'Prepare a table with extra columns that will appear in the merged
table'

def _compute_reduced_extra_data(self, var_extra_data):

    domain = self.data.domain
    extra_domain = self.extra_data.domain
    all_vars = set(chain(domain.variables, domain.metas))
    if (self.merging != MergeType.OUTER_JOIN):
        all_vars.add(var_extra_data)
    extra_vars = chain(extra_domain.variables, extra_domain.metas)
    return self.extra_data[:, [var for var in extra_vars if (var not in all_vars)]]
