RECORDS_PER_BLOCK参数用于设定每个BLOCK中记录数的最大值，其先找到当前表所有BLOCK中容纳的最大行数，并会把这个数字记录到数据字典，以后任何导致BLOCK行数超过这个数字的插入都会被拒绝。
RECORDS_PER_BLOCK参数是为位图索引而生的，能够改善位图索引的存储，减小位图索引的长度。这样，利用该位图索引的时候，就能获得比较好的效率了。
测试案例：
1、表默认的存储分析
|
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|
15
:
45
:
46
SCOTT@ prod >create table t3 (x int,y int);
Table created.
15
:
46
:
03
SCOTT@ prod >
insert
into
t3 values (
1
,
1
);
1
row created.
15
:
46
:
12
SCOTT@ prod >
insert
into
t3 values (
2
,
1
);
1
row created.
15
:
46
:
27
SCOTT@ prod >commit;
Commit complete.
15
:
48
:
01
SCOTT@ prod >
insert
into
t3 select rownum+
2
,
1
from
all_objects
where
rownum <=
254
;
254
rows created.
15
:
48
:
37
SCOTT@ prod >create index t3_indx
on
t3(x);
Index created.
15
:
48
:
57
SCOTT@ prod >exec dbms_stats.gather_table_stats(user,
'T3'
,cascade=>
true
);
PL/SQL procedure successfully completed.
15
:
49
:
54
SCOTT@ prod >select count(distinct dbms_rowid.rowid_block_number(rowid))
from
t3;
COUNT(DISTINCTDBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID))
---------------------------------------------------
1
15
:
53
:
09
SCOTT@ prod >col segment_name
for
a20
15
:
53
:
21
SCOTT@ prod >select segment_name,EXTENTS,BLOCKS,BYTES
from
user_segments
where
segment_name=
'T3'
;
SEGMENT_NAME EXTENTS BLOCKS BYTES
-------------------- ---------- ---------- ----------
T3
1
8
65536
默认值，T3表中的数据存储在一个数据块上。
|
2、通过RECORDS_PER_BLOCK参数分散数据块的存储
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|
15
:
57
:
47
SCOTT@ prod >drop table t3 purge;
Table dropped.
15
:
59
:
59
SCOTT@ prod >create table t3 (x int,y int);
Table created.
16
:
00
:
08
SCOTT@ prod >
insert
into
t3 values (
1
,
1
);
1
row created.
16
:
00
:
16
SCOTT@ prod >
insert
into
t3 values (
2
,
1
);
1
row created.
16
:
00
:
25
SCOTT@ prod >commit;
Commit complete.
16
:
00
:
37
SCOTT@ prod >alter table t3 minimize records_per_block;
Table altered.
16
:
00
:
54
SCOTT@ prod >
insert
into
t3 select rownum+
2
,
1
from
all_objects
where
rownum <=
254
;
254
rows created.
16
:
01
:
09
SCOTT@ prod >commit;
Commit complete.
17
:
15
:
14
SCOTT@ prod >create index t3_indx
on
t3(x);
Index created.
16
:
01
:
12
SCOTT@ prod >exec dbms_stats.gather_table_stats(user,
'T3'
);
PL/SQL procedure successfully completed.
16
:
01
:
58
SCOTT@ prod >select count(distinct dbms_rowid.rowid_block_number(rowid))
from
t3;
COUNT(DISTINCTDBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID))
---------------------------------------------------
128
16
:
21
:
29
SCOTT@ prod >select dbms_rowid.rowid_block_number(rowid),count(
0
)
from
t3
group by dbms_rowid.rowid_block_number(rowid);
DBMS_ROWID.ROWID_BLOCK_NUMBER(ROWID) COUNT(
0
)
------------------------------------ ----------
198
2
138
2
151
2
153
2
167
2
244
2
245
2
247
2
537
2
544
2
134
2
194
2
207
2
147
2
209
2
213
2
155
2
......
128
rows selected.
可以看出，T3表占用了
128
个数据块！
|
测试发现：执行alter table test minimize records_per_block;之后，目前BLOCK中的记录数（的最大值）会应用到以后的新增数据中，也就是，当以后再往表中INSERT数据时，每个BLOCK中可以包含的记录数将与设定records_per_block之前的最大值保持一致。
需要注意的是：
-
不能对空表设定此参数。
-
每个BLOCK中可以包含的记录数的最低下限是2。
-
不能在已经有 bitmap 的表中使用records_per_block参数，也就是说，如果要使用records_per_block参数，必须先alter table xxx minimize records_per_block，然后才能在表上建立索引。
如果字段的类型、大小、个数发生了改变，那么就会导致一个比较差的结果，这就说明了，这项功能只在于使用在静态的环境中，比如数据仓库。
主要用途：
-
通过减少同一个block中的记录数，使记录分布于更多的数据块中，可以优化等待块类型为data block的Buffer Busy Wait事件。
-
其主要用途是提高BITMAP INDEX的存储性能
3、对table访问分析
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|
15
:
44
:
39
SYS@ prod >alter system flush buffer_cache;
System altered.
16
:
07
:
01
SYS@ prod >show parameter mult
NAME TYPE VALUE
------------------------------------ -------------------------------- ------------------------------
db_file_multiblock_read_count integer
28
parallel_adaptive_multi_user boolean TRUE
17
:
32
:
42
SCOTT@ prod >col object_name
for
a20
17
:
32
:
49
SCOTT@ prod >select object_name,object_id
from
user_objects
where
object_name=
'T3'
;
OBJECT_NAME OBJECT_ID
-------------------- ----------
T3
76505
16
:
22
:
19
SCOTT@ prod >alter session set db_file_multiblock_read_count=
64
;
Session altered.
将数据块以间隔的方式读入内存
16
:
09
:
03
SCOTT@ prod >declare
16
:
09
:
20
2
num number;
16
:
09
:
25
3
begin
16
:
09
:
29
4
for
i
in
1.
.64
16
:
09
:
34
5
loop
16
:
09
:
37
6
select y
into
num
from
t3
where
x=i*
4
;
16
:
09
:
42
7
end loop;
16
:
09
:
48
8
end;
16
:
09
:
50
9
/
PL/SQL procedure successfully completed.
17
:
25
:
29
SYS@ prod >select file#,block#,status,objd
from
v$bh
where
file#=
4
;
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
521
free
76505
4
521
free
76505
4
521
free
76505
4
165
free
76505
4
165
free
76505
4
165
free
76505
4
542
free
76505
4
542
free
76505
4
542
free
76505
4
131
free
76505
4
131
free
76505
4
131
free
76505
4
131
xcur
76505
4
529
free
76505
4
529
free
76505
4
529
free
76505
4
529
xcur
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
550
free
76505
4
550
free
76505
4
550
free
76505
4
139
free
76505
4
139
free
76505
4
139
free
76505
4
139
xcur
76505
4
537
free
76505
4
537
free
76505
4
537
free
76505
4
3
free
4294967295
4
3
free
4294967295
4
147
free
76505
4
147
free
76505
4
147
free
76505
4
524
free
76505
4
524
free
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
524
free
76505
4
545
free
76505
4
545
free
76505
4
545
free
76505
4
545
xcur
76505
4
134
free
76505
4
134
free
76505
4
134
free
76505
4
134
xcur
76505
4
155
free
76505
4
155
free
76505
4
155
free
76505
4
155
xcur
76505
4
532
free
76505
4
532
free
76505
4
532
free
76505
4
532
xcur
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
553
free
76506
4
142
free
76505
4
142
free
76505
4
142
free
76505
4
163
free
76505
4
163
free
76505
4
163
free
76505
4
540
free
76505
4
540
free
76505
4
540
free
76505
4
129
free
76505
4
129
free
76505
4
129
free
76505
4
150
free
76505
4
150
free
76505
4
150
free
76505
4
150
xcur
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
527
free
76505
4
527
free
76505
4
527
free
76505
4
527
xcur
76505
4
548
free
76505
4
548
free
76505
4
548
free
76505
4
137
free
76505
4
137
free
76505
4
158
free
76505
4
158
free
76505
4
535
free
76505
4
535
free
76505
4
145
free
76505
4
145
free
76505
4
145
xcur
76505
4
522
free
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
522
free
76505
4
522
xcur
76505
4
166
free
76505
4
166
free
76505
4
166
xcur
76505
4
543
free
76505
4
543
free
76505
4
543
xcur
76505
4
132
free
76505
4
132
free
76505
4
153
free
76505
4
153
free
76505
4
530
free
76505
4
530
free
76505
4
551
free
76505
4
551
free
76505
4
551
xcur
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
140
free
76505
4
140
free
76505
4
161
free
76505
4
161
free
76505
4
161
xcur
76505
4
538
free
76505
4
538
free
76505
4
538
xcur
76505
4
148
free
76505
4
148
free
76505
4
148
xcur
76505
4
525
free
76505
4
525
free
76505
4
525
xcur
76505
4
546
free
76505
4
546
free
76505
4
135
free
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
135
free
76505
4
156
free
76505
4
156
free
76505
4
533
free
76505
4
533
free
76505
4
554
free
76506
4
143
free
76505
4
143
free
76505
4
143
xcur
76505
4
164
free
76505
4
164
free
76505
4
164
xcur
76505
4
541
free
76505
4
541
free
76505
4
541
xcur
76505
4
130
free
76505
4
130
free
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
151
free
76505
4
151
free
76505
4
549
free
76505
4
549
free
76505
4
549
xcur
76505
4
138
free
76505
4
138
free
76505
4
138
xcur
76505
4
159
free
76505
4
159
free
76505
4
159
xcur
76505
4
2
free
4294967295
4
146
free
76505
4
146
free
76505
4
523
free
76505
4
523
free
76505
4
523
xcur
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
167
free
76505
4
167
free
76505
4
544
free
76505
4
544
free
76505
4
133
free
76505
4
133
free
76505
4
154
free
76505
4
154
free
76505
4
154
xcur
76505
4
531
free
76505
4
531
free
76505
4
552
free
76506
4
141
free
76505
4
141
free
76505
4
141
xcur
76505
4
162
free
76505
4
162
free
76505
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
539
free
76505
4
539
free
76505
4
539
xcur
76505
4
149
free
76505
4
149
free
76505
4
526
free
76505
4
526
free
76505
4
547
free
76505
4
547
free
76505
4
547
xcur
76505
4
157
free
76505
4
157
free
76505
4
157
xcur
76505
4
534
free
76505
4
534
free
76505
4
534
xcur
76505
4
555
free
76506
FILE# BLOCK# STATUS OBJD
---------- ---------- ---------- ----------
4
555
xcur
76506
188
rows selected.
16
:
14
:
20
SYS@ prod >grant alter session to scott;
Grant succeeded.
16
:
14
:
39
SYS@ prod >conn scott/tiger
Connected.
16
:
14
:
42
SCOTT@ prod >alter session set events
'10046 trace name context forever,level 12'
;
Session altered.
16
:
15
:
31
SCOTT@ prod >set autotrace trace
16
:
15
:
37
SCOTT@ prod >select *
from
t3 ;
256
rows selected.
Elapsed:
00
:
00
:
00.02
Execution Plan
----------------------------------------------------------
Plan hash value:
4161002650
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
|
0
| SELECT STATEMENT | |
256
|
1792
|
68
(
0
)|
00
:
00
:
01
|
|
1
| TABLE ACCESS FULL| T3 |
256
|
1792
|
68
(
0
)|
00
:
00
:
01
|
--------------------------------------------------------------------------
Statistics
----------------------------------------------------------
1
recursive calls
0
db block gets
196
consistent gets
0
physical reads
0
redo size
4829
bytes sent via SQL*Net to client
606
bytes received via SQL*Net
from
client
19
SQL*Net roundtrips to/
from
client
0
sorts (memory)
0
sorts (disk)
256
rows processed
[oracle@RH6 ~]$ ls -lt /u01/app/oracle/diag/rdbms/prod/prod/trace/|more
total
12056
-rw-r-----
1
oracle oinstall
51244
Nov
19
17
:
28
prod_ora_3681.trc
-rw-r-----
1
oracle oinstall
199
Nov
19
17
:
28
prod_ora_3681.trm
-rw-r--r--
1
oracle oinstall
430401
Nov
19
17
:
22
alert_prod.log
-rw-r-----
1
oracle oinstall
8230
Nov
19
17
:
18
prod_ora_3629.trc
[oracle@RH6 ~]$ grep sequen /u01/app/oracle/diag/rdbms/prod/prod/trace/prod_ora_3681.trc
WAIT #
10
: nam=
'db file sequential read'
ela=
13
file#=
4
block#=
130
blocks=
1
obj#=
76505
tim=
1416389324098217
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
4
block#=
135
blocks=
1
obj#=
76505
tim=
1416389324098716
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
6
block#=
193
blocks=
1
obj#=
76505
tim=
1416389324098758
WAIT #
10
: nam=
'db file sequential read'
ela=
0
file#=
6
block#=
195
blocks=
1
obj#=
76505
tim=
1416389324098837
WAIT #
10
: nam=
'db file sequential read'
ela=
0
file#=
6
block#=
197
blocks=
1
obj#=
76505
tim=
1416389324098837
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
199
blocks=
1
obj#=
76505
tim=
1416389324098874
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
4
block#=
137
blocks=
1
obj#=
76505
tim=
1416389324098917
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
140
blocks=
1
obj#=
76505
tim=
1416389324099100
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
142
blocks=
1
obj#=
76505
tim=
1416389324099144
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
200
blocks=
1
obj#=
76505
tim=
1416389324099188
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
202
blocks=
1
obj#=
76505
tim=
1416389324099230
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
204
blocks=
1
obj#=
76505
tim=
1416389324099395
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
6
block#=
206
blocks=
1
obj#=
76505
tim=
1416389324099439
WAIT #
10
: nam=
'db file sequential read'
ela=
223
file#=
4
block#=
149
blocks=
1
obj#=
76505
tim=
1416389324100699
WAIT #
10
: nam=
'db file sequential read'
ela=
13
file#=
4
block#=
151
blocks=
1
obj#=
76505
tim=
1416389324100962
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
209
blocks=
1
obj#=
76505
tim=
1416389324101019
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
211
blocks=
1
obj#=
76505
tim=
1416389324101319
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
6
block#=
213
blocks=
1
obj#=
76505
tim=
1416389324101384
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
215
blocks=
1
obj#=
76505
tim=
1416389324101418
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
153
blocks=
1
obj#=
76505
tim=
1416389324101459
WAIT #
10
: nam=
'db file sequential read'
ela=
10
file#=
4
block#=
156
blocks=
1
obj#=
76505
tim=
1416389324101664
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
4
block#=
158
blocks=
1
obj#=
76505
tim=
1416389324101716
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
216
blocks=
1
obj#=
76505
tim=
1416389324101770
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
218
blocks=
1
obj#=
76505
tim=
1416389324101813
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
220
blocks=
1
obj#=
76505
tim=
1416389324101992
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
222
blocks=
1
obj#=
76505
tim=
1416389324102036
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
4
block#=
165
blocks=
1
obj#=
76505
tim=
1416389324102276
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
4
block#=
167
blocks=
1
obj#=
76505
tim=
1416389324102309
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
233
blocks=
1
obj#=
76505
tim=
1416389324102355
WAIT #
10
: nam=
'db file sequential read'
ela=
32
file#=
6
block#=
235
blocks=
1
obj#=
76505
tim=
1416389324102705
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
237
blocks=
1
obj#=
76505
tim=
1416389324102931
WAIT #
10
: nam=
'db file sequential read'
ela=
27
file#=
6
block#=
239
blocks=
1
obj#=
76505
tim=
1416389324103182
WAIT #
10
: nam=
'db file sequential read'
ela=
10
file#=
6
block#=
256
blocks=
1
obj#=
76505
tim=
1416389324103344
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
129
blocks=
1
obj#=
76505
tim=
1416389324103389
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
257
blocks=
1
obj#=
76505
tim=
1416389324103423
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
521
blocks=
1
obj#=
76505
tim=
1416389324103466
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
524
blocks=
1
obj#=
76505
tim=
1416389324103678
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
4
block#=
526
blocks=
1
obj#=
76505
tim=
1416389324103722
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
240
blocks=
1
obj#=
76505
tim=
1416389324103766
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
242
blocks=
1
obj#=
76505
tim=
1416389324103808
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
244
blocks=
1
obj#=
76505
tim=
1416389324103872
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
246
blocks=
1
obj#=
76505
tim=
1416389324103918
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
533
blocks=
1
obj#=
76505
tim=
1416389324104170
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
4
block#=
535
blocks=
1
obj#=
76505
tim=
1416389324104206
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
249
blocks=
1
obj#=
76505
tim=
1416389324104250
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
6
block#=
251
blocks=
1
obj#=
76505
tim=
1416389324104449
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
253
blocks=
1
obj#=
76505
tim=
1416389324104512
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
6
block#=
255
blocks=
1
obj#=
76505
tim=
1416389324104544
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
537
blocks=
1
obj#=
76505
tim=
1416389324104584
WAIT #
10
: nam=
'db file sequential read'
ela=
9
file#=
4
block#=
540
blocks=
1
obj#=
76505
tim=
1416389324104759
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
4
block#=
542
blocks=
1
obj#=
76505
tim=
1416389324104802
WAIT #
10
: nam=
'db file sequential read'
ela=
8
file#=
4
block#=
544
blocks=
1
obj#=
76505
tim=
1416389324104845
WAIT #
10
: nam=
'db file sequential read'
ela=
76
file#=
4
block#=
546
blocks=
1
obj#=
76505
tim=
1416389324105604
WAIT #
10
: nam=
'db file sequential read'
ela=
7
file#=
4
block#=
548
blocks=
1
obj#=
76505
tim=
1416389324105805
WAIT #
10
: nam=
'db file sequential read'
ela=
6
file#=
4
block#=
550
blocks=
1
obj#=
76505
tim=
1416389324105834
......
|
以上向我们展示了Oracle多个数据块读取的工作机制，当内存中已经有了某个数据块时，Oracle将不再从磁盘中读取它。这里使用一个循环来通过索引块访问的方式（每次读取一个数据块），将间隔的数据块读入到内存中。这样，当我们对T3表执行全表扫描时，尽管设置了参数：
16:22:19 SCOTT@ prod >alter session set db_file_multiblock_read_count=64;
但是由于没有连续的数据块可以读取了，所以Oracle每次也只能将一个数据块读取到内存。在等待事件中每一个WAIT#中 blocks=1说明每次I/O读取的数据块都为1，而且数据块的序号正好间隔为1，说明她们之间的那个数据块已经读取到内存中了。因为需要读取的数据块不再连续，所以此时不能一次读取多个数据块。
多数据块读取一般发生在：
FTS(FULL TABLE SCAN)
INDEX_FFS(INDEX FAST FULL SCAN)
本文转自 客居天涯 51CTO博客，原文链接：http://blog.51cto.com/tiany/1579707，如需转载请自行联系原作者
