MAKE IT SO: Java DB Connections & Transactions

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Large JDBC ResultSets

A topic which you will encounter sooner or later in real-life: You want to query a bigger table, let’s say millions of rows and your query will return tens of thousands of them.

You can of course load the whole resultset into memory at once, providing you have enough. Or you do not query your database in one go, but execute several "LIMIT"-like sql queries.

But another viable option is to use the setter "setFetchSize" on your JDBC statement - if your jdbc driver supports it properly (MySQL and H2 for example don’t and pretty much ignore it, Oracle supports it just fine).

When you use setFetchSize in combination with an Oracle db, you will get a real server side cursor and the jdbc driver will make multiple round-trips to the database, but only fetch as many rows as you specified each round-trip. This can be a huge memory saver!

Our example below shows you the correct usage, but unfortunately H2 will ignore setting the fetchSize. Nevertheless you should know about this setting.

Create This .java File And Run It

package part_02_additional.code;

import org.h2.jdbcx.JdbcDataSource;
import org.jdbcdslog.JDBCDSLogException;
import org.junit.Before;
import org.junit.Test;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

import static;

 * @author Marco Behler <>
public class LargeResultSetExercise {

    private DataSource ds;

    public void setUp() {
        ds = getDataSource();

        try (Connection connection = ds.getConnection()) {
        } catch (SQLException e) {

    public void exercise() throws JDBCDSLogException {
        /* this would register it with a JNDI service. ignore for now
        Context ctx = new InitialContext();
        ctx.bind("jdbc/datasource", ds);*/
        System.out.println("We are about to load thousands of records...");
        try (Connection connection = ds.getConnection()) {
            // for fetchSize below to work properly, you have to make
            // sure to start a tx/setAutocommit to false

            Statement statement = connection.createStatement();

            // we only want to query the server for batches of 1000 !
            // but make sure to read your JDBC driver documentation
            // regarding this property. your driver is allowed to ignore
            // it!

            ResultSet results = statement.executeQuery(
                    "SELECT my_magic_number from numbers");
            while ( {
                int number = results.getInt("my_magic_number");
            // these two would also be automatically closed once you close
            // the connection
        } catch (SQLException e) {

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Console Output After Running The Test
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