11G新特性，explain plan 可以評估出索引大小

plan for
  2  create index ttt on aaaa(id);

Explained.

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 2861610204

-------------------------------------------------------------------------------
| Id  | Operation              | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | CREATE INDEX STATEMENT |      |   409 |  5317 |     4   (0)| 00:00:01 |
|   1 |  INDEX BUILD NON UNIQUE| TTT  |       |       |            |          |
|   2 |   SORT CREATE INDEX    |      |   409 |  5317 |            |          |
|   3 |    TABLE ACCESS FULL   | AAAA |   409 |  5317 |     3   (0)| 00:00:01 |
-------------------------------------------------------------------------------

Note
-----
   - estimated index size: 65536  bytes

14 rows selected.