Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
28 consistent gets
0 physical reads
0 redo size
479 bytes sent via SQL*Net to client
513 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
上面的查询可以通过索引000来得到，并且where后面没有用到索引列，而且返回的行数很少（。）所以cbo选择index skip scan
select owner, object_name,object_type from test where object_type='INDEX';
Execution Plan
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Plan hash value: 3464522019
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| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1971 | 78840 | 168 (1)| 00:00:03 |
|* 1 | INDEX FAST FULL SCAN| OOO | 1971 | 78840 | 168 (1)| 00:00:03 |
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Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_TYPE"='INDEX')
Statistics
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0 recursive calls
0 db block gets
957 consistent gets
0 physical reads
0 redo size
199834 bytes sent via SQL*Net to client
4253 bytes received via SQL*Net from client
341 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
5088 rows processed
同上，但是这里返回行数较多，cbo选择了index fast full scan，避免了全表扫描