我们都知道,数据块是oracle最基本的读写单位,但用户所需要的数据,并不是整个块,而是块中的行,或列.当用户发出SQL语句时,此语句被解析执行完毕,就开始了数据的抓取阶段,在此阶段,服务器进程会先将行所在的数据块从数据文件中读入buffer cache,这个过程叫做物理读.物理读,每读取一个块,就算一次物理读.当块被送进buffer cache后,并不能立即将块传给用户,因为用户所需要的并不整个块,而是块中的行.从buffer cache的块中读取行的过程,就是逻辑读.为了完成一次逻辑读,服务器进程先要在hash表中查找块所在的cache buffer 链.找到之后,需要在这个链上加一个cache buffer chains 闩,加闩成功之后,就在这个链中寻找指定的块,并在块上加一个pin锁.并释放cache buffer chains闩.然后就可以访问块中的行了.服务器进程不会将块中所有满足条件的行一次取出,而是根据你的抓取命令,每次取一定数量的行.这些行取出之后,会经由PGA传给客户端用户.行一旦从buffer cache中取出,会话要释放掉在块上所加的PIN.本次逻辑读就算结束.如果还要再抓取块中剩余的行,服务器进程要再次申请获得cache bufffer链闩.再次在块上加PIN.这就算是另外一次逻辑读咯.也就是说,服务器进程每申请一次cache buffer链闩,就是一次逻辑读.而每次逻辑读所读取的行的数量,可以在抓取命令中进行设置.逻辑读和Cache buffer chains闩关系密切,TOM曾有文章提到,进程每申请一次Cache buffer chains闩,就是一次逻辑读。但是,逻辑读并不等同于Cache buffer chains闩,每次逻辑读,在9i中至少需要获得两Cache buffer chains闩。逻辑读是指在Hash表中定位块的这个过程。
--创建测试表
SQL> create table t (id int,name varchar2(10));
Table created.
--插入100行数据
SQL> begin
2 for i in 1..100 loop
3 insert into t values (i,'aaa');
4 end loop;
5 commit;
6 end;
7 /
PL/SQL procedure successfully completed.
--查看表中行分布
SQL> select bno,max(id),min(id) from (select dbms_rowid.rowid_block_number(rowid) bno, id from t) group by bno;
BNO MAX(ID) MIN(ID)
---------- ---------- ----------
91545 100 1
可以看到这个表的所有数据全部存放在91545块上
--查询一行观察执行计划统计信息
SQL> set autotrace trace stat;
SQL> select * from t where id <= 1;
Statistics
----------------------------------------------------------
28 recursive calls
0 db block gets
8 consistent gets --逻辑读为10
0 physical reads
0 redo size
590 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--在执行一次
SQL> select * from t where id <= 1;
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets --逻辑读减小到4
0 physical reads
0 redo size
590 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--把id换成1到15的值,逻辑读和上面结果一模一样,下面给出了id=12的比较
SQL> select * from t where id <= 12;
12 rows selected.
Statistics
----------------------------------------------------------
5 recursive calls
0 db block gets
8 consistent gets
0 physical reads
0 redo size
728 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
12 rows processed
SQL> /
12 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
728 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
12 rows processed
--当把id换成16时逻辑读多了1 bytes sent via SQL*Net to client也多了200多
SQL> select * from t where id <=16;
16 rows selected.
Statistics
----------------------------------------------------------
5 recursive calls
0 db block gets
9 consistent gets
0 physical reads
0 redo size
896 bytes sent via SQL*Net to client
535 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
16 rows processed
SQL> /
16 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
5 consistent gets
0 physical reads
0 redo size
896 bytes sent via SQL*Net to client
535 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
16 rows processed
从上面的实验看出,在读取数据的时候,逻辑读的次数与读取的数据量有很大的关系,oracle是通过批读取数据行的,批越大逻辑读就越少,而且服务端和客户端交互的次数也越少,由网络传输的数据也可以减少,下面继续实验
--批默认值
SQL> show arraysize;
arraysize 15
--设置批大小为1
SQL> select * from t;
100 rows selected.
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
57 consistent gets
0 physical reads
0 redo size
10946 bytes sent via SQL*Net to client
1063 bytes received via SQL*Net from client
51 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
100 rows processed
--设置批大小为100
SQL> set arraysize 100
SQL> select * from t;
100 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
1832 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
100 rows processed
可以看出逻辑的相差了10多倍,而bytes sent via SQL*Net to client也相差了6倍左右,继续下面的实验
--插入数据
SQL> begin
2 for i in 501..2000 loop
3 insert into t values (i,'aaa');
4 end loop;
5 commit;
6 end;
7 /
PL/SQL procedure successfully completed.
--查看表中行分布
SQL> select bno,max(id),min(id),count(*) from (select dbms_rowid.rowid_block_number(rowid) bno, id from t) group by bno;
BNO MAX(ID) MIN(ID) COUNT(*)
---------- ---------- ---------- ----------
91545 567 1 567
91548 2000 1686 315
91546 1126 568 559
91547 1685 1127 559
这时候数据存入了4个快中,默认arraysize为15,那么从91545这个块查询的次数就是567/15=37.8 我们估算约38次,验证下
SQL> select * from t where rownum < 568 --读取91548块的所有行
567 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
41 consistent gets --和我们估算相差很接近
0 physical reads
0 redo size
14781 bytes sent via SQL*Net to client
931 bytes received via SQL*Net from client
39 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
567 rows processed
比我们估算的多了3个逻辑读,如果将批大小改为517,让oracle一次性读出这个块上的所有的行,根据上面的结果推算应该有4个逻辑读,是否是这样呢?
SQL> set arraysize 567
SQL> /
567 rows selected.
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
4 consistent gets
0 physical reads
0 redo size
7899 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
567 rows processed
实验看出刚好为4个,那么这3个逻辑读究竟用在什么地方了呢?继续下面实验
--重建表别且插入数据
SQL> create table t1 (id int, name varchar2(10));
Table created.
SQL> insert into t1 values (1,'aaa');
1 row created.
SQL> commit;
Commit complete.
--查看行分布
SQL> select bno,max(id),min(id),count(*) from (select dbms_rowid.rowid_block_number(rowid) bno, id from t1) group by bno;
BNO MAX(ID) MIN(ID) COUNT(*)
---------- ---------- ---------- ----------
91697 1 1 1
--查看区分布
SQL> select extent_id,file_id,block_id,blocks from dba_extents where segment_name='T1' and owner='SYS';
EXTENT_ID FILE_ID BLOCK_ID BLOCKS
---------- ---------- ---------- ----------
0 1 91696 8
--查看高水位点
SQL> select header_file,header_block from dba_segments where segment_name='T1' and owner='SYS';
HEADER_FILE HEADER_BLOCK
----------- ------------
1 91696
SQL> alter system dump datafile 1 block 91696
在dump文件中找到Highwater:: 0x00416632行,查出高水位点
SQL> select dbms_utility.data_block_address_block(to_number('00416632','xxxxxxxx')) from dual;
DBMS_UTILITY.DATA_BLOCK_ADDRESS_BLOCK(TO_NUMBER('00416632','XXXXXXXX'))
-----------------------------------------------------------------------
91698
得出91696为数据段头部分,91697 91698为数据块
--执行查询
SQL> select * from t1 where rownum=1;
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
594 bytes sent via SQL*Net to client
524 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
逻辑读是3,难道这里刚好读到了这3个块,我只是推测,希望哪位大师帮我解说下,最好能够通过实验证明下!!!!!!!!!!
将批大小设置的越高,速度就越快,否则,Oracle直接将它设置为一个最大的值,我们知道一个session连接到实例会为这个session分配PGA,当行从Buffercache中读出来后,会先缓存在PGA中(具体是在游标的运行时区),然后再传给客户端。如
果批大小过大,在PGA、客户端占用的内存也会增大。而且,如果渐歇性的在网络上传输大量数据,对网
络也会有一定影响。下面来观察一下批大小对PGA的影响:
--创建一个100多万数据的测试表
SQL> create table t2 as select * from dba_objects;
Table created.
SQL> insert into t2 select * from t2;
73884 rows created.
SQL> /
147768 rows created.
SQL> /
295536 rows created.
SQL> /
591072 rows created.
SQL> /
1182144 rows created.
SQL> commit;
Commit complete.
--退出会话重新登录下,查询下sid
SQL> select distinct sid from v$mystat;
SID
----------
140
--在另外一个会话查询sid=140分配的PGA
SQL> SELECT
a.pga_used_mem "PGA Used"
, a.pga_alloc_mem "PGA Alloc"
, a.pga_max_mem "PGA Max"
FROM
v$process a
, v$session b
WHERE
a.addr = b.paddr
AND b.sid= 140
PGA Used PGA Alloc PGA Max
---------- ---------- ----------
711016 1376576 1376576
--返回sid=140的会话执行下面过程
declare
type test is table of t2.object_id%type;
v_test test;
cursor c is select object_id from t2;
begin
open c;
loop
fetch c bulk collect into v_test limit 500; --通过游标将批大小限制到500
exit when c%notfound;
end loop;
dbms_output.put_line(c%rowcount);
close c;
end;
/
我的这个表数据有点大,执行时间比较长,在执行完成后重新查看PGA大小还是一样
--将批大小改为5000
declare
type test is table of t2.object_id%type;
v_test test;
cursor c is select object_id from t2;
begin
open c;
loop
fetch c bulk collect into v_test limit 5000;
exit when c%notfound;
end loop;
dbms_output.put_line(c%rowcount);
close c;
end;
/
第一个执行我花了很久很久。。。这一个大概2秒钟,执行速度非常快。
查看PGA使用如下。可以看到扩大了一倍多。
PGA Used PGA Alloc PGA Max
---------- ---------- ----------
1038872 2031936 2949440
上面的实验很好的证明了设置arrraysize可以降低逻辑读,使用它还需要根据实际情况进行设置。
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