Limit GroupByCombineOperator to use 2 * numCores threads instead of creating one task per operator

pinot-core/src/main/java/org/apache/pinot/core/operator/combine/GroupByCombineOperator.java

@@ -39,6 +39,7 @@
 import org.apache.pinot.core.query.aggregation.groupby.AggregationGroupByResult;
 import org.apache.pinot.core.query.aggregation.groupby.GroupKeyGenerator;
 import org.apache.pinot.core.query.request.context.QueryContext;
+import org.apache.pinot.core.query.scheduler.resources.ResourceManager;
 import org.apache.pinot.core.util.GroupByUtils;
 import org.apache.pinot.spi.trace.Tracing;
 import org.slf4j.Logger;
@@ -47,8 +48,6 @@
 
 /**
  * Combine operator for group-by queries.
- * TODO: Use CombineOperatorUtils.getNumThreadsForQuery() to get the parallelism of the query instead of using
- *       all threads
  */
 @SuppressWarnings("rawtypes")
 public class GroupByCombineOperator extends BaseSingleBlockCombineOperator<GroupByResultsBlock> {
@@ -80,12 +79,13 @@ public GroupByCombineOperator(List<Operator> operators, QueryContext queryContex
   }
 
   /**
-   * For group-by queries, when maxExecutionThreads is not explicitly configured, create one task per operator.
+   * For group-by queries, when maxExecutionThreads is not explicitly configured, override it to create as many tasks
+   * as the default number of query worker threads (or the number of operators / segments if that's lower).
    */
   private static QueryContext overrideMaxExecutionThreads(QueryContext queryContext, int numOperators) {
     int maxExecutionThreads = queryContext.getMaxExecutionThreads();
     if (maxExecutionThreads <= 0) {
-      queryContext.setMaxExecutionThreads(numOperators);
+      queryContext.setMaxExecutionThreads(Math.min(numOperators, ResourceManager.DEFAULT_QUERY_WORKER_THREADS));
     }
     return queryContext;
   }

pinot-perf/src/main/java/org/apache/pinot/perf/BenchmarkQueries.java

@@ -20,7 +20,6 @@
 
 import java.io.File;
 import java.util.ArrayList;
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
@@ -35,7 +34,6 @@
 import org.apache.pinot.segment.local.segment.creator.impl.SegmentIndexCreationDriverImpl;
 import org.apache.pinot.segment.local.segment.index.loader.IndexLoadingConfig;
 import org.apache.pinot.segment.spi.AggregationFunctionType;
-import org.apache.pinot.segment.spi.ImmutableSegment;
 import org.apache.pinot.segment.spi.IndexSegment;
 import org.apache.pinot.segment.spi.creator.SegmentGeneratorConfig;
 import org.apache.pinot.segment.spi.index.startree.AggregationFunctionColumnPair;
@@ -81,8 +79,7 @@ public static void main(String[] args)
 
   private static final File INDEX_DIR = new File(FileUtils.getTempDirectory(), "FilteredAggregationsTest");
   private static final String TABLE_NAME = "MyTable";
-  private static final String FIRST_SEGMENT_NAME = "firstTestSegment";
-  private static final String SECOND_SEGMENT_NAME = "secondTestSegment";
+  private static final String SEGMENT_NAME_TEMPLATE = "testSegment%d";
   private static final String INT_COL_NAME = "INT_COL";
   private static final String SORTED_COL_NAME = "SORTED_COL";
   private static final String RAW_INT_COL_NAME = "RAW_INT_COL";
@@ -103,9 +100,9 @@ public static void main(String[] args)
       .setStarTreeIndexConfigs(
           Collections.singletonList(
               new StarTreeIndexConfig(List.of(SORTED_COL_NAME, INT_COL_NAME), null,
-              Collections.singletonList(
-                  new AggregationFunctionColumnPair(AggregationFunctionType.SUM, RAW_INT_COL_NAME).toColumnName()),
-              null, Integer.MAX_VALUE))).build();
+                  Collections.singletonList(
+                      new AggregationFunctionColumnPair(AggregationFunctionType.SUM, RAW_INT_COL_NAME).toColumnName()),
+                  null, Integer.MAX_VALUE))).build();
 
   //@formatter:off
   private static final Schema SCHEMA = new Schema.SchemaBuilder()
@@ -199,6 +196,8 @@ public static void main(String[] args)
       + "FromDateTime(dateTimeConvert(TSTMP_COL, '1:MILLISECONDS:EPOCH', '1:DAYS:SIMPLE_DATE_FORMAT:yyyy-MM-dd HH:mm:ss"
       + ".SSSZ tz(CET)', '1:DAYS'), 'yyyy-MM-dd HH:mm:ss.SSSZ') = 120000000";
 
+  @Param({"1", "2", "10", "50"})
+  private int _numSegments;
   @Param("1500000")
   private int _numRows;
   @Param({"EXP(0.001)", "EXP(0.5)", "EXP(0.999)"})
@@ -222,16 +221,14 @@ public void setUp()
     _supplier = Distribution.createSupplier(42, _scenario);
     FileUtils.deleteQuietly(INDEX_DIR);
 
-    buildSegment(FIRST_SEGMENT_NAME);
-    buildSegment(SECOND_SEGMENT_NAME);
-
+    _indexSegments = new ArrayList<>();
     IndexLoadingConfig indexLoadingConfig = new IndexLoadingConfig(TABLE_CONFIG, SCHEMA);
-    ImmutableSegment firstImmutableSegment =
-        ImmutableSegmentLoader.load(new File(INDEX_DIR, FIRST_SEGMENT_NAME), indexLoadingConfig);
-    ImmutableSegment secondImmutableSegment =
-        ImmutableSegmentLoader.load(new File(INDEX_DIR, SECOND_SEGMENT_NAME), indexLoadingConfig);
-    _indexSegment = firstImmutableSegment;
-    _indexSegments = Arrays.asList(firstImmutableSegment, secondImmutableSegment);
+    for (int i = 0; i < _numSegments; i++) {
+      buildSegment(String.format(SEGMENT_NAME_TEMPLATE, i));
+      _indexSegments.add(ImmutableSegmentLoader.load(new File(INDEX_DIR, String.format(SEGMENT_NAME_TEMPLATE, i)),
+          indexLoadingConfig));
+    }
+    _indexSegment = _indexSegments.get(0);
   }
 
   @TearDown