*** a/src/backend/catalog/index.c
--- b/src/backend/catalog/index.c
***************
*** 1320,1325 **** index_drop(Oid indexId, bool concurrent)
--- 1320,1337 ----
  	 * In the concurrent case we make sure that nobody can be looking at the
  	 * indexes by dropping the index in multiple steps, so we don't need a full
  	 * fledged AccessExlusiveLock yet.
+ 	 *
+ 	 * All predicate locks on the index are about to be made invalid. Promote
+ 	 * them to relation locks on the heap. For correctness the index must not
+ 	 * be seen with indisvalid = true during query planning after the move
+ 	 * starts, so that the index will not be used for a scan after the
+ 	 * predicate lock move, as this could create new predicate locks on the
+ 	 * index which would not ensure a heap relation lock. Also, the index must
+ 	 * not be seen during execution of a heap tuple insert with indisready =
+ 	 * false before the move is complete, since the conflict with the
+ 	 * predicate lock on the index gap could be missed before the lock on the
+ 	 * heap relation is in place to detect a conflict based on the heap tuple
+ 	 * insert.
  	 */
  	heapId = IndexGetRelation(indexId, false);
  	if (concurrent)
***************
*** 1439,1444 **** index_drop(Oid indexId, bool concurrent)
--- 1451,1464 ----
  		}
  
  		/*
+ 		 * No more predicate locks will be acquired on this index, and we're
+ 		 * about to stop doing inserts into the index which could show
+ 		 * conflicts with existing predicate locks, so now is the time to move
+ 		 * them to the heap relation.
+ 		 */
+ 		TransferPredicateLocksToHeapRelation(userIndexRelation);
+ 
+ 		/*
  		 * now we are sure that nobody uses the index for queries, they just
  		 * might have it opened for updating it. So now we can unset
  		 * ->indisready and wait till nobody could update the index anymore.
***************
*** 1507,1518 **** index_drop(Oid indexId, bool concurrent)
  		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
  		userIndexRelation = index_open(indexId, AccessExclusiveLock);
  	}
! 
! 	/*
! 	 * All predicate locks on the index are about to be made invalid. Promote
! 	 * them to relation locks on the heap.
! 	 */
! 	TransferPredicateLocksToHeapRelation(userIndexRelation);
  
  	/*
  	 * Schedule physical removal of the files
--- 1527,1534 ----
  		userHeapRelation = heap_open(heapId, ShareUpdateExclusiveLock);
  		userIndexRelation = index_open(indexId, AccessExclusiveLock);
  	}
! 	else
! 		TransferPredicateLocksToHeapRelation(userIndexRelation);
  
  	/*
  	 * Schedule physical removal of the files