[hive] branch master updated: HIVE-26184: COLLECT_SET with GROUP BY is very slow when some keys are highly skewed (#3253) (okumin reviewed by Zoltan Haindrich)

[kgyrtkirk]

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The following commit(s) were added to refs/heads/master by this push:
     new c4f6eb2f914 HIVE-26184: COLLECT_SET with GROUP BY is very slow when 
some keys are highly skewed (#3253) (okumin reviewed by Zoltan Haindrich)
c4f6eb2f914 is described below

commit c4f6eb2f91478152c89070a7455df9a1b8980c75
Author: okumin <g...@okumin.com>
AuthorDate: Tue Jun 14 00:26:36 2022 +0900

    HIVE-26184: COLLECT_SET with GROUP BY is very slow when some keys are 
highly skewed (#3253) (okumin reviewed by Zoltan Haindrich)
---
 .../udf/generic/GenericUDAFMkCollectionEvaluator.java  | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git 
a/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFMkCollectionEvaluator.java
 
b/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFMkCollectionEvaluator.java
index b05023dd37a..cffc7f76510 100644
--- 
a/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFMkCollectionEvaluator.java
+++ 
b/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFMkCollectionEvaluator.java
@@ -95,11 +95,27 @@ public class GenericUDAFMkCollectionEvaluator extends 
GenericUDAFEvaluator
         throw new RuntimeException("Buffer type unknown");
       }
     }
+
+    private void reset() {
+      if (bufferType == BufferType.LIST) {
+        container.clear();
+      } else if (bufferType == BufferType.SET) {
+        // Don't reuse a container because HashSet#clear can be very slow. The 
operation takes O(N)
+        // and N is the capacity of the internal hash table. The internal 
capacity grows based on
+        // the number of elements and it never shrinks. Thus, HashSet#clear 
takes O(N) every time
+        // once a skewed key appears.
+        // In order to avoid too many resizing in average cases, we set the 
initial capacity to the
+        // number of elements of the previous aggregation.
+        container = new LinkedHashSet<>(container.size());
+      } else {
+        throw new RuntimeException("Buffer type unknown");
+      }
+    }
   }
 
   @Override
   public void reset(AggregationBuffer agg) throws HiveException {
-    ((MkArrayAggregationBuffer) agg).container.clear();
+    ((MkArrayAggregationBuffer) agg).reset();
   }
 
   @Override