how to use query_tree_walker to get all relations used in a query

Hello,

I am trying to get the list of all relations used in a SELECT query using  the post parse analyze hook.

I can get all relations from top level FROM clause but I cannot get them for a simple subquery like:

select * from t1 where x1=(select max(x2) from t2);

My current code is:

static bool pgds_tree_walker(Query *node, void *context)
{

/*
* from setrefs.c
* extract_query_dependencies_walker
*/

if (node == NULL)
         return false;

  if (IsA(node, Query))
    {
         Query      *query = (Query *) node;
         ListCell   *lc;
 
         /* Collect relation OIDs in this Query's rtable */
         foreach(lc, query->rtable)
         {
             RangeTblEntry *rte = (RangeTblEntry *) lfirst(lc);
 
             if (rte->rtekind == RTE_RELATION ||
                 (rte->rtekind == RTE_SUBQUERY && OidIsValid(rte->relid)) ||
                 (rte->rtekind == RTE_NAMEDTUPLESTORE && OidIsValid(rte->relid)))

                     elog(INFO, "pgds_tree_walker: relid=%d", rte->relid);

         }

/*
* from rewriteHandler.c
* AcquireRewriteLocks
*/
if (rte->rtekind == RTE_SUBQUERY)
return query_tree_walker(rte->subquery, pgds_tree_walker, (void *) context, QTW_EXAMINE_RTES_BEFORE);

         }
 
         /* And recurse  ...*/
         query_tree_walker(query, pgds_tree_walker, (void *) context, QTW_EXAMINE_RTES_BEFORE);
}
}

/*
 * build_rel_array
 */
static void pgds_build_rel_array(Query *query)
{
ListCell *cell;
Oid rel_id;
void * context = NULL;
bool result;

foreach(cell, query->rtable)
{
RangeTblEntry *rte = (RangeTblEntry *) lfirst(cell);
rel_id = rte->relid;
if (pgds_rel_index < MAX_REL )
{
pgds_rel_array[pgds_rel_index] = rel_id;
pgds_rel_index++;
}
else elog(ERROR, "pgds_build_rel_array: too many relations (%d)", MAX_REL);
}

result = query_tree_walker(query, pgds_tree_walker, context, QTW_EXAMINE_RTES_BEFORE);
}

Could someone help me ?

Thanks