[20210224]fetch r=0算邏輯讀嗎.txt
--//我一直以為fetch r=0時依舊算1次邏輯讀.測驗發現我理解錯了.通過測驗說明問題.
1.環境:
SCOTT@book> @ ver1
PORT_STRING VERSION BANNER
------------------- -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0 Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
SCOTT@book> create table empx as select * from emp;
Table created.
--//分析略.該表共14條記錄.
2.測驗:
SCOTT@book> set arraysize 13
SCOTT@book> @ seg empx
SCOTT@book> @ prxx
==============================
SEG_MB : 0
SEG_OWNER : SCOTT
SEG_SEGMENT_NAME : EMPX
SEG_PARTITION_NAME :
SEG_SEGMENT_TYPE : TABLE
SEG_TABLESPACE_NAME : USERS
BLOCKS : 8
HDRFIL : 4
HDRBLK : 554
PL/SQL procedure successfully completed.
SCOTT@book> alter session set statistics_level=all;
Session altered.
SCOTT@book> @ 10046on 12
Session altered.
SCOTT@book> select * from empx;
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
7369 SMITH CLERK 7902 1980-12-17 00:00:00 800 20
7499 ALLEN SALESMAN 7698 1981-02-20 00:00:00 1600 300 30
7521 WARD SALESMAN 7698 1981-02-22 00:00:00 1250 500 30
7566 JONES MANAGER 7839 1981-04-02 00:00:00 2975 20
7654 MARTIN SALESMAN 7698 1981-09-28 00:00:00 1250 1400 30
7698 BLAKE MANAGER 7839 1981-05-01 00:00:00 2850 30
7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10
7788 SCOTT ANALYST 7566 1987-04-19 00:00:00 3000 20
7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10
7844 TURNER SALESMAN 7698 1981-09-08 00:00:00 1500 0 30
7876 ADAMS CLERK 7788 1987-05-23 00:00:00 1100 20
7900 JAMES CLERK 7698 1981-12-03 00:00:00 950 30
7902 FORD ANALYST 7566 1981-12-03 00:00:00 3000 20
7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300 10
14 rows selected.
SCOTT@book> @ 10046off
Session altered.
SCOTT@book> select * from empx;
Plan hash value: 722738080
--------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 14 |00:00:00.01 | 3 |
| 1 | TABLE ACCESS FULL| EMPX | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 3 |
--------------------------------------------------------------------------------------------------------------------
--//看一下轉儲檔案:
$ grep FETCH /u01/app/oracle/diag/rdbms/book/book/trace/book_ora_14841.trc
FETCH #140627174044680:c=0,e=72,p=0,cr=2,cu=0,mis=0,r=1,dep=0,og=1,plh=722738080,tim=1614150824077788
FETCH #140627174044680:c=0,e=77,p=0,cr=1,cu=0,mis=0,r=13,dep=0,og=1,plh=722738080,tim=1614150824078538
FETCH #140627174044680:c=0,e=11,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=722738080,tim=1614150824079938
--//第1次 r=1,第2次r=13,第3次 r=0.
--//如果算第3次的fetch,執行計劃的邏輯讀應該是4,而現在是3.
--//可以使用我建立的gdb腳本確定,腳本參考鏈接:http://blog.itpub.net/267265/viewspace-2757990/=>[20210220]gdb跟蹤邏輯讀2.txt
Breakpoint 1 at 0x994df72
Breakpoint 2 at 0xfcafda
Breakpoint 3 at 0x947da78
Breakpoint 4 at 0xfc97f0
Breakpoint 5 at 0x95ed0ca
Breakpoint 6 at 0x94471a4
(gdb) c
Continuing.
2021/02/24 15:16:42.990936245 :kteinpscan 0x100022a
2021/02/24 15:16:42.996402852 :kteinmap 0x100022a
2021/02/24 15:16:43.016412801 :kdst_fetch 0x100022b
2021/02/24 15:16:43.035717000 :kdst_fetch 0x100022b
--//kteinmap 不算1次邏輯讀.這樣邏輯讀3次.
3.再換一種測驗,使用12c的客戶端:
SCOTT@78> show sqlpluscompatibility
sqlpluscompatibility 12.2.0
SCOTT@78> set rowprefetch 14
--//這樣1次fetch全部記錄,這樣邏輯讀可以減少1個.
SCOTT@78> alter session set statistics_level=all;
Session altered.
SCOTT@78> select * from empx;
SCOTT@78> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID 4ag7sc82kdhh4, child number 0
-------------------------------------
select * from empx
Plan hash value: 722738080
--------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time | A-Rows | A-Time | Buffers |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | | 3 (100)| | 14 |00:00:00.01 | 2 |
| 1 | TABLE ACCESS FULL| EMPX | 1 | 14 | 532 | 3 (0)| 00:00:01 | 14 |00:00:00.01 | 2 |
--------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / EMPX@SEL$1
--//邏輯讀為2.
--//gdb跟蹤顯示:
(gdb) c
Continuing.
2021/02/24 15:21:32.389548656 :kteinpscan 0x100022a
2021/02/24 15:21:32.395058076 :kteinmap 0x100022a
2021/02/24 15:21:32.406183568 :kdst_fetch 0x100022b
4.總結:
--//這些都是一些細節問題,可能在實際的作業中不不是太重要.
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