Berkeley DB Reference Version 5.3.35

BTree Configuration

Allowing Duplicate Records

Setting Comparison Functions

In going through the previous chapters in this book, you may notice that we touch on some topics that are specific to BTree, but we do not cover those topics in any real detail. In this section, we will discuss configuration issues that are unique to BTree.

Specifically, in this section we describe:

Allowing Duplicate Records

BTree databases can contain duplicate records. One record is considered to be a duplicate of another when both records use keys that compare as equal to one another.

By default, keys are compared using a lexicographical comparison, with shorter keys collating higher than longer keys. You can override this default using the DatabaseConfig.setBtreeComparator() method. See the next section for details.

By default, DB databases do not allow duplicate records. As a result, any attempt to write a record that uses a key equal to a previously existing record results in the previously existing record being overwritten by the new record.

Allowing duplicate records is useful if you have a database that contains records keyed by a commonly occurring piece of information. It is frequently necessary to allow duplicate records for secondary databases.

For example, suppose your primary database contained records related to automobiles. You might in this case want to be able to find all the automobiles in the database that are of a particular color, so you would index on the color of the automobile. However, for any given color there will probably be multiple automobiles. Since the index is the secondary key, this means that multiple secondary database records will share the same key, and so the secondary database must support duplicate records.

Sorted Duplicates

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.

Unsorted Duplicates

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:

Configuring a Database to Support 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);
}

Setting Comparison Functions

By default, DB uses a lexicographical comparison function where shorter records collate before longer records. For the majority of cases, this comparison works well and you do not need to manage it in any way.

However, in some situations your application's performance can benefit from setting a custom comparison routine. You can do this either for database keys, or for the data if your database supports sorted duplicate records.

Some of the reasons why you may want to provide a custom sorting function are:

Creating Java Comparators

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://download.oracle.com/javase/1.4.2/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
}