Verify that the variables of a grouped DESCRIBE query are grouped,
as for the projection of queryProjectionRespectsGrouping.
DESCRIBE * is not checked, since toAlgebra expands it to the variables in scope,
which are the grouped ones in a grouped query (see getGroupedVariables).
Verify that the variables of a grouped DESCRIBE query are grouped, as for the projection of queryProjectionRespectsGrouping. DESCRIBE * is not checked, since toAlgebra expands it to the variables in scope, which are the grouped ones in a grouped query (see getGroupedVariables).