Method sql_function_to_expr

datafusion/sql/src/expr/function.rs:230–913

(
        &self,
        function: SQLFunction,
        schema: &DFSchema,
        planner_context: &mut PlannerContext,
    )

impl<S: ContextProvider> SqlToRel<'_, S> {
    pub(super) fn sql_function_to_expr(
        &self,
        function: SQLFunction,
        schema: &DFSchema,
        planner_context: &mut PlannerContext,
    ) -> Result<Expr> {
        let function_args = FunctionArgs::try_new(function)?;
        let FunctionArgs {
            name: object_name,
            args,
            order_by,
            over,
            filter,
            null_treatment,
            distinct,
            within_group,
            function_without_parentheses,
        } = function_args;

        if over.is_some() && !within_group.is_empty() {
            return plan_err!(
                "OVER and WITHIN GROUP clause cannot be used together. \
                OVER is for window functions, whereas WITHIN GROUP is for ordered set aggregate functions"
            );
        }

        if !order_by.is_empty() && !within_group.is_empty() {
            return plan_err!(
                "ORDER BY and WITHIN GROUP clauses cannot be used together in the same aggregate function"
            );
        }

        // If function is a window function (it has an OVER clause),
        // it shouldn't have ordering requirement as function argument
        // required ordering should be defined in OVER clause.
        let is_function_window = over.is_some();
        let sql_parser_span = object_name.0[0].span();
        let name = if object_name.0.len() > 1 {
            // DF doesn't handle compound identifiers
            // (e.g. "foo.bar") for function names yet
            object_name.to_string()
        } else {
            match object_name.0[0].as_ident() {
                Some(ident) => crate::utils::normalize_ident(ident.clone()),
                None => {
                    return plan_err!(
                        "Expected an identifier in function name, but found {:?}",
                        object_name.0[0]
                    );
                }
            }
        };

        // handle make_map and map functions
        // make_map always uses plan_make_map: make_map(k1, v1, k2, v2, ...)
        // map has 2 syntaxes:
        //     1. map([keys], [values]) - two arrays that get zipped
        //     2. map(k1, v1, k2, v2, ...) - variadic pairs (uses plan_make_map)