PostgreSQL索引扫描时为什么index only scan不返回ctid

SQL> select rowid,id from t1 where id = 1;
---------------------------------------------------------------------------
| Id  | Operation        | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT |        |     1 |    25 |     1   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| IDX_T1 |     1 |    25 |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------

bill=# explain analyze select c1 from t1 where c1 = 10;
                                                     QUERY PLAN
---------------------------------------------------------------------------------------------------------------------
 Index Only Scan using idx_t1 on t1  (cost=0.29..10.74 rows=523 width=4) (actual time=0.021..0.117 rows=523 loops=1)
   Index Cond: (c1 = 10)
   Heap Fetches: 0
 Planning Time: 0.076 ms
 Execution Time: 0.196 ms
(5 rows)

bill=# explain analyze select ctid,c1 from t1 where c1 = 10;
                                                   QUERY PLAN
-----------------------------------------------------------------------------------------------------------------
 Index Scan using idx_t1 on t1  (cost=0.29..81.71 rows=523 width=10) (actual time=0.038..0.447 rows=523 loops=1)
   Index Cond: (c1 = 10)
 Planning Time: 0.098 ms
 Execution Time: 0.537 ms
(4 rows)

for (i = 0; i < index->ncolumns; i++)
{
	int			attno = index->indexkeys[i];
	/*
	 * For the moment, we just ignore index expressions.  It might be nice
	 * to do something with them, later.
	 */
	if (attno == 0)
		continue;
	if (index->canreturn[i])
		index_canreturn_attrs =
			bms_add_member(index_canreturn_attrs,
						   attno - FirstLowInvalidHeapAttributeNumber);
	else
		index_cannotreturn_attrs =
			bms_add_member(index_cannotreturn_attrs,
						   attno - FirstLowInvalidHeapAttributeNumber);
}
index_canreturn_attrs = bms_del_members(index_canreturn_attrs,
										index_cannotreturn_attrs);
/* Do we have all the necessary attributes? */
result = bms_is_subset(attrs_used, index_canreturn_attrs);