Oracle中"TABLE ACCESS FULL"的”欺骗时刻“

Oracle中"TABLE ACCESS FULL"的”欺骗时刻“

基础表信息

缓存前提下，全表扫描表zkmbak需要1.69s，约229000个逻辑读。

14:53:18 ZKM@dev-app73/pdb(400)> select count(*) from zkmbak;

  COUNT(*)
----------
 150994944

Elapsed: 00:00:01.69
14:53:29 ZKM@dev-app73/pdb(400)> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------
SQL_ID  f7mhuvxws84fv, child number 0
-------------------------------------
select count(*) from zkmbak

Plan hash value: 2160384922

---------------------------------------------------------------------------------------
| Id  | Operation          | Name   | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
---------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |        |      1 |        |      1 |00:00:01.68 |     229K|
|   1 |  SORT AGGREGATE    |        |      1 |      1 |      1 |00:00:01.68 |     229K|
|   2 |   TABLE ACCESS FULL| ZKMBAK |      1 |    150M|    150M|00:00:01.65 |     229K|
---------------------------------------------------------------------------------------


14 rows selected.

Elapsed: 00:00:00.06

案例1

14:53:32 ZKM@dev-app73/pdb(400)> select count(*) from zkmbak where rownum<=10;

  COUNT(*)
----------
        10

Elapsed: 00:00:00.00
14:56:30 ZKM@dev-app73/pdb(400)> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------------------------------
SQL_ID  22uwfszq8a9v7, child number 0
-------------------------------------
select count(*) from zkmbak where rownum<=10

Plan hash value: 3390400203

----------------------------------------------------------------------------------------
| Id  | Operation           | Name   | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |        |      1 |        |      1 |00:00:00.01 |       3 |
|   1 |  SORT AGGREGATE     |        |      1 |      1 |      1 |00:00:00.01 |       3 |
|*  2 |   COUNT STOPKEY     |        |      1 |        |     10 |00:00:00.01 |       3 |
|   3 |    TABLE ACCESS FULL| ZKMBAK |      1 |     10 |     10 |00:00:00.01 |       3 |
----------------------------------------------------------------------------------------

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

   2 - filter(ROWNUM<=10)


20 rows selected.

Elapsed: 00:00:00.06

加了rownum<=10后，虽然有"TABLE ACCESS FULL"但是实际上扫描出符合条件的前10行数据后Oracle就停止扫描了。

因此并不会产生229000个逻辑读。

案例2

15:02:44 ZKM@dev-app73/pdb(400)> select count(*) from zkm where id in (select id from zkmbak);

  COUNT(*)
----------
      1000

Elapsed: 00:00:00.00
15:02:44 ZKM@dev-app73/pdb(400)> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------------------------------------------------
SQL_ID  7ypyj251q62s2, child number 0
-------------------------------------
select count(*) from zkm where id in (select id from zkmbak)

Plan hash value: 783349724

-------------------------------------------------------------------------------------------------------------------
| Id  | Operation           | Name   | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |        |      1 |        |      1 |00:00:00.01 |       6 |       |       |          |
|   1 |  SORT AGGREGATE     |        |      1 |      1 |      1 |00:00:00.01 |       6 |       |       |          |
|*  2 |   HASH JOIN SEMI    |        |      1 |   1000 |   1000 |00:00:00.01 |       6 |  2546K|  2546K| 1416K (0)|
|   3 |    TABLE ACCESS FULL| ZKM    |      1 |   1000 |   1000 |00:00:00.01 |       3 |       |       |          |
|   4 |    TABLE ACCESS FULL| ZKMBAK |      1 |    150M|    256 |00:00:00.01 |       3 |       |       |          |
-------------------------------------------------------------------------------------------------------------------

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

   2 - access("ID"="ID")


21 rows selected.

Elapsed: 00:00:00.06

其中，id=4这一步实际消耗逻辑读3个，实际取的行数为256（这个就不懂CBO是怎么判定的了）。

所以也没有进行实际上执行计划显示的全表扫描。