gianm commented on a change in pull request #10053:
URL: https://github.com/apache/druid/pull/10053#discussion_r442988947
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File path:
processing/src/main/java/org/apache/druid/query/topn/TopNQueryEngine.java
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@@ -158,6 +156,40 @@ private TopNMapFn getMapFn(
return new TopNMapFn(query, topNAlgorithm);
}
+ /**
+ * {@link PooledTopNAlgorithm} (and {@link
AggregateTopNMetricFirstAlgorithm} which utilizes the pooled
+ * algorithm) are optimized off-heap algorithms for aggregating dictionary
encoded string columns. These algorithms
+ * rely on dictionary ids being unique so to aggregate on the dictionary ids
directly and defer
+ * {@link org.apache.druid.segment.DimensionSelector#lookupName(int)} until
as late as possible in query processing.
+ *
+ * When these conditions are not true, we have an on-heap fall-back
algorithm, the {@link HeapBasedTopNAlgorithm}
+ * (and {@link TimeExtractionTopNAlgorithm} for a specialized form for long
columns) which aggregates on values of
+ * selectors.
+ */
+ private static boolean requiresHeapAlgorithm(
Review comment:
I think this method would be easier to read if it was called
`canUsePooledAlgorithm` and the checks were all flipped. This is because IMO it
makes sense to view the heap algorithm as the base case, and the pooled
algorithm as a special case, meaning the logic should be "can we do the special
case".
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