner_path)
  *
  * 'joinrel' is the join relation.
  * 'jointype' is the type of join required
+ * 'unique_inner' is the inner side of the join unique on the join condition
  * 'workspace' is the result from initial_cost_nestloop
  * 'sjinfo' is extra info about the join for selectivity estimation
  * 'semifactors' contains valid data if jointype is SEMI or ANTI
@@ -1556,6 +1557,7 @@ NestPath *
 create_nestloop_path(PlannerInfo *root,
 					 RelOptInfo *joinrel,
 					 JoinType jointype,
+					 bool unique_inner,
 					 JoinCostWorkspace *workspace,
 					 SpecialJoinInfo *sjinfo,
 					 SemiAntiJoinFactors *semifactors,
@@ -1608,6 +1610,7 @@ create_nestloop_path(PlannerInfo *root,
 	pathnode->jointype = jointype;
 	pathnode->outerjoinpath = outer_path;
 	pathnode->innerjoinpath = inner_path;
+	pathnode->unique_inner = unique_inner;
 	pathnode->joinrestrictinfo = restrict_clauses;
 
 	final_cost_nestloop(root, pathnode, workspace, sjinfo, semifactors);
@@ -1622,6 +1625,7 @@ create_nestloop_path(PlannerInfo *root,
  *
  * 'joinrel' is the join relation
  * 'jointype' is the type of join required
+  * 'unique_inner' is the inner side of the join unique on the join condition
  * 'workspace' is the result from initial_cost_mergejoin
  * 'sjinfo' is extra info about the join for selectivity estimation
  * 'outer_path' is the outer path
@@ -1638,6 +1642,7 @@ MergePath *
 create_mergejoin_path(PlannerInfo *root,
 					  RelOptInfo *joinrel,
 					  JoinType jointype,
+					  bool unique_inner,
 					  JoinCostWorkspace *workspace,
 					  SpecialJoinInfo *sjinfo,
 					  Path *outer_path,
@@ -1665,6 +1670,7 @@ create_mergejoin_path(PlannerInfo *root,
 	pathnode->jpath.jointype = jointype;
 	pathnode->jpath.outerjoinpath = outer_path;
 	pathnode->jpath.innerjoinpath = inner_path;
+	pathnode->jpath.unique_inner = unique_inner;
 	pathnode->jpath.joinrestrictinfo = restrict_clauses;
 	pathnode->path_mergeclauses = mergeclauses;
 	pathnode->outersortkeys = outersortkeys;
@@ -1682,6 +1688,7 @@ create_mergejoin_path(PlannerInfo *root,
  *
  * 'joinrel' is the join relation
  * 'jointype' is the type of join required
+ * 'unique_inner' is the inner side of the join unique on the join condition
  * 'workspace' is the result from initial_cost_hashjoin
  * 'sjinfo' is extra info about the join for selectivity estimation
  * 'semifactors' contains valid data if jointype is SEMI or ANTI
@@ -1696,6 +1703,7 @@ HashPath *
 create_hashjoin_path(PlannerInfo *root,
 					 RelOptInfo *joinrel,
 					 JoinType jointype,
+					 bool unique_inner,
 					 JoinCostWorkspace *workspace,
 					 SpecialJoinInfo *sjinfo,
 					 SemiAntiJoinFactors *semifactors,
@@ -1733,6 +1741,7 @@ create_hashjoin_path(PlannerInfo *root,
 	pathnode->jpath.jointype = jointype;
 	pathnode->jpath.outerjoinpath = outer_path;
 	pathnode->jpath.innerjoinpath = inner_path;
+	pathnode->jpath.unique_inner = unique_inner;
 	pathnode->jpath.joinrestrictinfo = restrict_clauses;
 	pathnode->path_hashclauses = hashclauses;
 	/* final_cost_hashjoin will fill in pathnode->num_batches */
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 5796de8..348ea3b 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -1642,6 +1642,7 @@ typedef struct JoinState
 	PlanState	ps;
 	JoinType	jointype;
 	List	   *joinqual;		/* JOIN quals (in addition to ps.qual) */
+	bool		unique_inner;	/* inner rel is unique on the join condition */
 } JoinState;
 
 /* ----------------
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index 0654d02..c11269e 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -591,6 +591,7 @@ typedef struct Join
 	Plan		plan;
 	JoinType	jointype;
 	List	   *joinqual;		/* JOIN quals (in addition to plan.qual) */
+	bool		unique_inner;	/* inner rel is unique on the join condition */
 } Join;
 
 /* ----------------
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 5dc23d9..77881cf 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -1052,6 +1052,7 @@ typedef struct JoinPath
 
 	Path	   *outerjoinpath;	/* path for the outer side of the join */
 	Path	   *innerjoinpath;	/* path for the inner side of the join */
+	bool		unique_inner;	/* inner rel is unique on the join condition */
 
 	List	   *joinrestrictinfo;		/* RestrictInfos to apply to join */
 
@@ -1428,6 +1429,7 @@ typedef struct SpecialJoinInfo
 	JoinType	jointype;		/* always INNER, LEFT, FULL, SEMI, or ANTI */
 	bool		lhs_strict;		/* joinclause is strict for some LHS rel */
 	bool		delay_upper_joins;		/* can't commute with upper RHS */
+	bool		is_unique_join; /* matches a max of 1 row per outer join row */
 	/* Remaining fields are set only for JOIN_SEMI jointype: */
 	bool		semi_can_btree; /* true if semi_operators are all btree */
 	bool		semi_can_hash;	/* true if semi_operators are all hash */
@@ -1706,6 +1708,7 @@ typedef struct JoinPathExtraData
 	SemiAntiJoinFactors semifactors;
 	Relids		param_source_rels;
 	Relids		extra_lateral_rels;
+	bool		unique_inner;
 } JoinPathExtraData;
 
 /*
diff --git a/src/include/optimizer/cost.h b/src/include/optimizer/cost.h
index dd43e45..87c0d1e 100644
--- a/src/include/optimizer/cost.h
+++ b/src/include/optimizer/cost.h
@@ -115,7 +115,7 @@ extern void cost_group(Path *path, PlannerInfo *root,
 		   double input_tuples);
 extern void initial_cost_nestloop(PlannerInfo *root,
 					  JoinCostWorkspace *workspace,
-					  JoinType jointype,
+					  JoinType jointype, bool unique_inner,
 					  Path *outer_path, Path *inner_path,
 					  SpecialJoinInfo *sjinfo,
 					  SemiAntiJoinFactors *semifactors);
diff --git a/src/include/optimizer/pathnode.h b/src/include/optimizer/pathnode.h
index 161644c..e9b547d 100644
--- a/src/include/optimizer/pathnode.h
+++ b/src/include/optimizer/pathnode.h
@@ -91,6 +91,7 @@ extern Relids calc_non_nestloop_required_outer(Path *outer_path, Path *inner_pat
 extern NestPath *create_nestloop_path(PlannerInfo *root,
 					 RelOptInfo *joinrel,
 					 JoinType jointype,
+					 bool unique_inner,
 					 JoinCostWorkspace *workspace,
 					 SpecialJoinInfo *sjinfo,
 					 SemiAntiJoinFactors *semifactors,
@@ -103,6 +104,7 @@ extern NestPath *create_nestloop_path(PlannerInfo *root,
 extern MergePath *create_mergejoin_path(PlannerInfo *root,
 					  RelOptInfo *joinrel,
 					  JoinType jointype,
+					  bool unique_inner,
 					  JoinCostWorkspace *workspace,
 					  SpecialJoinInfo *sjinfo,
 					  Path *outer_path,
@@ -117,6 +119,7 @@ extern MergePath *create_mergejoin_path(PlannerInfo *root,
 extern HashPath *create_hashjoin_path(PlannerInfo *root,
 					 RelOptInfo *joinrel,
 					 JoinType jointype,
+					 bool unique_inner,
 					 JoinCostWorkspace *workspace,
 					 SpecialJoinInfo *sjinfo,
 					 SemiAntiJoinFactors *semifactors,
diff --git a/src/include/optimizer/planmain.h b/src/include/optimizer/planmain.h
index 52b077a..d7d373b 100644
--- a/src/include/optimizer/planmain.h
+++ b/src/include/optimizer/planmain.h
@@ -124,7 +124,11 @@ extern RestrictInfo *build_implied_join_equality(Oid opno,
 /*
  * prototypes for plan/analyzejoins.c
  */
+extern void mark_unique_joins(PlannerInfo *root, List *joinlist);
 extern List *remove_useless_joins(PlannerInfo *root, List *joinlist);
+extern bool rel_is_distinct_for(PlannerInfo *root, RelOptInfo *rel,
+								List *clause_list);
+extern bool rel_supports_distinctness(PlannerInfo *root, RelOptInfo *rel);
 extern bool query_supports_distinctness(Query *query);
 extern bool query_is_distinct_for(Query *query, List *colnos, List *opids);
 
diff --git a/src/test/regress/expected/equivclass.out b/src/test/regress/expected/equivclass.out
index 0391b8e..6e32a7e 100644
--- a/src/test/regress/expected/equivclass.out
+++ b/src/test/regress/expected/equivclass.out
@@ -186,7 +186,7 @@ explain (costs off)
   select * from ec1, ec2 where ff = x1 and x1 = '42'::int8alias2;
                QUERY PLAN                
 -----------------------------------------
- Nested Loop
+ Nested Loop (inner unique)
    ->  Seq Scan on ec2
          Filter: (x1 = '42'::int8alias2)
    ->  Index Scan using ec1_pkey on ec1
@@ -310,7 +310,7 @@ explain (costs off)
          ->  Index Scan using ec1_expr3 on ec1 ec1_5
          ->  Index Scan using ec1_expr4 on ec1 ec1_6
    ->  Materialize
-         ->  Merge Join
+         ->  Merge Join (inner unique)
                Merge Cond: ((((ec1_1.ff + 2) + 1)) = ec1.f1)
                ->  Merge Append
                      Sort Key: (((ec1_1.ff + 2) + 1))
@@ -365,7 +365,7 @@ explain (costs off)
   where ss1.x = ec1.f1 and ec1.ff = 42::int8;
                      QUERY PLAN                      
 -----------------------------------------------------
- Merge Join
+ Merge Join (inner unique)
    Merge Cond: ((((ec1_1.ff + 2) + 1)) = ec1.f1)
    ->  Merge Append
          Sort Key: (((ec1_1.ff + 2) + 1))
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index 9d605a2..0a028ef 100644
--- a/src/test/regress/expected/join.out
+++ b/src/test/regress/expected/join.out
@@ -2728,8 +2728,8 @@ from nt3 as nt3
 where nt3.id = 1 and ss2.b3;
                   QUERY PLAN                   
 -----------------------------------------------
- Nested Loop
-   ->  Nested Loop
+ Nested Loop (inner unique)
+   ->  Nested Loop (inner unique)
          ->  Index Scan using nt3_pkey on nt3
                Index Cond: (id = 1)
          ->  Index Scan using nt2_pkey on nt2
@@ -3816,7 +3816,7 @@ explain (costs off)
     on (p.k = ss.k);
            QUERY PLAN            
 ---------------------------------
- Hash Left Join
+ Hash Left Join (inner unique)
    Hash Cond: (p.k = c.k)
    ->  Seq Scan on parent p
    ->  Hash
@@ -5043,3 +5043,247 @@ ERROR:  invalid reference to FROM-clause entry for table "xx1"
 LINE 1: ...xx1 using lateral (select * from int4_tbl where f1 = x1) ss;
                                                                 ^
 HINT:  There is an entry for table "xx1", but it cannot be referenced from this part of the query.
+--
+-- test planner's ability to mark joins as unique.
+--
+create table j1 (id int primary key);
+create table j2 (id int primary key);
+create table j3 (id int);
+insert into j1 values(1),(2),(3);
+insert into j2 values(1),(2),(3);
+insert into j3 values(1),(1);
+analyze j1;
+analyze j2;
+analyze j3;
+-- Ensure join is marked as unique
+explain (verbose, costs off)
+select * from j1 inner join j2 on j1.id = j2.id;
+            QUERY PLAN             
+-----------------------------------
+ Hash Join (inner unique)
+   Output: j1.id, j2.id
+   Hash Cond: (j1.id = j2.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Hash
+         Output: j2.id
+         ->  Seq Scan on public.j2
+               Output: j2.id
+(9 rows)
+
+-- Ensure join not marked as unique when not using =
+explain (verbose, costs off)
+select * from j1 inner join j2 on j1.id > j2.id;
+            QUERY PLAN             
+-----------------------------------
+ Nested Loop
+   Output: j1.id, j2.id
+   Join Filter: (j1.id > j2.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Materialize
+         Output: j2.id
+         ->  Seq Scan on public.j2
+               Output: j2.id
+(9 rows)
+
+-- j3 has no unique index or pk on id
+explain (verbose, costs off)
+select * from j1 inner join j3 on j1.id = j3.id;
+            QUERY PLAN             
+-----------------------------------
+ Hash Join
+   Output: j1.id, j3.id
+   Hash Cond: (j1.id = j3.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Hash
+         Output: j3.id
+         ->  Seq Scan on public.j3
+               Output: j3.id
+(9 rows)
+
+-- ensure left join is marked as unique
+explain (verbose, costs off)
+select * from j1 left join j2 on j1.id = j2.id;
+            QUERY PLAN             
+-----------------------------------
+ Hash Left Join (inner unique)
+   Output: j1.id, j2.id
+   Hash Cond: (j1.id = j2.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Hash
+         Output: j2.id
+         ->  Seq Scan on public.j2
+               Output: j2.id
+(9 rows)
+
+-- ensure right join is marked as unique
+explain (verbose, costs off)
+select * from j1 right join j2 on j1.id = j2.id;
+            QUERY PLAN             
+-----------------------------------
+ Hash Left Join (inner unique)
+   Output: j1.id, j2.id
+   Hash Cond: (j2.id = j1.id)
+   ->  Seq Scan on public.j2
+         Output: j2.id
+   ->  Hash
+         Output: j1.id
+         ->  Seq Scan on public.j1
+               Output: j1.id
+(9 rows)
+
+-- cross joins can't be proved unique
+explain (verbose, costs off)
+select * from j1 cross join j2;
+            QUERY PLAN             
+-----------------------------------
+ Nested Loop
+   Output: j1.id, j2.id
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Materialize
+         Output: j2.id
+         ->  Seq Scan on public.j2
+               Output: j2.id
+(8 rows)
+
+-- ensure natural join is marked as unique
+explain (verbose, costs off)
+select * from j1 natural join j2;
+            QUERY PLAN             
+-----------------------------------
+ Hash Join (inner unique)
+   Output: j1.id
+   Hash Cond: (j1.id = j2.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Hash
+         Output: j2.id
+         ->  Seq Scan on public.j2
+               Output: j2.id
+(9 rows)
+
+-- ensure distinct clause uniquifies the join
+explain (verbose, costs off)
+select * from j1
+inner join (select distinct id from j3) j3 on j1.id = j3.id;
+               QUERY PLAN                
+-----------------------------------------
+ Nested Loop (inner unique)
+   Output: j1.id, j3.id
+   Join Filter: (j1.id = j3.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Materialize
+         Output: j3.id
+         ->  HashAggregate
+               Output: j3.id
+               Group Key: j3.id
+               ->  Seq Scan on public.j3
+                     Output: j3.id
+(12 rows)
+
+-- ensure group by clause uniquifies the join
+explain (verbose, costs off)
+select * from j1
+inner join (select id from j3 group by id) j3 on j1.id = j3.id;
+               QUERY PLAN                
+-----------------------------------------
+ Nested Loop (inner unique)
+   Output: j1.id, j3.id
+   Join Filter: (j1.id = j3.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Materialize
+         Output: j3.id
+         ->  HashAggregate
+               Output: j3.id
+               Group Key: j3.id
+               ->  Seq Scan on public.j3
+                     Output: j3.id
+(12 rows)
+
+-- a subquery with an empty FROM clause should be marked as unique.
+explain (verbose, costs off)
+select * from j1
+inner join (select 1 id offset 0) j3 on j1.id = j3.id;
+         QUERY PLAN          
+-----------------------------
+ Hash Join
+   Output: j1.id, (1)
+   Hash Cond: (j1.id = (1))
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Hash
+         Output: (1)
+         ->  Result
+               Output: 1
+(9 rows)
+
+explain (verbose, costs off)
+select * from j1 full join j2 on j1.id = j2.id;
+            QUERY PLAN             
+-----------------------------------
+ Hash Full Join
+   Output: j1.id, j2.id
+   Hash Cond: (j1.id = j2.id)
+   ->  Seq Scan on public.j1
+         Output: j1.id
+   ->  Hash
+         Output: j2.id
+         ->  Seq Scan on public.j2
+               Output: j2.id
+(9 rows)
+
+drop table j1;
+drop table j2;
+drop table j3;
+-- test a more complex permutations of unique joins
+create table j1 (id1 int, id2 int, primary key(id1,id2));
+create table j2 (id1 int, id2 int, primary key(id1,id2));
+create table j3 (id1 int, id2 int, primary key(id1,id2));
+insert into j1 values(1,1),(2,2);
+insert into j2 values(1,1);
+insert into j3 values(1,1);
+analyze j1;
+analyze j2;
+analyze j3;
+-- ensure no unique joins when not all columns which are part of
+-- the unique index are part of the join clause.
+explain (verbose, costs off)
+select * from j1
+inner join j2 on j1.id1 = j2.id1;
+                QUERY PLAN                
+------------------------------------------
+ Nested Loop
+   Output: j1.id1, j1.id2, j2.id1, j2.id2
+   Join Filter: (j1.id1 = j2.id1)
+   ->  Seq Scan on public.j2
+         Output: j2.id1, j2.id2
+   ->  Seq Scan on public.j1
+         Output: j1.id1, j1.id2
+(7 rows)
+
+-- ensure unique joins work with multiple columns
+explain (verbose, costs off)
+select * from j1
+inner join j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2;
+                        QUERY PLAN                        
+----------------------------------------------------------
+ Nested Loop (inner unique)
+   Output: j1.id1, j1.id2, j2.id1, j2.id2
+   Join Filter: ((j1.id1 = j2.id1) AND (j1.id2 = j2.id2))
+   ->  Seq Scan on public.j1
+         Output: j1.id1, j1.id2
+   ->  Materialize
+         Output: j2.id1, j2.id2
+         ->  Seq Scan on public.j2
+               Output: j2.id1, j2.id2
+(9 rows)
+
+drop table j1;
+drop table j2;
+drop table j3;
diff --git a/src/test/regress/expected/rowsecurity.out b/src/test/regress/expected/rowsecurity.out
index 6fc80af..a79f497 100644
--- a/src/test/regress/expected/rowsecurity.out
+++ b/src/test/regress/expected/rowsecurity.out
@@ -276,7 +276,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM document WHERE f_leak(dtitle);
 EXPLAIN (COSTS OFF) SELECT * FROM document NATURAL JOIN category WHERE f_leak(dtitle);
                      QUERY PLAN                     
 ----------------------------------------------------
- Nested Loop
+ Nested Loop (inner unique)
    ->  Subquery Scan on document
          Filter: f_leak(document.dtitle)
          ->  Seq Scan on document document_1
diff --git a/src/test/regress/expected/select_views.out b/src/test/regress/expected/select_views.out
index 7f57526..a238a62 100644
--- a/src/test/regress/expected/select_views.out
+++ b/src/test/regress/expected/select_views.out
@@ -1411,7 +1411,7 @@ NOTICE:  f_leak => 9801-2345-6789-0123
 EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_normal WHERE f_leak(cnum);
                        QUERY PLAN                        
 ---------------------------------------------------------
- Hash Join
+ Hash Join (inner unique)
    Hash Cond: (r.cid = l.cid)
    ->  Seq Scan on credit_card r
          Filter: f_leak(cnum)
@@ -1432,7 +1432,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_secure WHERE f_leak(cnum);
 ---------------------------------------------------------------
  Subquery Scan on my_credit_card_secure
    Filter: f_leak(my_credit_card_secure.cnum)
-   ->  Hash Join
+   ->  Hash Join (inner unique)
          Hash Cond: (r.cid = l.cid)
          ->  Seq Scan on credit_card r
          ->  Hash
@@ -1466,7 +1466,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_usage_normal
    ->  Materialize
          ->  Subquery Scan on l
                Filter: f_leak(l.cnum)
-               ->  Hash Join
+               ->  Hash Join (inner unique)
                      Hash Cond: (r_1.cid = l_1.cid)
                      ->  Seq Scan on credit_card r_1
                      ->  Hash
@@ -1497,7 +1497,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_usage_secure
          ->  Seq Scan on credit_usage r
                Filter: ((ymd >= '10-01-2011'::date) AND (ymd < '11-01-2011'::date))
          ->  Materialize
-               ->  Hash Join
+               ->  Hash Join (inner unique)
                      Hash Cond: (r_1.cid = l.cid)
                      ->  Seq Scan on credit_card r_1
                      ->  Hash
diff --git a/src/test/regress/expected/select_views_1.out b/src/test/regress/expected/select_views_1.out
index 5275ef0..5f21ac2 100644
--- a/src/test/regress/expected/select_views_1.out
+++ b/src/test/regress/expected/select_views_1.out
@@ -1411,7 +1411,7 @@ NOTICE:  f_leak => 9801-2345-6789-0123
 EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_normal WHERE f_leak(cnum);
                        QUERY PLAN                        
 ---------------------------------------------------------
- Hash Join
+ Hash Join (inner unique)
    Hash Cond: (r.cid = l.cid)
    ->  Seq Scan on credit_card r
          Filter: f_leak(cnum)
@@ -1432,7 +1432,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_secure WHERE f_leak(cnum);
 ---------------------------------------------------------------
  Subquery Scan on my_credit_card_secure
    Filter: f_leak(my_credit_card_secure.cnum)
-   ->  Hash Join
+   ->  Hash Join (inner unique)
          Hash Cond: (r.cid = l.cid)
          ->  Seq Scan on credit_card r
          ->  Hash
@@ -1466,7 +1466,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_usage_normal
    ->  Materialize
          ->  Subquery Scan on l
                Filter: f_leak(l.cnum)
-               ->  Hash Join
+               ->  Hash Join (inner unique)
                      Hash Cond: (r_1.cid = l_1.cid)
                      ->  Seq Scan on credit_card r_1
                      ->  Hash
@@ -1497,7 +1497,7 @@ EXPLAIN (COSTS OFF) SELECT * FROM my_credit_card_usage_secure
          ->  Seq Scan on credit_usage r
                Filter: ((ymd >= '10-01-2011'::date) AND (ymd < '11-01-2011'::date))
          ->  Materialize
-               ->  Hash Join
+               ->  Hash Join (inner unique)
                      Hash Cond: (r_1.cid = l.cid)
                      ->  Seq Scan on credit_card r_1
                      ->  Hash
diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql
index 528e1ef..bf94e9e 100644
--- a/src/test/regress/sql/join.sql
+++ b/src/test/regress/sql/join.sql
@@ -1600,3 +1600,98 @@ update xx1 set x2 = f1 from xx1, lateral (select * from int4_tbl where f1 = x1)
 delete from xx1 using (select * from int4_tbl where f1 = x1) ss;
 delete from xx1 using (select * from int4_tbl where f1 = xx1.x1) ss;
 delete from xx1 using lateral (select * from int4_tbl where f1 = x1) ss;
+
+--
+-- test planner's ability to mark joins as unique.
+--
+
+create table j1 (id int primary key);
+create table j2 (id int primary key);
+create table j3 (id int);
+
+insert into j1 values(1),(2),(3);
+insert into j2 values(1),(2),(3);
+insert into j3 values(1),(1);
+
+analyze j1;
+analyze j2;
+analyze j3;
+
+-- Ensure join is marked as unique
+explain (verbose, costs off)
+select * from j1 inner join j2 on j1.id = j2.id;
+
+-- Ensure join not marked as unique when not using =
+explain (verbose, costs off)
+select * from j1 inner join j2 on j1.id > j2.id;
+
+-- j3 has no unique index or pk on id
+explain (verbose, costs off)
+select * from j1 inner join j3 on j1.id = j3.id;
+
+-- ensure left join is marked as unique
+explain (verbose, costs off)
+select * from j1 left join j2 on j1.id = j2.id;
+
+-- ensure right join is marked as unique
+explain (verbose, costs off)
+select * from j1 right join j2 on j1.id = j2.id;
+
+-- cross joins can't be proved unique
+explain (verbose, costs off)
+select * from j1 cross join j2;
+
+-- ensure natural join is marked as unique
+explain (verbose, costs off)
+select * from j1 natural join j2;
+
+-- ensure distinct clause uniquifies the join
+explain (verbose, costs off)
+select * from j1
+inner join (select distinct id from j3) j3 on j1.id = j3.id;
+
+-- ensure group by clause uniquifies the join
+explain (verbose, costs off)
+select * from j1
+inner join (select id from j3 group by id) j3 on j1.id = j3.id;
+
+-- a subquery with an empty FROM clause should be marked as unique.
+explain (verbose, costs off)
+select * from j1
+inner join (select 1 id offset 0) j3 on j1.id = j3.id;
+
+explain (verbose, costs off)
+select * from j1 full join j2 on j1.id = j2.id;
+
+drop table j1;
+drop table j2;
+drop table j3;
+
+-- test a more complex permutations of unique joins
+
+create table j1 (id1 int, id2 int, primary key(id1,id2));
+create table j2 (id1 int, id2 int, primary key(id1,id2));
+create table j3 (id1 int, id2 int, primary key(id1,id2));
+
+insert into j1 values(1,1),(2,2);
+insert into j2 values(1,1);
+insert into j3 values(1,1);
+
+analyze j1;
+analyze j2;
+analyze j3;
+
+-- ensure no unique joins when not all columns which are part of
+-- the unique index are part of the join clause.
+explain (verbose, costs off)
+select * from j1
+inner join j2 on j1.id1 = j2.id1;
+
+-- ensure unique joins work with multiple columns
+explain (verbose, costs off)
+select * from j1
+inner join j2 on j1.id1 = j2.id1 and j1.id2 = j2.id2;
+
+drop table j1;
+drop table j2;
+drop table j3;