关于对db_block_gets的理解与实验

实验

一、自己手动创建的小表

创建一个区大小为 40k
SYS@ORCL>show parameter db_block_size

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_block_size integer

SYS@ORCL>create tablespace tyger1 datafile '/u01/app/oracle/oradata/ORCL/tyger1.dbf' size 10m
2 extent management local uniform size 40k;

Tablespace created.

SYS@ORCL>create table test_db1(x int) tablespace tyger1;

Table created.

SYS@ORCL>set autotrace on
SYS@ORCL>insert into test_db1 values(1);

1 row created.

Execution Plan
----------------------------------------------------------

-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | INSERT STATEMENT | | 1 | 100 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
19 db block gets
1 consistent gets
3 physical reads
964 redo size
675 bytes sent via SQL*Net to client
562 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

SYS@ORCL>insert into test_db1 values(2);

1 row created.

Execution Plan
----------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
3 db block gets
1 consistent gets
0 physical reads
244 redo size
675 bytes sent via SQL*Net to client
562 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

2. 创建一个区大小为80k
SYS@ORCL>create tablespace tyger2 datafile '/u01/app/oracle/oradata/ORCL/tyger2.dbf' size 10m
2 extent management local uniform size 80k;

Tablespace created.

SYS@ORCL>create table test_db2(x int) tablespace tyger2;

Table created.

SYS@ORCL>insert into test_db2 values(1);

1 row created.

Execution Plan
----------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
29 db block gets
1 consistent gets
28 physical reads
1364 redo size
675 bytes sent via SQL*Net to client
562 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

SYS@ORCL>insert into test_db2 values(2);

1 row created.

Execution Plan
----------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
3 db block gets
1 consistent gets
0 physical reads
288 redo size
677 bytes sent via SQL*Net to client
562 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

结论：对于新创建的表来说，因为创建的是空表就没有对表里的空间进行分配，当插入第一
条数据时，就需要对区上的块进行空间分配和对数据字典的一些操作，就会有比较大的db_block_size。如果再次插入数据的话就基本没有对空间的分
配啥的，就会有比较少的db_block_size产生。

所以对于extent指定的区大小来说同样的空表插入同样的数据 db_block_size 可能不同。

对插入更新、删除的实验：
SYS@ORCL>update test_db1 set x=3 where x=1;

1 row updated.

Execution Plan
----------------------------------------------------------
Plan hash value: 2185639234

-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | UPDATE STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |
| 1 | UPDATE | TEST_DB1 | | | | |
|* 2 | TABLE ACCESS FULL| TEST_DB1 | 1 | 13 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("X"=1)

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
28 recursive calls
1 db block gets
11 consistent gets
0 physical reads
388 redo size
678 bytes sent via SQL*Net to client
565 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

SYS@ORCL>delete test_db1 where x=2;

1 row deleted.

Execution Plan
----------------------------------------------------------
Plan hash value: 3135214910

-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | DELETE STATEMENT | | 1 | 13 | 2 (0)| 00:00:01 |
| 1 | DELETE | TEST_DB1 | | | | |
|* 2 | TABLE ACCESS FULL| TEST_DB1 | 1 | 13 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("X"=2)

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
5 recursive calls
1 db block gets
9 consistent gets
0 physical reads
288 redo size
678 bytes sent via SQL*Net to client
557 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

SYS@ORCL>insert into test_db1 values(&x);
Enter value for x: 1
old 1: insert into test_db1 values(&x)
new 1: insert into test_db1 values(1)

1 row created.

。。。。
SYS@ORCL>commit;

Commit complete.

SYS@ORCL>select * from test_db1;

X
----------
3
1
2
3
4
5
6
7
8
9
19
10
1
11
12
13
14
15
16
17
18

21 rows selected.

SYS@ORCL>alter system flush buffer_cache;

System altered.
SYS@ORCL>update test_db1 set x=21 where x=18;

1 row updated.

Execution Plan
----------------------------------------------------------
Plan hash value: 2185639234

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("X"=18)

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
5 recursive calls
1 db block gets
9 consistent gets
0 physical reads
412 redo size
678 bytes sent via SQL*Net to client
567 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

二、对于比较大的表来说

SYS@ORCL>create table test_db1 as select * from dba_objects;

Table created.

SYS@ORCL>insert into test_db1 values('tyger','tyger','tyger',22,23,'tyger','04-SEP-14','04-SEP-14','tyger','t','t','t','t');

1 row created.

Execution Plan
----------------------------------------------------------

Statistics
----------------------------------------------------------
1 recursive calls
15 db block gets
1 consistent gets
5 physical reads
1144 redo size
677 bytes sent via SQL*Net to client
646 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed

SYS@ORCL>alter system flush buffer_cache;

System altered.

SYS@ORCL>update test_db1 set OBJECT_NAME='tom' where owner='tyger';

3 rows updated.

Execution Plan
----------------------------------------------------------
Plan hash value: 2185639234

-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | UPDATE STATEMENT | | 8 | 664 | 154 (2)| 00:00:02 |
| 1 | UPDATE | TEST_DB1 | | | | |
|* 2 | TABLE ACCESS FULL| TEST_DB1 | 8 | 664 | 154 (2)| 00:00:02 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("OWNER"='tyger')

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
5 recursive calls
3 db block gets
769 consistent gets
687 physical reads
824 redo size
679 bytes sent via SQL*Net to client
589 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
3 rows processed
SYS@ORCL>delete test_db1 where owner='tyger';

3 rows deleted.

Execution Plan
----------------------------------------------------------
Plan hash value: 3135214910

-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | DELETE STATEMENT | | 8 | 136 | 154 (2)| 00:00:02 |
| 1 | DELETE | TEST_DB1 | | | | |
|* 2 | TABLE ACCESS FULL| TEST_DB1 | 8 | 136 | 154 (2)| 00:00:02 |
-------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter("OWNER"='tyger')

Note
-----
- dynamic sampling used for this statement

Statistics
----------------------------------------------------------
4 recursive calls
3 db block gets
769 consistent gets
0 physical reads
1064 redo size
679 bytes sent via SQL*Net to client
567 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
3 rows processed

结论：对于占用多个段的大表来说，可能对数据修改时对数据字典或者对于区、块的分配都包含在 physical reads中。

感想：

对于生产库来说，这个值一般不会太考虑到底数字是怎么来的，因为数字都比较大，一般只在乎它的大小数量级。

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