[GitHub] [druid] maytasm commented on a change in pull request #10948: If ingested data has sparse columns, the ingested data with forceGuaranteedRollup=true can result in imperfect rollup and final dimension ordering can be different from dimensionSpec ordering in the ingestionSpec

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maytasm commented on a change in pull request #10948:
URL: https://github.com/apache/druid/pull/10948#discussion_r597272564



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File path: processing/src/main/java/org/apache/druid/segment/IndexMerger.java
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@@ -106,6 +118,41 @@
       return null;
     }
 
+    if (isDimensionOrderingValid(indexes, orderingCandidate)) {
+      return ImmutableList.copyOf(orderingCandidate);
+    } else {
+      log.info("Indexes have incompatible dimension orders, try falling back on dimension ordering from ingestionSpec");
+      // Check if there is a valid dimension ordering in the ingestionSpec to fall back on
+      if (dimensionsSpec == null || CollectionUtils.isNullOrEmpty(dimensionsSpec.getDimensionNames())) {
+        log.info("Cannot fall back on dimension ordering from ingestionSpec as it does not exist");
+        return null;
+      }
+      List<String> candidate = dimensionsSpec.getDimensionNames();

Review comment:
       Good point. From the Guava docs, seems like "The returned list is a transformed view of fromList; changes to fromList will be reflected in the returned list and vice versa." and "To avoid lazy evaluation when the returned list doesn't need to be a view, copy the returned list into a new list of your choosing."
   
   Changed this to make a copy of the returned list from `dimensionsSpec.getDimensionNames()`




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