Question about the 'A-Time' in dbms_xplan.display_cursor's output

Hi,
I use /*+ gather_plan_statistics */ and dbms_xplan.display_cursor(null,
null, 'ALLSTATS LAST')) to show a query's plan statistics.

There's a A-Time column in the output. Is it the actual time taken in an
operation? Does it equal to last_elapsed_time column in
v$sql_plan_statistics?

I believe so, but I observed that for some long queries, A-Times are little.
It doesn't match the real wall clock time. For example,

SQL> set timing on

SQL> select /*+ gather_plan_statistics */ * from large_table where rownum <
10000;
....

9999 rows selected.

Elapsed: *00:01:11.59*

SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS
LAST'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------

SQL_ID  5gpp067wgrtsp, child number 0
-------------------------------------
select /*+ gather_statistics */ * from large_table where rownum < :"SYS_B_0"

Plan hash value: 9990311
-----------------------------------------------------------------------------------------------------

| Id  | Operation                    | Name                 | Starts |
E-Rows | A-Rows |   A-Time     | Buffers | Reads  |

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

|*  1 |  COUNT STOPKEY     |                           |      1 |
    |   9999   |*00:00:00.21*  |     791 |    127 |

|   2 |   TABLE ACCESS FULL| LARGE_TABLE |      1 |  11170     |   9999   |*
00:00:00.19*  |     791 |    127 |


Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<:SYS_B_0)

Note
-----
   - dynamic sampling used for this statement


22 rows selected.

Elapsed: 00:00:00.46


I am wondering why the A-Time does not match the wall clock time? Is there
any way to get the actual time taken in each step of an execution plan?


The database is 10.2.0.4.

Thanks,
Eric Geng

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