Duplicate records can be stored in sorted or unsorted order. You can cause DB to automatically sort your duplicate records by setting DatabaseConfig.setSortedDuplicates() to true. Note that this property must be set prior to database creation time and it cannot be changed afterwards.

If sorted duplicates are supported, then the java.util.Comparator implementation identified to DatabaseConfig.setDuplicateComparator() is used to determine the location of the duplicate record in its duplicate set. If no such function is provided, then the default lexicographical comparison is used.

For performance reasons, BTrees should always contain sorted records. (BTrees containing unsorted entries must potentially spend a great deal more time locating an entry than does a BTree that contains sorted entries). That said, DB provides support for suppressing automatic sorting of duplicate records because it may be that your application is inserting records that are already in a sorted order.

That is, if the database is configured to support unsorted duplicates, then the assumption is that your application will manually perform the sorting. In this event, expect to pay a significant performance penalty. Any time you place records into the database in a sort order not know to DB, you will pay a performance penalty

That said, this is how DB behaves when inserting records into a database that supports non-sorted duplicates:

Duplicates support can only be configured at database creation time. You do this by specifying the appropriate DatabaseConfig method before the database is opened for the first time.

The methods that you can use are:

The following code fragment illustrates how to configure a database to support sorted duplicate records:

package db.GettingStarted;

import java.io.FileNotFoundException;

import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseConfig;
import com.sleepycat.db.DatabaseException;
import com.sleepycat.db.DatabaseType;

...

Database myDb = null;

try {
    // Typical configuration settings
    DatabaseConfig myDbConfig = new DatabaseConfig();
    myDbConfig.setType(DatabaseType.BTREE);
    myDbConfig.setAllowCreate(true);

    // Configure for sorted duplicates
    myDbConfig.setSortedDuplicates(true);

   // Open the database
   myDb = new Database("mydb.db", null, myDbConfig);
} catch(DatabaseException dbe) {
    System.err.println("MyDbs: " + dbe.toString());
    System.exit(-1);
} catch(FileNotFoundException fnfe) {
    System.err.println("MyDbs: " + fnfe.toString());
    System.exit(-1);
} 

You set a BTree's key comparator using DatabaseConfig.setBtreeComparator(). You can also set a BTree's duplicate data comparison function using DatabaseConfig.setDuplicateComparator().

If the database already exists when it is opened, the comparator provided to these methods must be the same as that historically used to create the database or corruption can occur.

You override the default comparison function by providing a Java Comparator class to the database. The Java Comparator interface requires you to implement the Comparator.compare() method (see http://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html for details).

DB hands your Comparator.compare() method the byte arrays that you stored in the database. If you know how your data is organized in the byte array, then you can write a comparison routine that directly examines the contents of the arrays. Otherwise, you have to reconstruct your original objects, and then perform the comparison.

For example, suppose you want to perform unicode lexical comparisons instead of UTF-8 byte-by-byte comparisons. Then you could provide a comparator that uses String.compareTo(), which performs a Unicode comparison of two strings (note that for single-byte roman characters, Unicode comparison and UTF-8 byte-by-byte comparisons are identical – this is something you would only want to do if you were using multibyte unicode characters with DB). In this case, your comparator would look like the following:

package db.GettingStarted;

import java.util.Comparator;

public class MyDataComparator implements Comparator {

    public MyDataComparator() {}

    public int compare(Object d1, Object d2) {

        byte[] b1 = (byte[])d1;
        byte[] b2 = (byte[])d2;

        String s1 = new String(b1);
        String s2 = new String(b2);
        return s1.compareTo(s2);
    }
} 

To use this comparator:

package db.GettingStarted;

import java.io.FileNotFoundException;
import java.util.Comparator;
import com.sleepycat.db.Database;
import com.sleepycat.db.DatabaseConfig;
import com.sleepycat.db.DatabaseException;

...

Database myDatabase = null;
try {
    // Get the database configuration object
    DatabaseConfig myDbConfig = new DatabaseConfig();
    myDbConfig.setAllowCreate(true);

    // Set the duplicate comparator class
    MyDataComparator mdc = new MyDataComparator();
    myDbConfig.setDuplicateComparator(mdc);

    // Open the database that you will use to store your data
    myDbConfig.setSortedDuplicates(true);
    myDatabase = new Database("myDb", null, myDbConfig);
} catch (DatabaseException dbe) {
    // Exception handling goes here
} catch (FileNotFoundException fnfe) {
    // Exception handling goes here
}