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type_union_coercion

datafusion_expr::binary

Function type_union_coercion 

Source 
pub fn type_union_coercion(
    lhs_type: &DataType,
    rhs_type: &DataType,
) -> Option<DataType>
Expand description

Coerce lhs_type and rhs_type to a common type for type unification contexts — where two values must be brought to a common type but are not being compared. Examples: UNION, CASE THEN/ELSE branches, NVL2. For other contexts, comparison_coercion should typically be used instead.

The intuition is that we try to find the “widest” type that can represent all values from both sides. When one side is a string and the other is numeric, this prefers strings because every number has a textual representation but not every string can be parsed as a number (e.g., SELECT 1 UNION SELECT 'a' coerces both sides to a string). This is in contrast to comparison_coercion, which prefers numeric types so that ordering and equality follow numeric semantics.