Oracle排序汇总重置成本

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--Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
CREATE TABLE t(ID INTEGER PRIMARY KEY,flag VARCHAR2(4000));
INSERT /*+append*/ INTO t
SELECT LEVEL,LPAD('a',4000,' ') FROM dual CONNECT BY LEVEL <= 500000;
COMMIT;
CALL dbms_stats.gather_table_stats(USER,'T');
SELECT /*+gather_plan_statistics*/ *
FROM t
WHERE id = (SELECT MIN(id)
FROM t
WHERE flag LIKE 'a%');
SELECT PLAN_TABLE_OUTPUT FROM table (DBMS_XPLAN.DISPLAY_CURSOR(format => 'ALL RUNSTATS_LAST'));
SQL_ID 5jbcgzgkhfkay, child number 0
-------------------------------------
SELECT /*+gather_plan_statistics*/ * FROM t WHERE id = (SELECT
MIN(id) FROM t WHERE flag LIKE 'a%')
Plan hash value: 2119356367
------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers | Reads |
------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 0 |00:00:09.95 | 500K| 500K|
| 1 | TABLE ACCESS BY INDEX ROWID| T | 1 | 1 | 4006 | 3 (0)| 00:00:01 | 0 |00:00:09.95 | 500K| 500K|
|* 2 | INDEX UNIQUE SCAN | SYS_C00142636 | 1 | 1 | | 2 (0)| 00:00:01 | 0 |00:00:09.95 | 500K| 500K|
| 3 | SORT AGGREGATE | | 1 | 1 | 4006 | | | 1 |00:00:09.95 | 500K| 500K|
|* 4 | TABLE ACCESS FULL | T | 1 | 500K| 1910M| 109K (1)| 00:21:59 | 0 |00:00:09.95 | 500K| 500K|
------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / T@SEL$1
2 - SEL$1 / T@SEL$1
3 - SEL$2
4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("ID"=)
4 - filter("FLAG" LIKE 'a%')
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - "ID"[NUMBER,22], "T"."FLAG"[VARCHAR2,4000]
2 - "T".ROWID[ROWID,10], "ID"[NUMBER,22]
3 - (#keys=0) MIN("ID")[22]
4 - "ID"[NUMBER,22]

是否有任何合理的解释，为什么如此昂贵的计划的总成本为3，而它包含的完整扫描成本为109K，估计的00:21:59时间？