关于执行计划里recursive calls，db block gets和consistent gets参数的解释

我们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。