Method validate_set_operation_compatibility

graphlite/src/exec/executor.rs:7561–7602

Validate that two query results have compatible schemas for set operations

(
        &self,
        left: &QueryResult,
        right: &QueryResult,
    )

    /// Validate that two query results have compatible schemas for set operations
    fn validate_set_operation_compatibility(
        &self,
        left: &QueryResult,
        right: &QueryResult,
    ) -> Result<(), ExecutionError> {
        // Handle special case where one side has no results but should have the same number of variables
        // This happens when a query has a RETURN clause but matches no rows

        // If both sides have no variables, that's fine (both are empty queries)
        if left.variables.is_empty() && right.variables.is_empty() {
            return Ok(());
        }

        // If one side has no variables but the other has variables, use the non-empty side's variables
        // This handles cases where one query returns no rows but still has a RETURN clause
        let effective_left_count =
            if left.variables.is_empty() && left.rows.is_empty() && !right.variables.is_empty() {
                right.variables.len() // Assume same variable count as right side
            } else {
                left.variables.len()
            };

        let effective_right_count =
            if right.variables.is_empty() && right.rows.is_empty() && !left.variables.is_empty() {
                left.variables.len() // Assume same variable count as left side
            } else {
                right.variables.len()
            };

        if effective_left_count != effective_right_count {
            return Err(ExecutionError::RuntimeError(format!(
                "Set operation variable count mismatch: left has {} variables, right has {} variables",
                effective_left_count,
                effective_right_count
            )));
        }

        // Note: In a more complete implementation, we might also check variable types
        // For now, we just ensure the same number of variables in the RETURN clause

        Ok(())
    }

    /// Convert rows to positional format for set operations
    fn convert_to_positional_rows(&self, rows: Vec<Row>, variables: &[String]) -> Vec<Row> {