Java向PostgreSQL发送prepared statement 与 libpq 向PostgreSQL发送prepared statement之比较：

Java 代码，在数据库端，并没有当成 prepared statetment 被处理。

C代码通过libpq 访问数据库端，被当成了 prepared statement 处理。也许是因PostgreSQL对JDBC的支持毕竟是后期出现的：

下面看代码和运行结果：

Java 代码：

import java.sql.*;
 
public class Test01 {
 
        public static void main(String argsv[]){
        try
         {
           Class.forName("org.postgresql.Driver").newInstance();
           String url = "jdbc:postgresql://localhost:5432/postgres" ;
   
           Connection con = DriverManager.getConnection(url,"postgres","postgres" );
          
           ///Phase 1:-------------Select data from table-----------------------
 
          
           System.out.println("Phase 1------------------------start");
          
           String strsql = " select * from customers where cust_id = ?";
           PreparedStatement pst=con.prepareStatement(strsql);
 
           pst.setInt(1,3); //find the customer with cust_id of 3.
          
           ResultSet rs = pst.executeQuery();
          
           while (rs.next())
            {
               System.out.print("cust_id:"+rs.getInt( "cust_id"));
               System.out.println("...cust_name:"+rs.getString( "cust_name" ));
           }
           System.out.println("Phase 1------------------------end
");
          
          
          
           ///Phase 2:-------------Use connection again,to select data from data dictionary-----------------------
          
           System.out.println("Phase 2------------------------start");
          
           strsql = "select * from pg_prepared_statements";
           pst=con.prepareStatement(strsql);          
          
           rs = pst.executeQuery();
          
           while (rs.next())
           {
              System.out.println("statement:"+rs.getString( "statement"));
           }          
           System.out.println("Phase 2------------------------end
");          
          
           ///Phase 3:-------------Use connection again,to select data from table-----------------------
 
           System.out.println("Phase 3------------------------start");          
           strsql = "select * from customers";
           pst=con.prepareStatement(strsql);          
          
           rs = pst.executeQuery();
          
           while (rs.next())
           {
              System.out.print("cust_id:"+rs.getInt( "cust_id"));
              System.out.println("...cust_name:"+rs.getString( "cust_name" ));
          }           
         
          System.out.println("Phase 3------------------------end
");           
          
          rs.close();          
          pst.close();
          con.close();
          
       }
        catch (Exception ee)
        {
           System.out.print(ee.getMessage());
       }
        }
 
}

运行：

Phase 1------------------------start
cust_id:3...cust_name:Taylor
Phase 1------------------------end
 
Phase 2------------------------start
Phase 2------------------------end
 
Phase 3------------------------start
cust_id:1...cust_name:Smith
cust_id:2...cust_name:Brown
cust_id:3...cust_name:Taylor
Phase 3------------------------end

C 代码：

[root@lex tst]# cat testprepared.c
/*
 * testlibpq.c
 *  Test the C version of LIBPQ, the POSTGRES frontend library.
 */
#include <stdio.h>
#include <stdlib.h>
#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
 PQfinish(conn);
 exit(EXIT_SUCCESS);
}

int
main()
{
 int   nFields;
 int   i,
       j;

#ifdef DEBUG
 FILE    *debug;
#endif  /* DEBUG */

 ///////////////////////////////////////////////////////////////////////////////
 ///Step1: making connection


 PGconn    *conn;
 PGresult   *res;

 const char *conninfo="postgresql://postgres:postgres@localhost:5432/postgres";

 /* make a connection to the database */
 conn = PQconnectdb(conninfo);

 /* check to see that the backend connection was successfully made */
 if (PQstatus(conn) == CONNECTION_BAD)
 {
  fprintf(stderr, "Connection to database failed.
");
  fprintf(stderr, "%s", PQerrorMessage(conn));
  exit_nicely(conn);
 }

#ifdef DEBUG
 debug = fopen("/tmp/trace.out", "w");
 PQtrace(conn, debug);
#endif  /* DEBUG */
  
 ////////////////////////////////////////////////////////////////////////////////////
 ///Step 2, activating prepared statement

 /* start a transaction block */
 res = PQexec(conn, "BEGIN");
 if (PQresultStatus(res) != PGRES_COMMAND_OK)
 {
  fprintf(stderr, "BEGIN command failed
");
  PQclear(res);
  exit_nicely(conn);
 }

 PQclear(res);

 ////////////////////////////////////////////////////////////////////////////////////
 ///Step 2, activating prepared statement
 const char *stmt_name = "test_stmt";
 const char *stmt = "select * from customers where cust_id=$1";

 Oid param_types[1];
 param_types[0] = 0; ///let db to judge it.

 res = PQprepare(conn, stmt_name, stmt,1,param_types);
 if (PQresultStatus(res) != PGRES_COMMAND_OK)
 {
  fprintf(stderr, "PQprepare failed
");
  PQclear(res);
  exit_nicely(conn);
 }

 PQclear(res);

 const char* custid = "3";
 const char* param_values[1];
 param_values[0] =custid;

 int param_lengths[1];
 param_lengths[0] = 1;

 int param_formats[1];
 param_formats[0] = 0;
 
 res = PQexecPrepared(conn, stmt_name, 1, param_values, param_lengths,
                        param_formats, 0);

 if (PQresultStatus(res) != PGRES_TUPLES_OK)
 {
  fprintf(stderr, "PQexecPrepared statement didn't return tuples properly
");
  PQclear(res);
  exit_nicely(conn);
 }

 /* print out the attribute names */
 nFields = PQnfields(res);
 for (i = 0; i < nFields; i++)
  printf("%-15s", PQfname(res, i));

 printf("

");

 /* print out the instances */
 for (i = 0; i < PQntuples(res); i++)
 {
  for (j = 0; j < nFields; j++)
   printf("%-15s", PQgetvalue(res, i, j));
  printf("
");
 }

 PQclear(res);

 /* end the transaction */
 res = PQexec(conn, "END");
 PQclear(res);

 ////////////////////////////////////////////////////////////////////////////////////
 ///Step 3, looking for cached status of prepared statements

 /* start a transaction block */
 res = PQexec(conn, "BEGIN");
 if (PQresultStatus(res) != PGRES_COMMAND_OK)
 {
  fprintf(stderr, "BEGIN command failed
");
  PQclear(res);
  exit_nicely(conn);
 }

 PQclear(res);
 
 /* define cursor */
 res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_prepared_statements");
 if (PQresultStatus(res) != PGRES_COMMAND_OK)
 {
  fprintf(stderr, "DECLARE CURSOR command failed
");
  PQclear(res);
  exit_nicely(conn);
 }
 PQclear(res);

 /* fetch cursor */
 res = PQexec(conn, "FETCH ALL in myportal");
 if (PQresultStatus(res) != PGRES_TUPLES_OK)
 {
  fprintf(stderr, "FETCH ALL command didn't return tuples properly
");
  PQclear(res);
  exit_nicely(conn);
 }

 /* first, print out the attribute names */
 nFields = PQnfields(res);
 for (i = 0; i < nFields; i++)
  printf("%-15s", PQfname(res, i));

 printf("

");

 /* next, print out the instances */
 for (i = 0; i < PQntuples(res); i++)
 {
  for (j = 0; j < nFields; j++)
   printf("%-15s", PQgetvalue(res, i, j));
  printf("
");
 }

 PQclear(res);

 /* close the portal */
 res = PQexec(conn, "CLOSE myportal");
 PQclear(res);

 /* end the transaction */
 res = PQexec(conn, "END");
 PQclear(res);

 ////////////////////////////////////////////////////////////////////////////////////
 ///Step 4, close the connection

 /* close the connection to the database and cleanup */
 PQfinish(conn);

#ifdef DEBUG
 fclose(debug);
#endif  /* DEBUG */

 return 0;
}
[root@lex tst]# 

编译与运行：

[root@lex tst]# gcc -c -I/usr/local/pgsql/include testprepared.c
[root@lex tst]# gcc -o testprepared testprepared.o -L/usr/local/pgsql/lib -lpq

[root@lex tst]# ./testprepared
cust_id        cust_name      

3              Taylor         
name           statement      　　　　　　　　　　　　　　　　prepare_time   　　　　　　parameter_typesfrom_sql       

test_stmt      select * from customers where cust_id=$   12013-06-17 16:28:31.70059+08{integer}      f 

 追记：

为了解决Java中处理prepared statement 的问题。在Postgresql社区提问，得到的回答是：

采用threadshold：这个算是PostgreSQL自己的扩展。修改代码如下：

[root@lex src]# cat Test01.java
import java.sql.*;

public class Test01 {

        public static void main(String argsv[]) {
                try {
                        Class.forName("org.postgresql.Driver").newInstance();
                        String url = "jdbc:postgresql://localhost:5432/postgres";

                        Connection con = DriverManager.getConnection(url, "postgres",
                                        "postgres");

                        // /Phase 1:-------------Select data from
                        // table-----------------------

                        System.out.println("Phase 1------------------------start");

                        String strsql = " select * from customers where cust_id = ?";
                        PreparedStatement pst = con.prepareStatement(strsql);

                        org.postgresql.PGStatement pgt = (org.postgresql.PGStatement)pst;

                        pgt.setPrepareThreshold(1);

                        pst.setInt(1, 3); // find the customer with cust_id of 3.

                        ResultSet rs = pst.executeQuery();

                        while (rs.next()) {
                                System.out.print("cust_id:" + rs.getInt("cust_id"));
                                System.out.println("...cust_name:" + rs.getString("cust_name"));
                        }
                        System.out.println("Phase 1------------------------end
");

                        // /Phase 2:-------------Use connection again,to select data from
                        // data dictionary-----------------------

                        System.out.println("Phase 2------------------------start");

                        strsql = "select * from pg_prepared_statements";
                        pst = con.prepareStatement(strsql);

                        rs = pst.executeQuery();

                        while (rs.next()) {
                                System.out.println("statement:" + rs.getString("statement"));
                        }
                        System.out.println("Phase 2------------------------end
");

                        // /Phase 3:-------------Use connection again,to select data from
                        // table-----------------------

                        System.out.println("Phase 3------------------------start");
                        strsql = "select * from customers";
                        pst = con.prepareStatement(strsql);

                        rs = pst.executeQuery();

                        while (rs.next()) {
                                System.out.print("cust_id:" + rs.getInt("cust_id"));
                                System.out.println("...cust_name:" + rs.getString("cust_name"));
                        }

                        System.out.println("Phase 3------------------------end
");

                        rs.close();
                        pst.close();
                        con.close();

                } catch (Exception ee) {
                        System.out.print(ee.getMessage());
                }
        }

}
[root@lex src]# 

因为加入了这两句：

org.postgresql.PGStatement pgt = (org.postgresql.PGStatement)pst;

pgt.setPrepareThreshold(1);

所以，得到的运行结果不同了：

Phase 1------------------------start
cust_id:3...cust_name:Taylor
Phase 1------------------------end

Phase 2------------------------start
statement: select * from customers where cust_id = $1
Phase 2------------------------end

Phase 3------------------------start
cust_id:1...cust_name:Smith
cust_id:2...cust_name:Brown
cust_id:3...cust_name:Taylor
Phase 3------------------------end

不过，这么作的目的是什么呢，还是很让人困惑的。

http://jdbc.postgresql.org/documentation/head/server-prepare.html

The PostgreSQL™ server allows clients to compile sql statements that are expected to be reused to avoid the overhead of parsing and planning the statement for every execution. This functionality is available at the SQL level via PREPARE and EXECUTE beginning with server version 7.3, and at the protocol level beginning with server version 7.4, but as Java developers we really just want to use the standard PreparedStatement interface.

There are a number of ways to enable server side prepared statements depending on your application's needs. The general method is to set a threshold for a PreparedStatement. An internal counter keeps track of how many times the statement has been executed and when it reaches the threshold it will start to use server side prepared statements.

这里面最令我困惑的是这句话：

The PostgreSQL™ server allows clients to compile sql statements that are expected to be reused to avoid the overhead of parsing and planning the statement for every execution.

...server allows client to compile sql statements