diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index db1792b..c32ff2b 100644
*** a/src/backend/utils/adt/selfuncs.c
--- b/src/backend/utils/adt/selfuncs.c
*************** eqjoinsel_semi(Oid operator,
*** 2570,2575 ****
--- 2570,2585 ----
  	memset(&sslot2, 0, sizeof(sslot2));

  	/*
+ 	 * If we lack any fact-based estimate for nd2, it seems best to set it
+ 	 * equal to the inner relation size estimate, ie, assume the inner side of
+ 	 * the semijoin is unique.  This may lead to overestimating the size of
+ 	 * the join, but that's usually better than an underestimate.  We don't
+ 	 * make any comparable assumption for the outer side, though.
+ 	 */
+ 	if (isdefault2)
+ 		nd2 = inner_rel->rows;
+ 
+ 	/*
  	 * We clamp nd2 to be not more than what we estimate the inner relation's
  	 * size to be.  This is intuitively somewhat reasonable since obviously
  	 * there can't be more than that many distinct values coming from the
*************** eqjoinsel_semi(Oid operator,
*** 2583,2606 ****
  	 * We can apply this clamping both with respect to the base relation from
  	 * which the join variable comes (if there is just one), and to the
  	 * immediate inner input relation of the current join.
- 	 *
- 	 * If we clamp, we can treat nd2 as being a non-default estimate; it's not
- 	 * great, maybe, but it didn't come out of nowhere either.  This is most
- 	 * helpful when the inner relation is empty and consequently has no stats.
  	 */
  	if (vardata2->rel)
  	{
! 		if (nd2 >= vardata2->rel->rows)
! 		{
  			nd2 = vardata2->rel->rows;
- 			isdefault2 = false;
- 		}
  	}
! 	if (nd2 >= inner_rel->rows)
! 	{
  		nd2 = inner_rel->rows;
- 		isdefault2 = false;
- 	}

  	if (HeapTupleIsValid(vardata1->statsTuple))
  	{
--- 2593,2606 ----
  	 * We can apply this clamping both with respect to the base relation from
  	 * which the join variable comes (if there is just one), and to the
  	 * immediate inner input relation of the current join.
  	 */
  	if (vardata2->rel)
  	{
! 		if (nd2 > vardata2->rel->rows)
  			nd2 = vardata2->rel->rows;
  	}
! 	if (nd2 > inner_rel->rows)
  		nd2 = inner_rel->rows;

  	if (HeapTupleIsValid(vardata1->statsTuple))
  	{
*************** eqjoinsel_semi(Oid operator,
*** 2701,2723 ****
  		 * the uncertain rows that a fraction nd2/nd1 have join partners. We
  		 * can discount the known-matched MCVs from the distinct-values counts
  		 * before doing the division.
- 		 *
- 		 * Crude as the above is, it's completely useless if we don't have
- 		 * reliable ndistinct values for both sides.  Hence, if either nd1 or
- 		 * nd2 is default, punt and assume half of the uncertain rows have
- 		 * join partners.
  		 */
! 		if (!isdefault1 && !isdefault2)
! 		{
! 			nd1 -= nmatches;
! 			nd2 -= nmatches;
! 			if (nd1 <= nd2 || nd2 < 0)
! 				uncertainfrac = 1.0;
! 			else
! 				uncertainfrac = nd2 / nd1;
! 		}
  		else
! 			uncertainfrac = 0.5;
  		uncertain = 1.0 - matchfreq1 - nullfrac1;
  		CLAMP_PROBABILITY(uncertain);
  		selec = matchfreq1 + uncertainfrac * uncertain;
--- 2701,2713 ----
  		 * the uncertain rows that a fraction nd2/nd1 have join partners. We
  		 * can discount the known-matched MCVs from the distinct-values counts
  		 * before doing the division.
  		 */
! 		nd1 -= nmatches;
! 		nd2 -= nmatches;
! 		if (nd1 <= nd2 || nd2 < 0)
! 			uncertainfrac = 1.0;
  		else
! 			uncertainfrac = nd2 / nd1;
  		uncertain = 1.0 - matchfreq1 - nullfrac1;
  		CLAMP_PROBABILITY(uncertain);
  		selec = matchfreq1 + uncertainfrac * uncertain;
*************** eqjoinsel_semi(Oid operator,
*** 2730,2744 ****
  		 */
  		double		nullfrac1 = stats1 ? stats1->stanullfrac : 0.0;

! 		if (!isdefault1 && !isdefault2)
! 		{
! 			if (nd1 <= nd2 || nd2 < 0)
! 				selec = 1.0 - nullfrac1;
! 			else
! 				selec = (nd2 / nd1) * (1.0 - nullfrac1);
! 		}
  		else
! 			selec = 0.5 * (1.0 - nullfrac1);
  	}

  	free_attstatsslot(&sslot1);
--- 2720,2729 ----
  		 */
  		double		nullfrac1 = stats1 ? stats1->stanullfrac : 0.0;

! 		if (nd1 <= nd2 || nd2 < 0)
! 			selec = 1.0 - nullfrac1;
  		else
! 			selec = (nd2 / nd1) * (1.0 - nullfrac1);
  	}

  	free_attstatsslot(&sslot1);
diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out
index f47449b..eaffbad 100644
*** a/src/test/regress/expected/join.out
--- b/src/test/regress/expected/join.out
*************** where not exists (
*** 2431,2440 ****
  );
                         QUERY PLAN                        
  ---------------------------------------------------------
!  Hash Anti Join
!    Hash Cond: (t1.c1 = t2.c2)
     ->  Seq Scan on tt4x t1
!    ->  Hash
           ->  Merge Right Join
                 Merge Cond: (t5.c1 = t3.c2)
                 ->  Merge Join
--- 2431,2440 ----
  );
                         QUERY PLAN                        
  ---------------------------------------------------------
!  Nested Loop Anti Join
!    Join Filter: (t1.c1 = t2.c2)
     ->  Seq Scan on tt4x t1
!    ->  Materialize
           ->  Merge Right Join
                 Merge Cond: (t5.c1 = t3.c2)
                 ->  Merge Join