How can you call custom database functions with Hibernate?

Question

If I were to define some function in the database (perhaps Postgres, or any other database):

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Answer 1

As of Hibernate 5, if you don't want to depend on or customize the dialect, you can define a MetadataBuilderInitializer. For example, to use MySQL DATE_ADD with an INTERVAL from HQL, you can define a custom function called date_add_interval:

public class DateAddIntervalMetadataBuilderInitializer
        implements MetadataBuilderInitializer {
    @Override
    public void contribute(MetadataBuilder metadataBuilder,
            StandardServiceRegistry serviceRegistry) {
        metadataBuilder.applySqlFunction("date_add_interval",
            new SQLFunctionTemplate(DateType.INSTANCE,
                "DATE_ADD(?1, INTERVAL ?2 ?3)"));
    }
}

You would also need to put the name of the class in a JAR resource file called META-INF/services/org.hibernate.boot.spi.MetadataBuilderInitializer.

This approach is particularly useful when using Hibernate via a framework such as JPA and/or Spring, where the configuration is performed implicitly by the framework.

Answer 2

If you want to use your custom function in HQL, you'll need to define it in appropriate Dialect

Take a look at PostgreSQLDialect (or any other, really) source, and you'll see a bunch of registerFunction() calls. You'll need to add one more :-) - for your own custom function.

You'll then have to specify your own dialect in Hibernate configuration.