How to iterate a query with a large table

Question

Using Java with JDBC
Querying data from a postgres database that stores ship information.
I'm creating a vessel object from the data and adding coordinate data to the vessel object's coordinate list.

    ResultSet rs = stmt.executeQuery("SELECT mmsi, report_timestamp, position_geom, ST_X(position_geom) AS Long, "+ 
                "ST_Y(position_geom) AS Lat FROM reports3 WHERE position_geom IS NOT NULL ORDER by report_timestamp ASC"); 

 TreeMap <Long, Vessel> vessels = new TreeMap<Long, Vessel>(); 
                    long startTime2 = System.nanoTime();

                    while(rs.next()){
                        long mmsi = rs.getLong("mmsi");
                        java.util.Date time = rs.getTime("report_timestamp"); 
                        double longitude = rs.getDouble("Long");
                        double latitude = rs.getDouble("Lat");
                        Coordinate coordinate = new Coordinate(longitude, latitude, time);
                        Vessel vessel = new Vessel(mmsi); 

                        if(!vessels.containsKey(mmsi)) { //if vessel is not present in vessels
                            vessel.addCoor(coordinate);
                            vessels.put(mmsi, vessel);
                        }
                        else { //if vessel is already in vessels 
                            vessels.get(mmsi).addCoor(coordinate);
                        }
                    }

So I'm working with a table that contains a billion rows and it'll be impossible to store that many vessel objects on my machine.

I'm wondering how to iterate through the database by querying 1,000,000 rows at a time with each iteration I'll have enough information to run some methods and store important keys, then clear my vessels tree map and run the next 1,000,000 rows.

Answer 1

Try to use setFetchSize(int) method. More about it in this link