*** a/src/backend/commands/tablecmds.c
--- b/src/backend/commands/tablecmds.c
***************
*** 1747,1755 **** MergeAttributes(List *schema, List *supers, char relpersistence,
  		pfree(newattno);
  
  		/*
! 		 * Close the parent rel, but keep our AccessShareLock on it until xact
! 		 * commit.  That will prevent someone else from deleting or ALTERing
! 		 * the parent before the child is committed.
  		 */
  		heap_close(relation, NoLock);
  	}
--- 1747,1755 ----
  		pfree(newattno);
  
  		/*
! 		 * Close the parent rel, but keep our ShareUpdateExclusiveLock on it
! 		 * until xact commit.  That will prevent someone else from deleting or
! 		 * ALTERing the parent before the child is committed.
  		 */
  		heap_close(relation, NoLock);
  	}