同样是获取100条数据,但 Sending data的消耗让人费解,望高手解答

表中有200W条数据, 索引情况        Table: tblAlarm
  Non_unique: 1
    Key_name: tblAlarm_OccurTime
Seq_in_index: 1
 Column_name: OccurTime
   Collation: A
 Cardinality: 435583
    Sub_part: NULL
      Packed: NULL
        Null: YES
  Index_type: BTREE
     Comment: select * from tblAlarm  order by OccurTime desc limit 1, 100; 
+--------------------------------+----------+
| Status                         | Duration |
+--------------------------------+----------+
| starting                       | 0.000023 | 
| checking query cache for query | 0.000060 | 
| Opening tables                 | 0.000201 | 
| System lock                    | 0.000004 | 
| Table lock                     | 0.000013 | 
| init                           | 0.000021 | 
| optimizing                     | 0.000004 | 
| statistics                     | 0.000009 | 
| preparing                      | 0.000009 | 
| executing                      | 0.000003 | 
| Sorting result                 | 0.000006 | 
| Sending data                   | 0.001417 | 
| end                            | 0.000005 | 
| query end                      | 0.000003 | 
| freeing items                  | 0.000027 | 
| storing result in query cache  | 0.000005 | 
| logging slow query             | 0.000002 | 
| cleaning up                    | 0.000004 | 
+--------------------------------+----------+select * from tblAlarm  order by OccurTime desc limit 2000000, 100;
+--------------------------------+-----------+
| Status                         | Duration  |
+--------------------------------+-----------+
| starting                       |  0.000019 | 
| checking query cache for query |  0.000049 | 
| Opening tables                 |  0.000012 | 
| System lock                    |  0.000006 | 
| Table lock                     |  0.000018 | 
| init                           |  0.000022 | 
| optimizing                     |  0.000005 | 
| statistics                     |  0.000010 | 
| preparing                      |  0.000008 | 
| executing                      |  0.000003 | 
| Sorting result                 |  3.172254 | 
| Sending data                   | 18.321512 | 
| end                            |  0.000019 | 
| query end                      |  0.000005 | 
| freeing items                  |  0.000074 | 
| storing result in query cache  |  0.000009 | 
| logging slow query             |  0.000003 | 
| logging slow query             |  0.000002 | 
| cleaning up                    |  0.000005 | 表引擎为 MyIsam  数据库版本  5.1.37有人知道这是为什么么,或者,,有什么好办法可以减少Sending data这部分的开销?