Question

I want to use a standard deviation projection in a query that Im constructing using the criteria API. I can do something simply like this

public class StdDevProjection extends AggregateProjection {

    public StdDevProjection(String propertyName) {
        super("stddev", propertyName);
    }

    public Type[] getTypes(Criteria criteria, CriteriaQuery criteriaQuery)
    throws HibernateException {
        return new Type[] { Hibernate.DOUBLE };
   }

}

and then I can use it with my criteria as:

myCriteriea.setProjection(new StdDevProjection(myproperty));

Thats all good. But my problem is that I use HSQLDB for any db unit tests etc, whereas we use Oracle for deployment. The stddev function works perfectly in oracle, but its not there in HSQLDB. HSQLDB has stddev_pop and stddev_samp . So is there someway I can use a different function based on the dialect.

I maybe can extend the HSQL dialect to register the “stddev” to the appropriate HSQL function, but then im not sure how to use an hsql function in a query constructed using the Criteria API.

Any help would be gret.

Thanks

Answer 1

Using the dialect is the right approach (although I’ve got to say that using different database engines for testing vs deployment seems a bit iffy). You can do the following:

1. Extend HSQL dialect and use registerFunction() to register an appropriate stddev implementation.
2. Override toSqlString() method in your StdDevProjection class and have Dialect render function name.

Something like:

public String toSqlString(Criteria criteria, int loc, CriteriaQuery criteriaQuery) throws HibernateException {
  Dialect dialect = criteriaQuery.getFactory().getDialect();
  SQLFunction function = (SQLFunction) dialect.getFunctions().get(this.aggregate);
  //TODO: throw an exception if function is not registered

  //create function argument array
  List functionArgs = new ArrayList(1);
  functionArgs.add(criteriaQuery.getColumn(criteria, propertyName));

  return new StringBuffer()
    .append(function.render(functionArgs, criteriaQuery.getFactory()))
    .append(" as y").append(loc).append('_')
    .toString();
  }