Re: [POC] Allow flattening of subquery with a link to upper query

For contain_placeholders():

+   if (IsA(node, Query))
+       return query_tree_walker((Query *) node, contain_placeholders, context, 0);
+   else if (IsA(node, PlaceHolderVar))

The `else` is not needed.

For correlated_t struct, it would be better if the fields have comments.

+                    * (for grouping, as an example). So, revert its status to
+                    * a full valued entry.

full valued -> fully valued

Cheers