Method for_func

src/expr/src/interpret.rs:558–617

Returns the special-case handling for a particular function, if it exists.

(func: &UnaryFunc)

impl SpecialUnary {
    /// Returns the special-case handling for a particular function, if it exists.
    fn for_func(func: &UnaryFunc) -> Option<SpecialUnary> {
        /// Eager in the same sense as `func.rs` uses the term; this assumes that
        /// nulls and errors propagate up, and we only need to define the behaviour
        /// on values.
        fn eagerly<'b>(
            spec: ResultSpec<'b>,
            value_fn: impl FnOnce(Values<'b>) -> ResultSpec<'b>,
        ) -> ResultSpec<'b> {
            let result = match spec.values {
                Values::Empty => ResultSpec::nothing(),
                other => value_fn(other),
            };
            ResultSpec {
                fallible: spec.fallible || result.fallible,
                nullable: spec.nullable || result.nullable,
                values: result.values,
            }
        }
        match func {
            UnaryFunc::TryParseMonotonicIso8601Timestamp(_) => Some(SpecialUnary {
                map_fn: |specs, range| {
                    let expr = MirScalarExpr::CallUnary {
                        func: UnaryFunc::TryParseMonotonicIso8601Timestamp(
                            crate::func::TryParseMonotonicIso8601Timestamp,
                        ),
                        expr: Box::new(MirScalarExpr::column(0)),
                    };
                    let eval = |d| specs.eval_result(expr.eval(&[d], specs.arena));

                    eagerly(range, |values| {
                        match values {
                            Values::Within(a, b) if a == b => eval(a),
                            Values::Within(a, b) => {
                                let spec = eval(a).union(eval(b));
                                let values_spec = if spec.nullable {
                                    // At least one of the endpoints of the range wasn't a valid
                                    // timestamp. We can't compute a precise range in this case.
                                    // If we used the general is_monotone handling, that code would
                                    // incorrectly assume the whole range mapped to null if each
                                    // endpoint did.
                                    ResultSpec::value_all()
                                } else {
                                    spec
                                };
                                // A range of strings will always contain strings that don't parse
                                // as timestamps - so unlike the general case, we'll assume null
                                // is present in every range of output values.
                                values_spec.union(ResultSpec::null())
                            }
                            // Otherwise, assume the worst: this function may return either a valid
                            // value or null.
                            _ => ResultSpec::any_infallible(),
                        }
                    })
                },
                pushdownable: true,
            }),
            _ => None,