*** a/src/backend/storage/lmgr/predicate.c
--- b/src/backend/storage/lmgr/predicate.c
***************
*** 3635,3649 **** CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
  						 offsetof(PREDICATELOCK, targetLink));
  
  		sxact = predlock->tag.myXact;
! 		if (sxact == MySerializableXact)
  		{
  			/*
! 			 * If we're getting a write lock on the tuple, we don't need a
! 			 * predicate (SIREAD) lock. At this point our transaction already
! 			 * has an ExclusiveRowLock on the relation, so we are OK to drop
! 			 * the predicate lock on the tuple, if found, without fearing that
! 			 * another write against the tuple will occur before the MVCC
! 			 * information makes it to the buffer.
  			 */
  			if (GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
  			{
--- 3635,3654 ----
  						 offsetof(PREDICATELOCK, targetLink));
  
  		sxact = predlock->tag.myXact;
! 		if (sxact == MySerializableXact && !IsSubTransaction())
  		{
  			/*
! 			 * If we're getting a write lock on the tuple and we're not in a
! 			 * subtransaction, we don't need a predicate (SIREAD) lock.  We
! 			 * can't use this optimization within a subtransaction because
! 			 * the subtransaction could be rolled back, and we would be left
! 			 * without any lock at the top level.
! 			 * 
! 			 * At this point our transaction already has an ExclusiveRowLock
! 			 * on the relation, so we are OK to drop the predicate lock on
! 			 * the tuple, if found, without fearing that another write
! 			 * against the tuple will occur before the MVCC information
! 			 * makes it to the buffer.
  			 */
  			if (GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
  			{