统计信息及相关说明
统计信息：
0 recursive calls
20434 db block gets
317970511 consistent gets
0 physical reads
3759764 redo size
382 bytes sent via SQL*Net to client
1061 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
10001 sorts (memory)
0 sorts (disk)
10000 rows processed
317970511 consistent gets
0 physical reads
3759764 redo size
382 bytes sent via SQL*Net to client
1061 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
10001 sorts (memory)
0 sorts (disk)
10000 rows processed
recursive calls 统计在一段语句执行时，递归调用的次数。在执行SQL时，生成的执行计划可能牵涉到许多相关的调用执行，比如在执行Update时可能需要先select，这个select就是update的递归调用；在执行语句时可能还牵涉到对系统表的递归查询等操作，这些通通都算是递归调用。
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.
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：(DB Block Gets:请求的数据块在buffer能满足的个数)
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.
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：(Consistent Gets:数据请求总数在回滚段Buffer中)
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.
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. (Physical Reads:实例启动后，从磁盘读到Buffer Cache数据块数量)
sorts (memory)：
在内存中排序
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) )
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'。
