presto-main/src/main/java/io/prestosql/GroupByHashPageIndexerFactory.java/GroupByHashPageIndexerFactory/createPageIndexer
class GroupByHashPageIndexerFactory:    @Override
    public PageIndexer createPageIndexer(List<? extends Type> types)
    {
        if (types.isEmpty()) {
            return new NoHashPageIndexer();
        }
        return new GroupByHashPageIndexer(types, joinCompiler);
    }