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我是Oracle 11g的新手,我有一個關於執行計劃的問題。我運行了一個查詢,即使查詢中沒有ORDER BY子句,執行計劃的統計信息仍然顯示內存中有6種排序。任何人都可以向我解釋爲什麼發生這種情況?以下是查詢和統計:關於排序的優化器統計信息
SQL> SELECT ZIP FROM NOZIPSORT WHERE ZIP BETWEEN '10000' AND '29999' AND rownum < 26;
25 rows selected.
Elapsed: 00:00:00.07
Execution Plan
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Plan hash value: 4176934817
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 25 | 150 | 2 (0)| 00:00:01 |
|* 1 | COUNT STOPKEY | | | | | |
|* 2 | TABLE ACCESS FULL| NOZIPSORT | 28 | 168 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(ROWNUM<26)
2 - filter("ZIP"<='29999' AND "ZIP">='10000')
Statistics
----------------------------------------------------------
311 recursive calls
0 db block gets
47 consistent gets
38 physical reads
520 redo size
805 bytes sent via SQL*Net to client
431 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
25 rows processed
請注意,「統計」與「執行計劃」本身無關 - 它們是執行查詢本身的實際統計數據。再次運行它,即使執行計劃保持不變,每次都會看到不同的統計信息。 – 2011-05-12 07:50:46