Function build_join_schema

datafusion/expr/src/logical_plan/builder.rs:1662–1764

Creates a schema for a join operation. The fields from the left side are first

(
    left: &DFSchema,
    right: &DFSchema,
    join_type: &JoinType,
)

/// Creates a schema for a join operation.
/// The fields from the left side are first
pub fn build_join_schema(
    left: &DFSchema,
    right: &DFSchema,
    join_type: &JoinType,
) -> Result<DFSchema> {
    fn nullify_fields<'a>(
        fields: impl Iterator<Item = (Option<&'a TableReference>, &'a Arc<Field>)>,
    ) -> Vec<(Option<TableReference>, Arc<Field>)> {
        fields
            .map(|(q, f)| {
                // TODO: find a good way to do that
                let field = f.as_ref().clone().with_nullable(true);
                (q.cloned(), Arc::new(field))
            })
            .collect()
    }

    let right_fields = right.iter();
    let left_fields = left.iter();

    let qualified_fields: Vec<(Option<TableReference>, Arc<Field>)> = match join_type {
        JoinType::Inner => {
            // left then right
            let left_fields = left_fields
                .map(|(q, f)| (q.cloned(), Arc::clone(f)))
                .collect::<Vec<_>>();
            let right_fields = right_fields
                .map(|(q, f)| (q.cloned(), Arc::clone(f)))
                .collect::<Vec<_>>();
            left_fields.into_iter().chain(right_fields).collect()
        }
        JoinType::Left => {
            // left then right, right set to nullable in case of not matched scenario
            let left_fields = left_fields
                .map(|(q, f)| (q.cloned(), Arc::clone(f)))
                .collect::<Vec<_>>();
            left_fields
                .into_iter()
                .chain(nullify_fields(right_fields))
                .collect()
        }
        JoinType::Right => {
            // left then right, left set to nullable in case of not matched scenario
            let right_fields = right_fields
                .map(|(q, f)| (q.cloned(), Arc::clone(f)))
                .collect::<Vec<_>>();
            nullify_fields(left_fields)
                .into_iter()
                .chain(right_fields)
                .collect()
        }
        JoinType::Full => {
            // left then right, all set to nullable in case of not matched scenario
            nullify_fields(left_fields)
                .into_iter()
                .chain(nullify_fields(right_fields))
                .collect()
        }