add comment to check_nested_column_not_in_using

src/Analyzer/Passes/QueryAnalysisPass.cpp

@@ -3141,6 +3141,25 @@ QueryTreeNodePtr QueryAnalyzer::tryResolveIdentifierFromJoin(const IdentifierLoo
 
     auto check_nested_column_not_in_using = [&join_using_column_name_to_column_node, &identifier_lookup](const QueryTreeNodePtr & node)
     {
+        /** tldr: When an identifier is resolved into the function `nested` or `getSubcolumn`, and
+          * some column in its argument is in the USING list and its type has to be updated, we throw an error to avoid overcomplication.
+          *
+          * Identifiers can be resolved into functions in case of nested or subcolumns.
+          * For example `t.t.t` can be resolved into `getSubcolumn(t, 't.t')` function in case of `t` is `Tuple`.
+          * So, `t` in USING list is resolved from JOIN itself and has supertype of columns from left and right table.
+          * But `t` in `getSubcolumn` argument is still resolved from table and we need to update its type.
+          *
+          * It can be more complicated in case of Nested subcolumns, in that case in query:
+          *     SELECT t FROM ... JOIN ... USING (t.t)
+          * Here, `t` is resolved into function `nested(['t', 's'], t.t, t.s) so, `t.t` should be from JOIN and `t.s` should be from table.
+          *
+          * Updating type accordingly is pretty complicated, so just forbid such cases.
+          *
+          * While it still may work for storages that support selecting subcolumns directly without `getSubcolumn` function:
+          *     SELECT t, t.t, toTypeName(t), toTypeName(t.t) FROM t1 AS a FULL JOIN t2 AS b USING t.t;
+          * We just support it as a best-effort: `t` will have original type from table, but `t.t` will have super-type from JOIN.
+          * Probably it's good to prohibit such cases as well, but it's not clear how to check it in general case.
+          */
         if (node->getNodeType() != QueryTreeNodeType::FUNCTION)
             throw Exception(ErrorCodes::LOGICAL_ERROR, "Unexpected node type {}, expected function node", node->getNodeType());
 
@@ -3152,7 +3171,7 @@ QueryTreeNodePtr QueryAnalyzer::tryResolveIdentifierFromJoin(const IdentifierLoo
                 const auto & column_name = argument_node->as<ColumnNode &>().getColumnName();
                 if (join_using_column_name_to_column_node.contains(column_name))
                     throw Exception(ErrorCodes::AMBIGUOUS_IDENTIFIER,
-                        "Cannot select identifier '{}' while joining using nested column '{}'",
+                        "Cannot select subcolumn for identifier '{}' while joining using column '{}'",
                             identifier_lookup.identifier, column_name);
             }
             else if (argument_node->getNodeType() == QueryTreeNodeType::CONSTANT)