左右db_block_size了解和实验

关于db_block_gets了解和实验

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

----------------------------------------------------------





-------------------------------------------------------------------------

| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |

-------------------------------------------------------------------------

|   0 | INSERT STATEMENT |      |     1 |   100 |     1   (0)| 00:00:01 |

-------------------------------------------------------------------------









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

----------------------------------------------------------





-------------------------------------------------------------------------

| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |

-------------------------------------------------------------------------

|   0 | INSERT STATEMENT |      |     1 |   100 |     1   (0)| 00:00:01 |

-------------------------------------------------------------------------









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

----------------------------------------------------------





-------------------------------------------------------------------------

| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |

-------------------------------------------------------------------------

|   0 | INSERT STATEMENT |      |     1 |   100 |     1   (0)| 00:00:01 |

-------------------------------------------------------------------------









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。由于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，就须要对区上的块进行空间分配和对数据字典的一些操作，就会有比較大的db_block_size。

假设再次插入数据的话就基本没有对空间的分配啥的，就会有比較少的db_block_size产生。

所以对于extent指定的区大小来说  相同的空表插入相同的数据 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





-------------------------------------------------------------------------------

| 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"=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、对于比較大的表来说

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

----------------------------------------------------------





-------------------------------------------------------------------------

| Id  | Operation        | Name | Rows  | Bytes | Cost (%CPU)| Time     |

-------------------------------------------------------------------------

|   0 | INSERT STATEMENT |      |     1 |   100 |     1   (0)| 00:00:01 |

-------------------------------------------------------------------------









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。可能对数据改动时 对 数据字典  或者对于区、块的分配都包括在 physical reads中。

感想：

对于生产库来说，这个值一般不会太考虑究竟数字是怎么来的，由于数字都比较大，通常只关心它的尺寸大小。

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