adding doc

datafusion/expr/src/logical_plan/builder.rs

@@ -1502,6 +1502,25 @@ fn table_source(table_schema: &Schema) -> Arc<dyn TableSource> {
 }
 
 /// Wrap projection for a plan, if the join keys contains normal expression.
+///
+/// This function ensures that all expressions in `upper_exprs` are included in the output `LogicalPlan`.
+/// It handles cases where join keys contain expressions like `CAST` or `TRY_CAST`, which might otherwise
+/// result in schema conflicts due to identical field names.
+///
+/// # Parameters
+///
+/// - `join_keys`: A slice of expressions representing the join keys. These keys might include expressions
+///   like `CAST` or `TRY_CAST`, which need to be aliased to avoid conflicts in the projection schema.
+/// - `input`: The input `LogicalPlan` that provides the schema and data for the projection.
+/// - `upper_exprs`: A slice of references to expressions required by the parent join. This function
+///   ensures that all these expressions are included in the output `LogicalPlan`.
+///
+/// # Returns
+///
+/// A `Result` containing a tuple with:
+/// - The updated `LogicalPlan` with necessary projections.
+/// - A vector of `Column` representing the join columns.
+/// - A boolean indicating whether projection was needed.
 pub fn wrap_projection_for_join_if_necessary(
     join_keys: &[Expr],
     input: LogicalPlan,