一、背景

<span style="font-size:18px;"><select id=”queryUserById” returnType=”userResult”>  
   SELECT * FROM user WHERE id =#id#
</select> </span>

二、問題

       我的疑問有兩點：1.MySQL是否預設開啟了預編譯功能？若沒有，將如何開啟？ 2.預編譯是否能有效提升執行SQL的效能？

三、探究一

      MySQL是否預設開啟了預編譯？

<span style="font-size:18px;">public static void main(String[] args) throws Exception{  
	  String sql = "select * from userwhere id= ?";
	  Class.forName("com.mysql.jdbc.Driver");
	  Connection conn = null;  
	  try{  
	       conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/studb?user=root&password=root");  
	       PreparedStatement stmt = conn.prepareStatement(sql);
	       stmt.setString(1,5);
	       ResultSet rs = stmt.executeQuery();  
	       rs.close();  
	       stmt.close();  
	  }catch(Exception e){  
	       e.printStackTrace();  
	  }finally{  
	      conn.close();  
	  }  
}  </span>
 

<span style="font-size:18px;"> Query      SET NAMES utf8
 Query    SET character_set_results = NULL
 Query    SHOW VARIABLES
 Query    SHOW WARNINGS
 Query    SHOW COLLATION
 Query    SET autocommit=1
 Prepare  select *from user where id = ?
 Execute  select * from user where id= 5
 Close stmt   
 Quit </span>

<span style="font-size:18px;">DriverManager.getConnection("jdbc:mysql://localhost:3306/prepare_stmt_test?user=root&password=root&useServerPrepStmts=false")</span>

<span style="font-size:18px;">Query    SET NAMES utf8
Query    SET character_set_results = NULL
Query    SHOW VARIABLES
Query    SHOW WARNINGS
Query    SHOW COLLATION
Query    SET autocommit=1
Query    select * from user where id= 5
Quit   </span>

       果然，日誌沒有了prepare這一行，說明MySQL沒有進行預編譯。這意味著useServerPrepStmts這個引數是起效的，且預設值為true。
       最後意識到useServerPrepStmts這個引數是JDBC的連線引數，這說明此問題與JDBC驅動程式可能有關係。開啟MySQL官網，發現線上的官方文件很強大，支援全文檢索，於是我將“useServerPrepStmts”做為關鍵字，搜尋出了一些資訊，原文如下：

Important change: Due to a number ofissues with the use of server-side prepared statements, Connector/J5.0.5 

has disabled their use by default. The disabling of server-side prepared statements does not affect the operation of the connector in any way.To enable server-side prepared statements, add the following configuration property to your connectorstring:
useServerPrepStmts=true
The default value of thisproperty is false (that is,Connector/J does not use server-side prepared statements)

       這段文字說，Connector/J在5.0.5以後的版本，預設useServerPrepStmts引數為false，Connector/J就是我們熟知的JDBC驅動程式。看來，如果我們的驅動程式為5.0.5或之後的版本，想啟用mysql預編譯，就必須設定useServerPrepStmts=true。我的JDBC驅動用的是5.0.3，這個版本的useServerPrepStmts引數預設值是true。於是我將Java工程中的JDBC驅動程式替換為5.0.8的版本，去掉程式碼裡JDBC連線中的useServerPrepStmts引數，再執行，發現mysql_log.txt的日誌列印如下：

<span style="font-size:18px;"> Query    SHOW SESSIONVARIABLES
 Query    SHOW WARNINGS
 Query    SHOW COLLATION
 Query    SET NAMES utf8
 Query    SET character_set_results = NULL
 Query    SET autocommit=1
 Query    select * from user where id= 5
 Quit</span>

預編譯是否能有效提升執行SQL的效能？

<span style="font-size:18px;">public static void main(String []a) throws Exception{  
      String sql = "select * from user whereid = ?";  
      Class.forName("com.mysql.jdbc.Driver");  
      Connection conn = null;  
      try{  
	      conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/studb?user=root&password=root&useServerPrepStmts=true");  
	      PreparedStatement stmt = conn.prepareStatement(sql);  
	      stmt.setString(1,5);  
	      ResultSet rs1 = stmt.executeQuery(); //第一次執行  
	      s1.close();  
	      stmt.setString(1,9);  
	      ResultSet rs2 = stmt.executeQuery(); //第二次執行  
	      rs2.close();  
	      stmt.close();  
      }catch(Exception e){  
          e.printStackTrace();  
      }finally{  
          conn.close();  
      }  
}   </span>

<span style="font-size:18px;">Query    SHOW SESSION VARIABLES
Query    SHOW WARNINGS
Query    SHOW COLLATION
Query    SET NAMES utf8
Query    SET character_set_results = NULL
Query    SET autocommit=1
Prepare   select * from userwhere id = ?
Execute   select * from user where id = 5
Execute   select * from user where id = 9
Close stmt   
Quit</span>

<span style="font-size:18px;">public static void main(String []a) throws Exception{  
	String sql = "select * from userwhere id = ?";  
	Class.forName("com.mysql.jdbc.Driver");  
	Connection conn = null;  
	try{  
		conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/studb?user=root&password=root&useServerPrepStmts=true");  
		PreparedStatement stmt = conn.prepareStatement(sql);  
		stmt.setString(1,5);  
		ResultSet rs1 = stmt.executeQuery(); //第一次執行  
		rs1.close();  
		stmt.close();
		stmt = conn.prepareStatement(sql); //重新獲取一個statement  
		stmt.setString(1,9);  
		ResultSet rs2 = stmt.executeQuery(); //第二次執行  
		rs2.close();  
		stmt.close();  
	}catch(Exception e){  
		e.printStackTrace();  
	}finally{  
		conn.close();  
	}  
}  </span>

<span style="font-size:18px;">Query    SHOW SESSION VARIABLES
Query    SHOW WARNINGS
Query    SHOW COLLATION
Query    SET NAMES utf8
Query    SET character_set_results = NULL
Query    SET autocommit=1
Prepare   select * from user where id=?
Execute   select * from user where id= 5
Close stmt   
Prepare   select *from user where id = ?
Execute   select * from user where id = 9
Close stmt   
Quit </span>

<span style="font-size:18px;">conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/prepare_stmt_test?user=root&password=root&useServerPrepStmts=true&cachePrepStmts=true");</span>

<span style="font-size:18px;">Prepare  select * from user where id = ?
Execute  select * from user where id = 5
Execute  select * from user where id = 9</span>

<span style="font-size:18px;">12321，12173，12159，12132，12604，12349，12621，12356，12899，12287</span>

<span style="font-size:18px;"> 21349，22860，27237，26848，27772，28100，23114，22897，20010，23211</span>

<span style="font-size:18px;">8732，8655，8678，9693，8624，9874，8444，9660，8607，8780</span>

四、預編譯JDBC驅動原始碼剖析