Function resolve_positions_to_exprs

datafusion/sql/src/utils.rs:209–233

Given an expression that's literal int encoding position, lookup the corresponding expression in the select_exprs list, if the index is within the bounds and it is indeed a position literal, otherwise, returns planning error. If input expression is not an int literal, returns expression as-is.

(
    expr: Expr,
    select_exprs: &[Expr],
)

/// otherwise, returns planning error.
/// If input expression is not an int literal, returns expression as-is.
pub(crate) fn resolve_positions_to_exprs(
    expr: Expr,
    select_exprs: &[Expr],
) -> Result<Expr> {
    match expr {
        // sql_expr_to_logical_expr maps number to i64
        // https://github.com/apache/datafusion/blob/8d175c759e17190980f270b5894348dc4cff9bbf/datafusion/src/sql/planner.rs#L882-L887
        Expr::Literal(ScalarValue::Int64(Some(position)), _)
            if position > 0_i64 && position <= select_exprs.len() as i64 =>
        {
            let index = (position - 1) as usize;
            let select_expr = &select_exprs[index];
            Ok(match select_expr {
                Expr::Alias(Alias { expr, .. }) => *expr.clone(),
                _ => select_expr.clone(),
            })
        }
        Expr::Literal(ScalarValue::Int64(Some(position)), _) => plan_err!(
            "Cannot find column with position {} in SELECT clause. Valid columns: 1 to {}",
            position,
            select_exprs.len()
        ),
        _ => Ok(expr),
    }
}

/// Rebuilds an `Expr` with columns that refer to aliases replaced by the
/// alias' underlying `Expr`.