prepare statement cache size influence database

調整prepare statement cache size對資料庫的影響，簡單的說這個statement cache設定在應用端，快取了應用到資料庫的一些sql。[@more@]

關於這個引數的說明：

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
這次應用端做了引數調整，應用把只從30改到15，DB的這些指標明顯上升：

soft prase，

table scan(short tables),

logons,

bytes receive via sql*net from client