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src/java/org/apache/cassandra/cql3/selection/Selection.java

@@ -43,10 +43,24 @@ public abstract class Selection
     private static final Predicate<ColumnMetadata> STATIC_COLUMN_FILTER = (column) -> column.isStatic();
 
     private final TableMetadata table;
+
+    // Full list of columns needed for processing the query, including selected columns, ordering columns,
+    // and columns needed for restrictions.  Wildcard columns are fully materialized here.
+    //
+    // This also includes synthetic columns, because unlike all the other not-physical-columns selectables, they are
+    // computed on the replica instead of the coordinator and so, like physical columns, they need to be sent back
+    // as part of the result.
     private final List<ColumnMetadata> columns;
+
+    // maps ColumnSpecifications (columns, function calls, aliases) to the columns backing them
     private final SelectionColumnMapping columnMapping;
+
+    // metadata matching the ColumnSpcifications
     protected final ResultSet.ResultMetadata metadata;
+
+    // creates a ColumnFilter that breaks columns into `queried` and `fetched`
     protected final ColumnFilterFactory columnFilterFactory;
+
     protected final boolean isJson;
 
     // Columns used to order the result set for JSON queries with post ordering.