其實(shí)大家對(duì)v$bh這個(gè)視圖可能一直重視不夠.
我們來(lái)看一下這個(gè)視圖主要字段說(shuō)明:
status
varchar2(1)
status of the buffer:
free - not currently in use xcur - exclusive scur - shared current cr - consistent read read - being read from disk mrec - in media recovery mode irec - in instance recovery mode ?
dirty
varchar2(1)
y - block modified
temp
varchar2(1)
y - temporary block
ping
varchar2(1)
y - block pinged
stale
varchar2(1)
y - block is stale
direct
varchar2(1)
y - direct block
?
我們注意到v$dirty代表的就是block modified
?
如果我們進(jìn)一步向下追溯:
sql> select view_definition from v$fixed_view_definition where view_name = 'gv$bh';
view_definition
--------------------------------------------------------------------------------
select bh.inst_id, file#, dbablk, class, decode(state,0,'free',1,'xcur',2,'scur'
,3,'cr', 4,'read',5,'mrec',6,'irec',7,'write',8,'pi'), x_to_null, forced_reads,
forced_writes, bh.le_addr, name,le_class, decode(bitand(flag,1), 0, 'n', 'y'), d
ecode(bitand(flag,16), 0, 'n', 'y'), decode(bitand(flag,1536), 0, 'n', 'y'), dec
ode(bitand(flag,16384), 0, 'n', 'y'), decode(bitand(flag,65536), 0, 'n', 'y'), '
n', obj, ts# from x$bh bh, x$le le where bh.le_addr = le.le_addr (+)
我們可以看到v$bh的底層表示x$bh
其中v$bh中的dirty/temp/ping/stale/direct五個(gè)字段來(lái)源于x$bh中的flag字段
格式化一下輸出:
select bh.inst_id, file#, dbablk, class,
?????? decode (state,
?????????????? 0, 'free',
?????????????? 1, 'xcur',
?????????????? 2, 'scur',
?????????????? 3, 'cr',
?????????????? 4, 'read',
?????????????? 5, 'mrec',
?????????????? 6, 'irec',
?????????????? 7, 'write',
?????????????? 8, 'pi'
????????????? ),
?????? x_to_null, forced_reads, forced_writes, bh.le_addr, name, le_class,
?????? decode (bitand (flag, 1), 0, 'n', 'y'),
?????? decode (bitand (flag, 16), 0, 'n', 'y'),
?????? decode (bitand (flag, 1536), 0, 'n', 'y'),
?????? decode (bitand (flag, 16384), 0, 'n', 'y'),
?????? decode (bitand (flag, 65536), 0, 'n', 'y'), 'n', obj, ts#
? from x$bh bh, x$le le
?where bh.le_addr = le.le_addr(+)
由此我們得以窺視x$bh.flag的深層含義.
