Support for getting Scalar Values using Native Queries over Cassandra

What are Scalar Values?

Basically, a scalar value is one unit of data. This unit of data can be either a number or a chunk of text.

In relational database, a scalar value can also refer to a column.

Why support them?

Column values are always wrapped in an entity. This makes it difficult for getting values that does not have an entity defined (like COUNT, metadata queries, etc).

Using Scalar Querying, one can overcome this constraint.

Thus, retrieving scalar values using native queries over Cassandra has been facilitated from Kundera-2.17. No entity definition is required for retrieving the values.

Example:

• Define Persistence unit with Cassandra specific details:

	<persistence-unit name="CassandraScalarQueriesTest">
		<provider>com.impetus.kundera.KunderaPersistence</provider>
		<!-- test without entity definition -->
		<exclude-unlisted-classes>true</exclude-unlisted-classes>
		<properties>
			<property name="kundera.nodes" value="localhost" />
			<property name="kundera.port" value="9160" />
			<property name="kundera.keyspace" value="KunderaExamples" />
			<property name="kundera.dialect" value="cassandra" />
			<property name="kundera.client.lookup.class"
				value="com.impetus.client.cassandra.thrift.ThriftClientFactory" />
		</properties>
	</persistence-unit>

• Create EntityManagerFactory and EntityManager

      EntityManagerFactory emf = Persistence.createEntityManagerFactory("CassandraScalarQueriesTest");
      EntityManager entityManager = emf.createEntityManager();

• Use createNativeQuery and get the results

      String selectQuery = "SELECT key, state FROM users";
      Query q = entityManager.createNativeQuery(selectQuery);
      List results = q.getResultList();

results is an ArrayList of records from the database. Each record is a Map<String, Object> where key contains name of the column and value contains corresponding value of the column.

• Sample metadata queries:

      String useNativeSql = "SELECT * FROM system.schema_keyspaces WHERE keyspace_name = 'KunderaExamples'";
      Query q = entityManager.createNativeQuery(useNativeSql);
      results = q.getResultList();

Refer to test case CassandraScalarQueriesTest for more on this.