[GitHub] [incubator-druid] gianm edited a comment on issue #8194: HllSketch Merge/Build BufferAggregators: Speed up init with prebuilt sketch.

[GitBox <gi...@apache.org>]

gianm edited a comment on issue #8194: HllSketch Merge/Build BufferAggregators: Speed up init with prebuilt sketch.
URL: https://github.com/apache/incubator-druid/pull/8194#issuecomment-516636653
 
 
   I rewrote the comment and added it to "init":
   
   ```
       // Copy prebuilt empty union object.
       // Not necessary to cache a Union wrapper around the initialized memory, because:
       //  - It is cheap to reconstruct by re-wrapping the memory in "aggregate" and "get".
       //  - Unlike the HllSketch objects used by HllSketchBuildBufferAggregator, our Union objects never exceed the
       //    max size and therefore do not need to be potentially moved in-heap.
   ```
   
   I believe it is accurate but please let me know if I missed something.
   
   BTW, I removed the "never move" language because I believe it isn't accurate. "relocate" can still be called on the merge flavor -- it happens when e.g. the groupBy engine is resizing its internal hash table and moving objects around as a result. But the merge flavor shouldn't need to do anything special on a relocation, since it doesn't have any position based caches.

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