Function select_impl

src/sql/src/func.rs:998–1077

Selects the best implementation given the provided `args` using a process similar to [PostgreSQL's parser][pgparser], and returns the `ScalarExpr` to invoke that function. Inline comments prefixed with number are taken from the "Function Type Resolution" section of the aforelinked page. # Errors - When the provided arguments are not valid for any implementation, e.g. cannot be converted to the a

(
    ecx: &ExprContext,
    spec: FuncSpec,
    impls: &[FuncImpl<R>],
    args: Vec<CoercibleScalarExpr>,
    order_by: Vec<ColumnOrder>,
)

///
/// [pgparser]: https://www.postgresql.org/docs/current/typeconv-oper.html
pub fn select_impl<R>(
    ecx: &ExprContext,
    spec: FuncSpec,
    impls: &[FuncImpl<R>],
    args: Vec<CoercibleScalarExpr>,
    order_by: Vec<ColumnOrder>,
) -> Result<R, PlanError>
where
    R: fmt::Debug,
{
    let name = spec.to_string();
    let ecx = &ecx.with_name(&name);
    let mut types: Vec<_> = args.iter().map(|e| ecx.scalar_type(e)).collect();

    // PostgreSQL force coerces all record types before function selection. We
    // may want to do something smarter in the future (e.g., a function that
    // accepts multiple `RecordAny` parameters should perhaps coerce to the
    // result of calling `guess_best_common_type` on all those parameters), but
    // for now we just directly match PostgreSQL's behavior.
    for ty in &mut types {
        ty.force_coerced_if_record();
    }

    // 4.a. Discard candidate functions for which the input types do not
    // match and cannot be converted (using an implicit conversion) to
    // match. unknown literals are assumed to be convertible to anything for
    // this purpose.
    let impls: Vec<_> = impls
        .iter()
        .filter(|i| i.params.matches_argtypes(ecx, &types))
        .collect();

    let f = find_match(ecx, &types, impls).map_err(|candidates| {
        let arg_types: Vec<_> = types
            .into_iter()
            .map(|ty| match ty {
                // This will be used in error msgs, therefore we call with `postgres_compat` false.
                CoercibleScalarType::Coerced(ty) => ecx.humanize_sql_scalar_type(&ty, false),
                CoercibleScalarType::Record(_) => "record".to_string(),
                CoercibleScalarType::Uncoerced => "unknown".to_string(),
            })
            .collect();

        if candidates == 0 {
            match spec {
                FuncSpec::Func(name) => PlanError::UnknownFunction {
                    name: ecx
                        .qcx
                        .scx
                        .humanize_resolved_name(name)
                        .expect("resolved to object")
                        .to_string(),
                    arg_types,
                },
                FuncSpec::Op(name) => PlanError::UnknownOperator {
                    name: name.to_string(),
                    arg_types,
                },