Function sql::plan::typeconv::guess_best_common_type

pub fn guess_best_common_type(
    types: &[Option<ScalarType>], 
    type_hint: Option<&ScalarType>
) -> Option<ScalarType>

Guesses the most-common type among a set of ScalarTypes that all members can be cast to. Returns None if a common type cannot be deduced.

The returned type is not guaranteed to be accurate because we ignore type categories, e.g. on input [ScalarType::Date, ScalarType::Int32], will guess that Date is the common type.

However, if there is a common type among the input, it will correctly determine it, i.e. returns false positives but never false negatives.

The types parameter is meant to represent the types inferred from a Vec<CoercibleScalarExpr>. If no known types are present in the types parameter, it will try to use a provided type hint, instead.

Note that this function implements the type-determination components of Postgres’ “UNION, CASE, and Related Constructs” type conversion.

Type hints

Some types contain embedded values, e.g. ScalarType::Numeric contains the values’ scales. When choosing a common type, and the type_hint is of the type chosen, you should guess it because the given type hint can contain a distinct embedded value necessary to retain type invariants.