LakshSingla commented on a change in pull request #11809:
URL: https://github.com/apache/druid/pull/11809#discussion_r741615500



##########
File path: 
server/src/main/java/org/apache/druid/server/ClientQuerySegmentWalker.java
##########
@@ -431,6 +448,101 @@ private DataSource inlineIfNecessary(
         );
   }
 
+  /**
+   * This method returns the datasource by populating all the {@link 
QueryDataSource} with correct nesting level and
+   * sibling order of all the subqueries that are present.
+   * It also plumbs parent query's id and sql id in case the subqueries don't 
have it set by default
+   *
+   * @param dataSource       Datasource whose subqueries need to be populated
+   * @param parentQueryId    Parent Query's ID, can be null if do not need to 
update this in the subqueries
+   * @param parentSqlQueryId Parent Query's SQL Query ID, can be null if do 
not need to update this in the subqueries
+   * @return DataSource populated with the subqueries
+   */
+  private DataSource generateSubqueryIds(
+      DataSource dataSource,
+      @Nullable final String parentQueryId,
+      @Nullable final String parentSqlQueryId
+  )
+  {
+    Queue<DataSource> queue = new LinkedList<>();
+    queue.add(dataSource);
+
+    /*
+    Performs BFS on the datasource tree to find the nesting level, and the 
sibling order of the query datasource
+     */
+    Map<DataSource, Pair<Integer, Integer>> queryDataSourceToSubqueryIds = new 
HashMap<>();
+    int level = 1;
+    while (!queue.isEmpty()) {
+      int size = queue.size();
+      int siblingOrder = 1;
+      for (int i = 0; i < size; ++i) {
+        DataSource currentDataSource = queue.poll();
+        if (currentDataSource instanceof QueryDataSource) {
+          queryDataSourceToSubqueryIds.put(currentDataSource, new 
Pair<>(level, siblingOrder));

Review comment:
       The `Query#withSubqueryId()` method returns a new subquery with modified 
context, as does `Datasource#withChildren`. In order to actually modify the 
subquery id, we would have to go in a bottom's up manner (from the innermost 
datasource) and replace the child of the parent datasource, and then return 
this new parent datasource to be replaced in it's own parent datasource and so 
forth. I failed to come up with a way of doing that in a loop.




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