[GitHub] [incubator-druid] AlexanderSaydakov commented on a change in pull request #8194: HllSketch Merge/Build BufferAggregators: Speed up init with prebuilt sketch.

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AlexanderSaydakov commented on a change in pull request #8194: HllSketch Merge/Build BufferAggregators: Speed up init with prebuilt sketch.
URL: https://github.com/apache/incubator-druid/pull/8194#discussion_r308900477
 
 

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 File path: extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/hll/HllSketchBuildBufferAggregator.java
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 @@ -53,6 +55,14 @@
   private final IdentityHashMap<ByteBuffer, Int2ObjectMap<HllSketch>> sketchCache = new IdentityHashMap<>();
   private final Striped<ReadWriteLock> stripedLock = Striped.readWriteLock(NUM_STRIPES);
 
+  /**
+   * Used by {@link #init(ByteBuffer, int)}. We initialize by copying a prebuilt empty sketch object.
+   * {@link HllSketchMergeBufferAggregator} does something similar, but we don't share code to create emptySketch
+   * because it is not exactly the same (the "build" flavor initializes based on tgtHllType, but the "merge" flavor
+   * always initializes using HLL_8).
+   */
 
 Review comment:
   This is not so. HllSketchMergeBufferAggregator initializes HllUnion objects as opposed to HllSketch objects here. And HllUnion constructor does not even have the target type parameter (it is always in HLL_8 mode, but it is an implementation detail). The target type is used when the final result is obtained from the union in get().
   

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