Index on map key

Is there a way to create an Index with @Index on the key of a map?

Indexes on map keys as well as queries on map keys are currently not supported.

But as a workaround you can use a collection of embedded objects, as demonstrated below:

package test;


import java.util.*;

import javax.jdo.annotations.Index;
import javax.persistence.*;


public class T1255 {

    public static void main(String[] args) {
        EntityManagerFactory emf =
            Persistence.createEntityManagerFactory(
                "objectdb:test.tmp;drop");
       
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        for (int i = 0; i < 100000; i++) {
            MyEntry entry = new MyEntry("k" + i, "v" + i);
            em.persist(new MyEntity(Collections.singletonList(entry)));
        }
        em.getTransaction().commit();
        em.close();

        em = emf.createEntityManager();
        long ms = System.currentTimeMillis();
        Query query = em.createQuery(
            "SELECT o FROM MyEntity o JOIN o.map e WHERE e.key = 'k50000'");
        List resultList = query.getResultList();
        ms = System.currentTimeMillis() - ms;
        System.out.println("Result: " + resultList);
        System.out.println("Time: " + ms + "ms");
        em.close();
       
        emf.close();
    }

    @Entity
    @Index(members={"map.key"})
    public static class MyEntity {
        MyEntity(Collection<MyEntry> map) {
            this.map = map;
        }
        Collection<MyEntry> map;
        @Override
        public String toString() {
            return map.toString();
        }
    }
   
    @Embeddable
    public static class MyEntry {
        String key;
        String value;
        public MyEntry(String key, String value) {
            this.key = key;
            this.value = value;
        }
        @Override
        public String toString() {
            return key + ":" + value;
        }
    }
}

In this example, the field map is implemented as Collection<MyEntry>, i.e. a collection of a key-value embeddable objects.

An index on the key is defined using @Index(members={"map.key"}).

Running this example with and without the index definition shows that performance with index is better.