调整prepare statement cache size对数据库的影响，简单的说这个statement cache设置在应用端，缓存了应用到数据库的一些sql。[@more@]

关于这个参数的说明：

Statement caching improves performance by caching executable statements that are used repeatedly, such as in a loop or in a method that is called repeatedly. Starting from JDBC 3.0, JDBC standards define a statement-caching interface.

Statement caching can do the following:

   *   Prevent the overhead of repeated cursor creation
   *   Prevent repeated statement parsing and creation
   *   Reuse data structures in the client
这次应用端做了参数调整，应用把只从30改到15，DB的这些指标明显上升：

soft prase，

table scan(short tables),

logons,

bytes receive via sql*net from client