10046确认多块读无法跨区

SQL查询记录，记录是如何从磁盘中读取的呢？ 有几种访问方式？

本次测试探讨数据文件离散读无法跨区，因此单块读（顺序读）存在特殊场景,一个表的一个块在某个区中，Oracle查询记录及时全表扫描，也会产生单块读现象。

文档结构如下:

1.解释说明oracle数据文件从磁盘读取有几种方式

2.创建测试对象，并进行多块读测试解释说明

3.创建测试环境，一个表已分配多个区，存在一个区只使用一个块，次数对测试表全表扫描，验证是否存在单块读现象。

一、解释说明oracle数据文件从磁盘读取有几种方式

    EVENT#   EVENT_ID NAME                                PARAMETER1    PARAMETER2 PARAMETER3 WAIT_CLASS
---------- ---------- ----------------------------------- ------------- ---------- ---------- ----------
         9 2171045634 remote db file read                 clientid      count      intr       Network
       146 2652584166 db file sequential read             file#         block#     blocks     User I/O
       147  506183215 db file scattered read              file#         block#     blocks     User I/O
       151  834992820 db file parallel read               files         blocks     requests   User I/O
       197 3926164927 direct path read                    file number   first dba  block cnt  User I/O
       198  861319509 direct path read temp               file number   first dba  block cnt  User I/O
       269 3999678875 securefile direct-read completion                                       User I/O

7 rows selected.

三种：方式依次为 多块读，单块读，直接路径读

db file scattered read

db file sequential read

direct path read   

多块读：一次IO读取多个块，读取无法跨区，常见与全表扫描

单块读：一次IO读取单个块，常见索引rowid回表提取记录，非常见全表扫描有个块跨区落单了

直接路径读：读取的数据块，不放入buffer cache中，直接提取到会话PGA中，常见与LOB字段读取

　　

二、多块读测试

1）创建测试数据
SQL> create table scott.e(id char(2000),name char(2000),
deptno char(2000),salary char(2000));

Table created.

 2）插入测试数据

declare
v_id int;
begin
for v_id in 1 .. 7
loop
insert into scott.e values(v_id,v_id||'name',
v_id||'deptno',v_id||'salary');
end loop;
commit;
end;
/

3）查询数据存储记录

SQL> select tablespace_name,file_id,extent_id,block_id,
    bytes/1024 bytes_k,blocks from dba_extents
    where owner='SCOTT' and segment_name='E'

TABLESPACE_NAM    FILE_ID  EXTENT_ID   BLOCK_ID    BYTES_K     BLOCKS
-------------- ---------- ---------- ---------- ---------- ----------
USERS                   4          0       1768         64          8
USERS                   4          1       1776         64          8
USERS                   4          2       1784         64          8

4）查询rowid
SQL> select rowid,substr(id,1,2) from scott.e;

ROWID              SUBSTR(ID,1,2)
------------------ ----------------
AAAS5RAAEAAAAbsAAA 2
AAAS5RAAEAAAAbvAAA 1
AAAS5RAAEAAAAbwAAA 3
AAAS5RAAEAAAAbxAAA 4
AAAS5RAAEAAAAb0AAA 6
AAAS5RAAEAAAAb2AAA 5
AAAS5RAAEAAAAb/AAA 7
7 rows selected.
rowid解释说明，从左到有，可以发现表7行数据占用7个块

数据对象编号（6位显示）
相关数据文件编号（3位显示）
数据块编号（6位显示）
数据块中行编号（3位显示）

　　

5）根据rowid查询块编号
select rowid,
 dbms_rowid.rowid_object(rowid) "objct",
 dbms_rowid.rowid_relative_fno(rowid) "file",
 dbms_rowid.rowid_block_number(rowid) "block",
 dbms_rowid.rowid_row_number(rowid) "row"
from scott.e;
ROWID                  objct       file      block        row
------------------ --------- ---------- ---------- ----------
AAAS5RAAEAAAAbsAAA     77393          4       1772          0
AAAS5RAAEAAAAbvAAA     77393          4       1775          0
AAAS5RAAEAAAAbwAAA     77393          4       1776          0
AAAS5RAAEAAAAbxAAA     77393          4       1777          0
AAAS5RAAEAAAAb0AAA     77393          4       1780          0
AAAS5RAAEAAAAb2AAA     77393          4       1782          0
AAAS5RAAEAAAAb/AAA     77393          4       1791          0
7 rows selected.

　　6）数据文件离散读测试

SQL> alter system flush buffer_cache;

SQL> alter session set events '10046 trace name context forever,level 12';

Session altered.

SQL> select count(*) from scott.e;

COUNT(*)
----------
7

SQL> alter session set events '10046 trace name context off';

Session altered.

jx1:/u01/app/oracle/diag/rdbms/jx/jx1/trace$ cat jx1_ora_30782.trc |grep scatt
WAIT #140130911224448: nam='db file scattered read' ela= 713 file#=4 block#=1771 blocks=5 obj#=77393 tim=1545603274223872
WAIT #140130911224448: nam='db file scattered read' ela= 735 file#=4 block#=1776 blocks=8 obj#=77393 tim=1545603274224854
WAIT #140130911224448: nam='db file scattered read' ela= 1281 file#=4 block#=1785 blocks=7 obj#=77393 tim=1545603274226337 

可以发现执行一次全表扫描，数据文件多块读，读取了三次，每次返回不同的块数量

　　

三、创建测试环境，一个表已分配多个区，存在一个区只使用一个块，次数对测试表全表扫描，验证是否存在单块读现象

1）环境构建，使用笨拙的办法，不断insert,查询extent分配

SQL> insert into scott.e values(8,8,8,8);

SQL> insert into scott.e values(9,9,9,9);

SQL> insert into scott.e values(10,10,10,10);

SQL> insert into scott.e values(11,11,11,11);

SQL> insert into scott.e values(12,12,12,12);

SQL> r
1 select tablespace_name,file_id,extent_id,block_id,
2 bytes/1024 bytes_k,blocks from dba_extents
3 where owner='SCOTT' and segment_name='E'
4*

TABLESPACE_NAM FILE_ID EXTENT_ID BLOCK_ID BYTES_K BLOCKS
-------------- ---------- ---------- ---------- ---------- ----------
USERS 4 0 1768 64 8
USERS 4 1 1776 64 8
USERS 4 2 1784 64 8
USERS 4 3 3200 64 8  --终于新扩建一个区

SQL> select rowid from scott.e where id like '%12%';

ROWID
------------------
AAAS5RAAEAAAAyGAAA

select rowid,
 dbms_rowid.rowid_object(rowid) "objct",
 dbms_rowid.rowid_relative_fno(rowid) "file",
 dbms_rowid.rowid_block_number(rowid) "block",
 dbms_rowid.rowid_row_number(rowid) "row"
from scott.e;

ROWID                   objct       file      block        row
------------------ ---------- ---------- ---------- ----------
AAAS5RAAEAAAAbsAAA      77393          4       1772          0
AAAS5RAAEAAAAbvAAA      77393          4       1775          0
AAAS5RAAEAAAAbwAAA      77393          4       1776          0
AAAS5RAAEAAAAbxAAA      77393          4       1777          0
AAAS5RAAEAAAAb0AAA      77393          4       1780          0
AAAS5RAAEAAAAb2AAA      77393          4       1782          0
AAAS5RAAEAAAAb7AAA      77393          4       1787          0
AAAS5RAAEAAAAb9AAA      77393          4       1789          0
AAAS5RAAEAAAAb/AAA      77393          4       1791          0
AAAS5RAAEAAAAb/AAB      77393          4       1791          1
AAAS5RAAEAAAAb/AAC      77393          4       1791          2
AAAS5RAAEAAAAyGAAA      77393          4       3206          0     ---

12 rows selected.

　　

2)10046跟踪

SQL> alter system flush buffer_cache;

SQL> alter session set events '10046 trace name context forever,level 12';

Session altered.

SQL> select count(*) from scott.e;

COUNT(*)
----------
12

SQL> alter session set events '10046 trace name context off';

Session altered.

jx1:/u01/app/oracle/diag/rdbms/jx/jx1/trace$ cat jx1_ora_55917.trc|grep read
Redo thread mounted by this instance: 1
WAIT #139784104305960: nam='db file sequential read' ela= 15423 file#=4 block#=1770 blocks=1 obj#=77393 tim=1545605262033100 （OK）
WAIT #139784104305960: nam='db file scattered read' ela= 650 file#=4 block#=1771 blocks=5 obj#=77393 tim=1545605262034068
WAIT #139784104305960: nam='db file scattered read' ela= 708 file#=4 block#=1776 blocks=8 obj#=77393 tim=1545605262035050
WAIT #139784104305960: nam='db file scattered read' ela= 678 file#=4 block#=1785 blocks=7 obj#=77393 tim=1545605262036010
WAIT #139784104305960: nam='db file scattered read' ela= 12389 file#=4 block#=3200 blocks=8 obj#=77393 tim=1545605262048744

原谅我容易满足，得到单块读现象后，我现在停止了脚步，如有兴趣的朋友，可以继续研究，这些块都是什么对象，为什么这读取，是否存在什么规则。