Function record_fields 

pub fn record_fields(
    expr: &Expr,
    levels: &[&[Relation]],
) -> Result<Option<Vec<Column>>>

get_expr_result_tupdesc and expandRecordVariable: the fields of an expression of type record. A call of a function gives its OUT parameters. The whole row of a join gives the columns of the join. A column of a subquery in FROM gives the fields of its expression in the subquery, also through more subqueries. levels has the relations of the query of the expression, then the relations of each query outside it. None when the fields are not known before the query runs.

Errors

The error of a polymorphic type that the arguments of a call do not resolve.