[GitHub] [incubator-pinot] mcvsubbu commented on a change in pull request #4363: When initializing mutable dictionary, preserve 20% buffer for cardinality to reduce the chance of re-sizing the dictionary

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mcvsubbu commented on a change in pull request #4363: When initializing mutable dictionary, preserve 20% buffer for cardinality to reduce the chance of re-sizing the dictionary
URL: https://github.com/apache/incubator-pinot/pull/4363#discussion_r297260962
 
 

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 File path: pinot-core/src/main/java/org/apache/pinot/core/indexsegment/mutable/MutableSegmentImpl.java
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 @@ -165,9 +164,11 @@ public long getLatestIngestionTimestamp() {
           dictionaryColumnSize = dataType.size();
         }
         String allocationContext = buildAllocationContext(_segmentName, column, V1Constants.Dict.FILE_EXTENSION);
+        // NOTE: preserve 20% buffer for cardinality to reduce the chance of re-sizing the dictionary
+        int estimatedCardinality = (int) (_statsHistory.getEstimatedCardinality(column) * 1.2);
 
 Review comment:
   I think 10% should be sufficient, but then I agree that both 10 and 20 percent are arbitrary. Why not make this a per-machine config? Default can be 10, and we can play with it as needed.
   

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