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$bh结构: 13:07:20SYS@test1descv$bhNameNull?Type------------------------
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 NUMBER
STATUS
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 NUMBER
x$bh中“touch count”信息对应到x$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）。
