执行计划里statistics数据的解释

SQL> set autotrace on statistics
SQL> select * from tss;

        ID
----------
         1
         2
         3


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

现在来解释一下每一个statistics：

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'。

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) )。

select 1-physical_reads/(db_block_gets+consistent_gets) Buffer Cache
from
   (select value physical_reads from v$sysstat where name='physical reads'),
   (select value consistent_gets from v$sysstat where name='consistent gets'),
   (select value db_block_gets from v$sysstat where name='db block gets')

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。

4、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.

5、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.

最后我们看下db block gets和consistent gets的区别

buffer_gets=db block gets + consistent gets = LOGIC IO(逻辑读次数）(as opposed to physical io)

consistent gets : 通过不带for update的select 读的次数
db block gets : 通过update/delete/select for update读的次数.


consistent get ：在一致读模式下所读的快数，包括从回滚段读的快数。
db block gets ：在当前读模式下所读的快数，比较少和特殊，例如数据字典数据获取，在DML中，更改或删除数据是要用到当前读模式。

consistent gets：consistent_gets是从回滚段中读到的前映（或叫读取一致性影象）, 看见的数据是查询开始的时间点的，所以若存在block在查询开始后发生了变化的情况，则必须产生 before image 然后读数据，这就是一致读的含义
查询就是表示 consistent gets (query mode)，因为查询要保证所获取的数据的时间点的一致性，所以叫一致读，即使是从当前 buffer 获得的数据，也叫 consistent gets ，这仅仅表达一种模式一种期望，并不表示真实的是从 当前buffer 获得 还是从回滚段获取数据产生的 bufore image 。

db block gets：current mode , 不管这个块上的数据是否可能存在 before image ,也就是说不管是否存在回滚中数据可以 回滚，只看见当前最新块的数据，即使别人正在更新，也看见别人更新状态的数据，比如dml的时候就不需要看见别人更改前的数据，而是看见正在更改的，当然同时，若操作相同数据则被lock住。也就是说一次查询中看见的数据可能不在同一个时间点上，比如一个大的dml，当dml 开始更新一个非常大的表后，这个表更新的过程中，有一个进程去把该表末尾的一个记录更新了，然后这个大更新抵达该记录的时候会被阻塞的，若该进程事物提交，则大更新会覆盖该事务的更新，也就是说，这个大更新所看见的数据是当前的，不具有时间点的一致性，所以叫 current mode，个人认为db block gets这个词用的不好, 容易让人误解. 如果改成inconsistent gets可能会更准确一些。

转自：http://hi.baidu.com/learnfordba/blog/item/0dcb9858ef756b232934f022.html

英文原文：
· 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.
· 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.
· 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.
· 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.