void table_base::row_interface::get_fact(table_fact & result) const {
result.reset();
unsigned n=size();
for(unsigned i=0; i<n; i++) {
result.push_back((*this)[i]);
}
}
void group_by(table_fact const& in, table_fact& out) {
out.reset();
for (unsigned i = 0; i < m_group_by_cols.size(); ++i) {
out.push_back(in[m_group_by_cols[i]]);
}
}