Method rewrite_inputs_from_schema

datafusion/expr/src/logical_plan/plan.rs:2956–2985

When constructing a `UNION BY NAME`, we need to wrap inputs in an additional `Projection` to account for absence of columns in input schemas or differing projection orders.

(
        schema: &Arc<DFSchema>,
        inputs: Vec<Arc<LogicalPlan>>,
    )

    /// in an additional `Projection` to account for absence of columns
    /// in input schemas or differing projection orders.
    fn rewrite_inputs_from_schema(
        schema: &Arc<DFSchema>,
        inputs: Vec<Arc<LogicalPlan>>,
    ) -> Result<Vec<Arc<LogicalPlan>>> {
        let schema_width = schema.iter().count();
        let mut wrapped_inputs = Vec::with_capacity(inputs.len());
        for input in inputs {
            // Any columns that exist within the derived schema but do not exist
            // within an input's schema should be replaced with `NULL` aliased
            // to the appropriate column in the derived schema.
            let mut expr = Vec::with_capacity(schema_width);
            for column in schema.columns() {
                if input
                    .schema()
                    .has_column_with_unqualified_name(column.name())
                {
                    expr.push(Expr::Column(column));
                } else {
                    expr.push(
                        Expr::Literal(ScalarValue::Null, None).alias(column.name()),
                    );
                }
            }
            wrapped_inputs.push(Arc::new(LogicalPlan::Projection(
                Projection::try_new_with_schema(expr, input, Arc::clone(schema))?,
            )));
        }

        Ok(wrapped_inputs)
    }

    /// Constructs new Union instance deriving schema from inputs.
    ///