如何理解recursive calls，db block gets和consistent gets
Posted on 2012-08-29 17:48 徐正柱- 阅读(1155) 评论(0) 编辑 收藏 举报在实际工作中经常要看某个sql语句的执行计划，例如:
在sqlplus使用命令SET AUTOTRACE ON后，执行计划显示如下：
SQL>set autotrace on
SQL> select count(*) from emp;
COUNT(*)
----------
12
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'EMP'
Statistics
----------------------------------------------------------
0 recursive calls
2 db block gets
1 consistent gets
0 physical reads
0 redo size
383 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
其中recursive calls，db block gets，consistent gets的具体含义是什么？
现整理出的具体解释如下：
· Recursive Calls. Number of recursive calls generated at both the user and system level.
Oracle Database maintains tables used for internal processing. When it needs to change these tables, Oracle Database generates an internal SQL statement, which in turn generates a recursive call.
In short, recursive calls are basically SQL performed on behalf of your SQL. So, if you had to parse the query, for example, you might have had to run some other queries to get data dictionary information. These would be recursive calls. Space management, security checks, calling PL/SQL from SQL—all incur recursive SQL calls.
· DB Block Gets. Number of times a CURRENT block was requested.
Current mode blocks are retrieved as they exist right now, not in a consistent read fashion.
Normally, blocks retrieved for a query are retrieved as they existed when the query began. Current mode blocks are retrieved as they exist right now, not from a previous point in time.
During a SELECT, you might see current mode retrievals due to reading the data dictionary to find the extent information for a table to do a full scan (because you need the "right now" information, not the consistent read). During a modification, you will access the blocks in current mode in order to write to them.
(DB Block Gets:请求的数据块在buffer能满足的个数)
· Consistent Gets. Number of times a consistent read was requested for a block.
This is how many blocks you processed in "consistent read" mode. This will include counts of blocks read from the rollback segment in order to roll back a block.
This is the mode you read blocks in with a SELECT, for example.
Also, when you do a searched UPDATE/DELETE, you read the blocks in consistent read mode and then get the block in current mode to actually do the modification.
(Consistent Gets:数据请求总数在回滚段Buffer中)
· Physical Reads. Total number of data blocks read from disk. This number equals the value of "physical reads direct" plus all reads into buffer cache. (Physical Reads:实例启动后，从磁盘读到Buffer Cache数据块数量)
· Sorts (disk). Number of sort operations that required at least one disk write. Sorts that require I/O to disk are quite resource intensive. Try increasing the size of the initialization parameter SORT_AREA_SIZE.
（Sorts(disk):从磁盘上进行排序的数量)
Physical Reads通常是我们最关心的，如果这个值很高，说明要从磁盘请求大量的数据到Buffer Cache里，通常意味着系统里存在大量全表扫描的SQL语句，这会影响到数据库的性能，因此尽量避免语句做全表扫描，对于全表扫描的SQL语句，建议增 加相关的索引，优化SQL语句来解决。
关于physical reads ，db block gets 和consistent gets这三个参数之间有一个换算公式：
数据缓冲区的使用命中率=1 - ( physical reads / (db block gets + consistent gets) )。
在SQL语句里体现如下：
用以下语句可以查看数据缓冲区的命中率：
SQL>SELECT name, value FROM v$sysstat WHERE name IN ('db block gets', 'consistent gets','physical reads');
查询出来的结果Buffer Cache的命中率应该在90％以上，否则需要增加数据缓冲区的大小。
---------------------------------
b block gets：Number of times a CURRENT block was requested.
consistent gets：Number of times a consistent read was requested for a block.
physical reads：Total number of data blocks read from disk. This number equals the value of "physical reads direct" plus all reads into buffer cache.
也可查询:
select sid,value,name from v$sesstat x,v$statname y
where x.statistic#=y.statistic#
and ( name like '%db block%change%' or name like '%db block%get%'
or name like '%consistent%change%' or name like '%physical reads%'
or name like '%physical writes%' or name like '%scans%'
or name = 'index fetch by key' or name like '%read%'
)
and sid = 11 and value > 0
order by value
---------------------------------------------------------------
针对以上3个概念进行的说明解释及关系如下：
1、DB Block Gets（当前请求的块数目）
当前模式块意思就是在操作中正好提取的块数目，而不是在一致性读的情况下而产生的块数。正常的情况下，一个查询提取的块是在查询开始的那个时间点上存在的数据块，当前块是在这个时刻存在的数据块，而不是在这个时间点之前或者之后的数据块数目。
2、Consistent Gets（数据请求总数在回滚段Buffer中的数据一致性读所需要的数据块）
这里的概念是在处理你这个操作的时候需要在一致性读状态上处理多少个块，这些块产生的主要原因是因为由于在你查询的过程中，由于其他会话对数据块进行操 作，而对所要查询的块有了修改，但是由于我们的查询是在这些修改之前调用的，所以需要对回滚段中的数据块的前映像进行查询，以保证数据的一致性。这样就产 生了一致性读。
3、Physical Reads（物理读）
就是从磁盘上读取数据块的数量，其产生的主要原因是：
1、 在数据库高速缓存中不存在这些块
2、 全表扫描
3、 磁盘排序
它们三者之间的关系大致可概括为：
逻辑读指的是Oracle从内存读到的数据块数量。一般来说是'consistent gets' + 'db block gets'。当在内存中找不到所需的数据块的话就需要从磁盘中获取，于是就产生了'phsical reads'。
The consistent gets Oracle metric is the number of times a consistent read (a logical RAM buffer I/O) was requested to get data from a data block.
consistent gets在判断一段SQL的性能时非常有用，通常来讲比较两段SQL的性能好坏不是看谁的执行时间短，而是看谁的consistent gets小。不过这也不是绝对的，下面这个例子就是一个反例：
ETL@RACTEST> create table test( a int);
Table created.
Elapsed: 00:00:00.05
ETL@RACTEST> ETL@RACTEST> begin
PL/SQL procedure successfully completed.
Elapsed: 00:00:00.44
ETL@RACTEST> set autot trace
ETL@RACTEST> ETL@RACTEST> select * from test;
10000 rows selected.
Elapsed: 00:00:00.05
Execution Plan
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id
--------------------------------------------------------------------------
|
|
--------------------------------------------------------------------------
Note
-----
Statistics
----------------------------------------------------------
可以看到select *读了690个内存块。
ETL@RACTEST> select * from test order by 1;
10000 rows selected.
Elapsed: 00:00:00.04
Execution Plan
----------------------------------------------------------
Plan hash value: 2007178810
---------------------------------------------------------------------------
| Id
---------------------------------------------------------------------------
|
|
|
---------------------------------------------------------------------------
Note
-----
Statistics
----------------------------------------------------------