如以下程序所示，
PreparedStatement.executeQuery()始终从服务器检索结果集中的行。该程序还演示了语句获取大小如何影响行检索。如果该语句的默认访存大小为零，
executeQuery()则从服务器检索所有行，然后
ResultSet.next()从内存而不是从服务器检索并返回下一行。（程序甚至可以在执行查询后关闭连接，并且
next()仍然可以遍历所有行。）在提取大小不为零的情况下，
executeQuery()检索第一批行，其数量等于提取大小，并且
ResultSet.next()再次从内存中返回下一行，直到消耗完当前批处理中的所有行，这时它将从服务器中检索下一批行。重复此模式，直到
ResultSet.next()从服务器检索一个空批次（一个包含零行的批次）为止。
的SQL
-- Create table "test" and insert 2,000,000 integers from 1 up to 2,000,000.WITH RECURSIVE t(n) AS( VALUES (1) UNIOn ALL SELECt n+1 FROM t WHERe n < 2000000)SELECt n as valueINTO testFROM t;
爪哇
import java.sql.Connection;import java.sql.DriverManager;import java.sql.ResultSet;import java.sql.SQLException;import java.sql.Statement;import java.util.Date;import java.util.Properties;public class Start{ public static void main( String[] args ) throws InterruptedException, SQLException { try { Class.forName( "org.postgresql.Driver" ); } catch ( ClassNotFoundException e ) { System.out.println( "Where is your JDBC Driver?" ); e.printStackTrace(); return; } System.out.println( "Registered JDBC driver" ); Connection connection = null; try { final String databaseUrl = "jdbc:postgresql://localhost:5483/postgres"; final Properties properties = new Properties(); connection = DriverManager.getConnection( databaseUrl, properties ); connection.setAutoCommit(false); Statement statement = connection.createStatement(); // Default fetch size of 0 does not create a cursor. // Method executeQuery will retrieve all rows in result set. statement.setFetchSize( 0 ); // Fetch size of 1 creates a cursor with batch size of 1. // Method executeQuery will retrieve only 1 row in the result set. //statement.setFetchSize( 1 ); System.out.println( new Date() + ": Before execute query" ); ResultSet result = statement.executeQuery( "select * from test" ); System.out.println( new Date() + ": After execute query" ); System.out.println( new Date() + ": Sleep for 5 s" ); Thread.sleep( 5000 ); System.out.println( new Date() + ": Before process result set" ); while ( result.next() ); System.out.println( new Date() + ": After process result set" ); result.close(); statement.close(); } catch ( SQLException e ) { System.out.println( "Connection failed!" ); e.printStackTrace(); return; } finally { if ( connection != null ) connection.close(); } }}
