Function collect_subquery_cols

datafusion/expr/src/utils.rs:1323–1338

Determine the set of [`Column`]s produced by the subquery.

(
    exprs: &[Expr],
    subquery_schema: &DFSchema,
)

/// Determine the set of [`Column`]s produced by the subquery.
pub fn collect_subquery_cols(
    exprs: &[Expr],
    subquery_schema: &DFSchema,
) -> Result<BTreeSet<Column>> {
    exprs.iter().try_fold(BTreeSet::new(), |mut cols, expr| {
        let mut using_cols: Vec<Column> = vec![];
        for col in expr.column_refs().into_iter() {
            if subquery_schema.has_column(col) {
                using_cols.push(col.clone());
            }
        }

        cols.extend(using_cols);
        Result::<_>::Ok(cols)
    })
}

#[cfg(test)]
mod tests {