通过案例学调优之--动态视图（v$bh和x$bh)
v$bh与x$bh
v$bh:非常详细地记录了数据块在数据缓冲区内的使用情况，一条记录对应一个block的详细记录。
v$bh来自于基表x$bh与x$le
查看v$bh结构:
13:07:20 SYS@ test1 >desc v$bh
Name Null? Type
----------------------------------------------------------------- -------- --------------------------------------------
FILE# NUMBER
BLOCK# NUMBER
CLASS# NUMBER
STATUS VARCHAR2(10)
XNC NUMBER
FORCED_READS NUMBER
FORCED_WRITES NUMBER
LOCK_ELEMENT_ADDR RAW(4)
LOCK_ELEMENT_NAME NUMBER
LOCK_ELEMENT_CLASS NUMBER
DIRTY VARCHAR2(1)
TEMP VARCHAR2(1)
PING VARCHAR2(1)
STALE VARCHAR2(1)
DIRECT VARCHAR2(1)
NEW CHAR(1)
OBJD NUMBER
TS# NUMBER
LOBID NUMBER
CACHEHINT NUMBERSTATUS
free - not currently in use
xcur - exclusive current,表示该数据块处于排外模式,正在被当前Instance占用；
scur - shared current,在RAC环境中表示该数据库正在和其他实例共享数据。
cr - consistent read,一致性读。
read - being read from disk
mrec - in media recovery mode,表示数据块处于介质恢复模式；
irec - in instance recovery mode ,表示数据块处于实例恢复模式；
write - 表示数据库正在往磁盘写入数据；
13:07:19 SYS@ test1 >select * from v$fixed_view_definition t where t.view_name='GV$BH'
VIEW_NAME VIEW_DEFINITION
------------------------------ --------------------------------------------------
GV$BH select bh.inst_id, file#, dbablk, class, decode(st
ate,0,'free',1,'xcur',2,'scur',3,'cr', 4,'read',5,
'mrec',6,'irec',7,'write',8,'pi', 9,'memory',10,'m
write',11,'donated', 12,'protected', 13,'securefi
le', 14,'siop',15,'recckpt', 16, 'flashfree', 17,
'flashcur', 18, 'flashna'), 0, 0, 0, bh.le_addr,
le_id1, le_id2, decode(bitand(flag,1), 0, 'N', 'Y'
), decode(bitand(flag,16), 0, 'N', 'Y'), decode(bi
tand(flag,1536), 0, 'N', 'Y'), decode(bitand(flag,
16384), 0, 'N', 'Y'), decode(bitand(flag,65536), 0
, 'N', 'Y'), 'N', obj, ts#, lobid, bitand(OBJ_FLA
G, 240)/16 from x$bh bh, x$le le where bh.le_addr
= le.le_addr (+)13:08:20 SYS@ test1 >desc x$bh
Name Null? Type
----------------------------------------------------------------- -------- --------------------------------------------
ADDR RAW(4)
INDX NUMBER
INST_ID NUMBER
HLADDR RAW(4)
BLSIZ NUMBER
NXT_HASH RAW(4)
PRV_HASH RAW(4)
NXT_REPL RAW(4)
PRV_REPL RAW(4)
FLAG NUMBER
FLAG2 NUMBER
LOBID NUMBER
RFLAG NUMBER
SFLAG NUMBER
LRU_FLAG NUMBER
TS# NUMBER
FILE# NUMBER
DBARFIL NUMBER
DBABLK NUMBER
CLASS NUMBER
STATE NUMBER
MODE_HELD NUMBER
CHANGES NUMBER
CSTATE NUMBER
LE_ADDR RAW(4)
DIRTY_QUEUE NUMBER
SET_DS RAW(4)
OBJ NUMBER
BA RAW(4)
CR_SCN_BAS NUMBER
CR_SCN_WRP NUMBER
CR_XID_USN NUMBER
CR_XID_SLT NUMBER
CR_XID_SQN NUMBER
CR_UBA_FIL NUMBER
CR_UBA_BLK NUMBER
CR_UBA_SEQ NUMBER
CR_UBA_REC NUMBER
CR_SFL NUMBER
CR_CLS_BAS NUMBER
CR_CLS_WRP NUMBER
LRBA_SEQ NUMBER
LRBA_BNO NUMBER
HSCN_BAS NUMBER
HSCN_WRP NUMBER
HSUB_SCN NUMBER
US_NXT RAW(4)
US_PRV RAW(4)
WA_NXT RAW(4)
WA_PRV RAW(4)
OQ_NXT RAW(4)
OQ_PRV RAW(4)
AQ_NXT RAW(4)
AQ_PRV RAW(4)
OBJ_FLAG NUMBER
TCH NUMBER
TIM NUMBER
CR_RFCNT NUMBER
SHR_RFCNT NUMBERx$bh中“touch count”信息对应到x$bh的tch字段，而段的data_object_id信息对应到x$bh的obj，或者是v$bh的objd。
案例分析：
（1）对象有多少个数据块缓存到Data buffer中
13:29:01 SCOTT@ test1 >truncate table t1;
13:29:09 SCOTT@ test1 >begin
13:29:12 2 for i in 1..5000 loop
13:29:25 3 insert into t1 values (i);
13:29:35 4 end loop;
13:29:38 5 end;
13:29:39 6 /
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.67
13:29:41 SCOTT@ test1 >commit;创建一存储过程：show space读取数据块信息
13:29:43 SCOTT@ test1 >create or replace procedure show_space
13:34:44 2
13:34:44 3 ( p_segname in varchar2,
13:34:44 4
13:34:44 5 p_owner in varchar2 default user,
13:34:44 6
13:34:44 7 p_type in varchar2 default 'TABLE',
13:34:44 8
13:34:44 9 p_partition in varchar2 default NULL )
13:34:44 10
13:34:44 11 as
13:34:44 12
13:34:44 13 l_total_blocks number;
13:34:44 14
13:34:44 15 l_total_bytes number;
13:34:44 16
13:34:44 17 l_unused_blocks number;
13:34:44 18
13:34:44 19 l_unused_bytes number;
13:34:44 20
13:34:44 21 l_LastUsedExtFileId number;
13:34:44 22
13:34:44 23 l_LastUsedExtBlockId number;
13:34:44 24
13:34:44 25 l_last_used_block number;
13:34:44 26
13:34:44 27 procedure p( p_label in varchar2, p_num in number )
13:34:44 28
13:34:44 29 is
13:34:44 30
13:34:44 31 begin
13:34:44 32
13:34:44 33 dbms_output.put_line( rpad(p_label,40,'.') ||
13:34:44 34
13:34:45 35 p_num );
13:34:45 36
13:34:45 37 end;
13:34:45 38
13:34:45 39 begin
13:34:45 40
13:34:45 41
13:34:45 42
13:34:45 43 dbms_space.unused_space
13:34:45 44
13:34:45 45 ( segment_owner => p_owner,
13:34:45 46
13:34:45 47 segment_name => p_segname,
13:34:45 48
13:34:45 49 segment_type => p_type,
13:34:45 50
13:34:45 51 partition_name => p_partition,
13:34:45 52
13:34:45 53 total_blocks => l_total_blocks,
13:34:45 54
13:34:45 55 total_bytes => l_total_bytes,
13:34:45 56
13:34:45 57 unused_blocks => l_unused_blocks,
13:34:45 58
13:34:45 59 unused_bytes => l_unused_bytes,
13:34:45 60
13:34:45 61 last_used_extent_file_id => l_LastUsedExtFileId,
13:34:45 62
13:34:45 63 last_used_extent_block_id => l_LastUsedExtBlockId,
13:34:45 64
13:34:45 65 last_used_block => l_last_used_block );
13:34:45 66
13:34:45 67
13:34:45 68
13:34:45 69 p( 'Total Blocks', l_total_blocks );
13:34:45 70
13:34:45 71 p( 'Total Bytes', l_total_bytes );
13:34:45 72
13:34:45 73 p( 'Unused Blocks', l_unused_blocks );
13:34:45 74
13:34:45 75 p( 'Unused Bytes', l_unused_bytes );
13:34:45 76
13:34:45 77 p( 'Last Used Ext FileId', l_LastUsedExtFileId );
13:34:45 78
13:34:45 79 p( 'Last Used Ext BlockId', l_LastUsedExtBlockId );
13:34:45 80
13:34:45 81 p( 'Last Used Block', l_last_used_block );
13:34:45 82
13:34:45 83 end;
13:34:47 84 /13:34:49 SCOTT@ test1 >set serverout on
13:35:01 SCOTT@ test1 >exec show_space(p_segname=>'T1');
Total Blocks............................16
Total Bytes.............................131072
Unused Blocks...........................0
Unused Bytes............................0
Last Used Ext FileId....................4
Last Used Ext BlockId...................136
Last Used Block.........................8
PL/SQL procedure successfully completed.数据文件id为4，t1表总共使用了16个块
分析数据块的rowid：
13:35:40 SCOTT@ test1 >select f,b from (
13:41:15 2 select dbms_rowid.rowid_relative_fno(rowid) f,
13:41:44 3 dbms_rowid.rowid_block_number(rowid) b
13:42:04 4 from t1) group by f,b;
F B
---------- ----------
4 143
11 691
4 142
11 693
11 694
4 141
11 692
11 695
8 rows selected.Users 表空间有两个datafile，id为4，11
13:42:52 SYS@ test1 >select file#,name from v$datafile;
FILE# NAME
---------- --------------------------------------------------
1 /u01/app/oracle/oradata/test1/system01.dbf
2 /u01/app/oracle/oradata/test1/sysaux01.dbf
3 /u01/app/oracle/oradata/test1/undotbs01.dbf
4 /u01/app/oracle/oradata/test1/users01.dbf
10 /u01/app/oracle/oradata/test1/index01.dbf
11 /dsk1/oradata/test1/users02.dbf
6 rows selected.T1表总共为16个块，数据占用了8个块
13:47:43 SYS@ test1 >select file#,dbablk,tch from x$bh where obj=
2 (select data_object_id from dba_objects
3 where owner='SCOTT' AND object_name='T1')
4* order by dbablk
FILE# DBABLK TCH
---------- ---------- ----------
4 136 2
4 137 2
4 138 2
4 139 2
4 140 2
4 141 2
4 142 2
4 143 2
11 688 1
11 689 1
11 690 4
11 690 1
11 691 2
11 692 2
11 693 2
11 694 2
11 695 2
17 rows selected.13:48:59 SYS@ test1 >select file#,block#,status from v$bh where objd=
13:49:18 2 (select data_object_id from dba_objects
13:49:33 3 where owner='SCOTT' and object_name='T1')
13:49:51 4 order by block#;
FILE# BLOCK# STATUS
---------- ---------- ----------
4 136 xcur
4 137 xcur
4 138 xcur
4 139 xcur
4 140 xcur
4 141 xcur
4 142 xcur
4 143 xcur
11 688 xcur
11 689 xcur
11 690 xcur
11 690 cr
11 691 xcur
11 692 xcur
11 693 xcur
11 694 xcur
11 695 xcur
17 rows selected.查看x$bh,返回了block的touch count信息，表示了块的热点程度，现在最热的块是690，此块应该是段头块。
13:56:05 SYS@ test1 >select header_file,header_block from dba_segments
13:56:25 2 where owner='SCOTT' and segment_name='T1';
HEADER_FILE HEADER_BLOCK
----------- ------------
11 690手工清空buffer_cache：
14:03:30 SYS@ test1 >alter system flush buffer_cache;
System altered.
14:06:02 SYS@ test1 >select file#,dbablk,tch from x$bh where obj=(select data_object_id from dba_objects where owner='SCOTT' AND object_name='T1') order by dbablk;
no rows selected
14:06:16 SYS@ test1 >select file#,block#,status from v$bh where objd=(select data_object_id from dba_objects where owner='SCOTT' and object_name='T1') order by block#;
no rows selected统计一个object的非free状态的v$bh的记录数，基本就反映了一个对象再data buffer中被cache的块数。
14:10:29 SYS@ test1 >select count(*) from v$bh where objd=
14:10:45 2 (select data_object_id from dba_objects
14:10:59 3 where owner='SCOTT' and object_name='T1')
14:11:23 4 and status !='free';
COUNT(*)
----------
0
对t1表进行事务操作：
14:12:05 SCOTT@ test1 >insert into t1 values (5001);
1 row created.
Elapsed: 00:00:00.00
14:13:38 SCOTT@ test1 >commit;
Commit complete.
14:13:12 SYS@ test1 >select count(*) from v$bh where objd=
2 (select data_object_id from dba_objects
3 where owner='SCOTT' and object_name='T1')
4* and status !='free'
COUNT(*)
----------
414:13:43 SYS@ test1 >select file#,block#,status from v$bh where objd=(select data_object_id from dba_objects where owner='SCOTT' and object_name='T1') order by block#;
FILE# BLOCK# STATUS
---------- ---------- ----------
4 143 xcur
11 688 xcur
11 689 xcur
11 690 xcur（2）热点块问题
x$bh的touch count，这个数字将作为LRU算法的一个重要参考信息，如果一个块被多次访问，每次访问都会导致该块的记录加一。
1）查看t1的object_id
14:26:37 SYS@ test1 >select data_object_id from dba_objects
14:26:56 2 where owner='SCOTT' and object_name='T1';
DATA_OBJECT_ID
--------------
163992）查看block 143的tch
14:29:05 SYS@ test1 >select tch from x$bh
14:29:20 2 where obj=16399 and dbablk=143 and file#=4 and tch>0;
TCH
----------
13）访问block 143
采用了dbms_rowid.rowid_create函数来创建rowid，表示新创建的rowid类型为扩展rowid，类型为1；data_object_id为16399；数据文件id为4；块的id为143；行数为一行（0）。
14:30:47 SYS@ test1 >select count(*) from scott.t1
14:31:02 2 where rowid=dbms_rowid.rowid_create(1,16399,4,143,0);
COUNT(*)
----------
1多次访问后，block 143的tch在增加
14:32:19 SYS@ test1 >select tch from x$bh where obj=16399 and dbablk=143 and file#=4 and tch>0;
TCH
----------
2
14:32:35 SYS@ test1 >select count(*) from scott.t1 where rowid=dbms_rowid.rowid_create(1,16399,4,143,0);
COUNT(*)
----------
1
14:33:23 SYS@ test1 >select tch from x$bh where obj=16399 and dbablk=143 and file#=4 and tch>0;
TCH
----------
3block 143的初始tch=1，经过连续两次查询后，tch=3.根据这一变化，可以基本判断x$bh表中的tch大的块，一般都是热点块。
查询top 10热点块所在的对象：
14:43:43 SYS@ test1 >select owner,object_name from dba_objects
2 where data_object_id in
3 (select obj from
4 (select obj from x$bh order by tch desc )
5* where rownum <11)
OWNER OBJECT_NAME
------------------------------ ------------------------------
SYS I_OBJ#
SYS I_OBJ2
SYS I_ICOL1
SYS I_CDEF2
SYS I_OBJ#_INTCOL#
SYS I_HH_OBJ#_INTCOL#
SYS I_TAB_STATS$_OBJ#
SYS I_IND_STATS$_OBJ#
8 rows selected.
