add comment

Signed-off-by: Seaven <[email protected]>

fe/fe-core/src/main/java/com/starrocks/statistic/base/ColumnClassifier.java

@@ -50,6 +50,7 @@ private void classifyColumnStats(List<String> columnNames, List<Type> columnType
                     unSupportStats.add(new ComplexTypeColumnStats(columnName, columnType));
                 }
             } else {
+                // to split & valid struct subfield column, like 'a.b.c.d'
                 int start = 0;
                 int end;
                 List<String> names = Lists.newArrayList();