fix flush average points num may be mixed in one region (#18460)
diff --git a/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/service/metrics/WritingMetrics.java b/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/service/metrics/WritingMetrics.java
index 88197d0..bd2f124 100644
--- a/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/service/metrics/WritingMetrics.java
+++ b/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/service/metrics/WritingMetrics.java
@@ -45,6 +45,8 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Objects;
+import java.util.concurrent.atomic.DoubleAdder;
+import java.util.concurrent.atomic.LongAdder;
 
 public class WritingMetrics implements IMetricSet {
   private static final WritingMetrics INSTANCE = new WritingMetrics();
@@ -461,7 +463,9 @@
   private Counter manualFlushMemtableCounter = DoNothingMetricManager.DO_NOTHING_COUNTER;
   private Counter memControlFlushMemtableCounter = DoNothingMetricManager.DO_NOTHING_COUNTER;
 
-  private Histogram avgPointHistogram = DoNothingMetricManager.DO_NOTHING_HISTOGRAM;
+  // TVList preallocation needs a process-lifetime, node-wide mean independent of metric lifecycle.
+  private final DoubleAdder avgSeriesPointNumSum = new DoubleAdder();
+  private final LongAdder flushedMemTableCount = new LongAdder();
 
   private AutoGauge tableDiskUsageCacheBlockedRequestNumGauge =
       DoNothingMetricManager.DO_NOTHING_AUTO_GAUGE;
@@ -603,6 +607,7 @@
             MEM_TABLE_SIZE,
             SERIES_NUM,
             POINTS_NUM,
+            AVG_SERIES_POINT_NUM,
             COMPRESSION_RATIO,
             NULL_VALUE_RATIO,
             FLUSH_TSFILE_SIZE)
@@ -616,15 +621,6 @@
                         name,
                         Tag.REGION.toString(),
                         dataRegionId.toString()));
-    avgPointHistogram =
-        MetricService.getInstance()
-            .getOrCreateHistogram(
-                Metric.FLUSHING_MEM_TABLE_STATUS.toString(),
-                MetricLevel.IMPORTANT,
-                Tag.NAME.toString(),
-                AVG_SERIES_POINT_NUM,
-                Tag.REGION.toString(),
-                dataRegionId.toString());
   }
 
   public Counter createWalFlushMemTableCounterMetrics() {
@@ -746,7 +742,6 @@
                         name,
                         Tag.REGION.toString(),
                         dataRegionId.toString()));
-    avgPointHistogram = DoNothingMetricManager.DO_NOTHING_HISTOGRAM;
   }
 
   public void recordWALNodeEffectiveInfoRatio(String walNodeId, double ratio) {
@@ -845,7 +840,17 @@
             POINTS_NUM,
             Tag.REGION.toString(),
             dataRegionId.toString());
-    avgPointHistogram.update(avgSeriesNum);
+    MetricService.getInstance()
+        .histogram(
+            avgSeriesNum,
+            Metric.FLUSHING_MEM_TABLE_STATUS.toString(),
+            MetricLevel.IMPORTANT,
+            Tag.NAME.toString(),
+            AVG_SERIES_POINT_NUM,
+            Tag.REGION.toString(),
+            dataRegionId.toString());
+    avgSeriesPointNumSum.add(avgSeriesNum);
+    flushedMemTableCount.increment();
   }
 
   public void recordFlushTsFileSize(String storageGroup, long size) {
@@ -1046,7 +1051,8 @@
     return INSTANCE;
   }
 
-  public Histogram getAvgPointHistogram() {
-    return avgPointHistogram;
+  public double getGlobalAvgSeriesPointNum() {
+    long count = flushedMemTableCount.sum();
+    return count == 0 ? 0 : avgSeriesPointNumSum.sum() / count;
   }
 }
diff --git a/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/datastructure/TVList.java b/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/datastructure/TVList.java
index a7aa2ff..506f82d 100644
--- a/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/datastructure/TVList.java
+++ b/iotdb-core/datanode/src/main/java/org/apache/iotdb/db/utils/datastructure/TVList.java
@@ -1462,8 +1462,7 @@
     if (System.currentTimeMillis() - defaultArrayNumLastUpdatedTimeMs > 10_000) {
       defaultArrayNumLastUpdatedTimeMs = System.currentTimeMillis();
       defaultArrayNum =
-          ((int) WritingMetrics.getInstance().getAvgPointHistogram().takeSnapshot().getMean()
-              / ARRAY_SIZE);
+          ((int) WritingMetrics.getInstance().getGlobalAvgSeriesPointNum() / ARRAY_SIZE);
     }
     return defaultArrayNum;
   }
diff --git a/iotdb-core/datanode/src/test/java/org/apache/iotdb/db/service/metrics/WritingMetricsTest.java b/iotdb-core/datanode/src/test/java/org/apache/iotdb/db/service/metrics/WritingMetricsTest.java
new file mode 100644
index 0000000..4e889c5
--- /dev/null
+++ b/iotdb-core/datanode/src/test/java/org/apache/iotdb/db/service/metrics/WritingMetricsTest.java
@@ -0,0 +1,87 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.iotdb.db.service.metrics;
+
+import org.apache.iotdb.commons.consensus.DataRegionId;
+import org.apache.iotdb.commons.service.metric.MetricService;
+import org.apache.iotdb.commons.service.metric.enums.Metric;
+import org.apache.iotdb.commons.service.metric.enums.Tag;
+import org.apache.iotdb.metrics.config.MetricConfigDescriptor;
+import org.apache.iotdb.metrics.type.Histogram;
+import org.apache.iotdb.metrics.utils.MetricLevel;
+
+import org.junit.AfterClass;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import static org.apache.iotdb.db.service.metrics.WritingMetrics.AVG_SERIES_POINT_NUM;
+import static org.junit.Assert.assertEquals;
+
+public class WritingMetricsTest {
+
+  private static final DataRegionId FIRST_REGION = new DataRegionId(1);
+  private static final DataRegionId SECOND_REGION = new DataRegionId(2);
+  private static final MetricService METRIC_SERVICE = MetricService.getInstance();
+  private static final WritingMetrics WRITING_METRICS = WritingMetrics.getInstance();
+
+  @BeforeClass
+  public static void setUp() {
+    MetricConfigDescriptor.getInstance().getMetricConfig().setMetricLevel(MetricLevel.IMPORTANT);
+    METRIC_SERVICE.startService();
+    WRITING_METRICS.createFlushingMemTableStatusMetrics(FIRST_REGION);
+    WRITING_METRICS.createFlushingMemTableStatusMetrics(SECOND_REGION);
+  }
+
+  @AfterClass
+  public static void tearDown() {
+    WRITING_METRICS.removeFlushingMemTableStatusMetrics(FIRST_REGION);
+    WRITING_METRICS.removeFlushingMemTableStatusMetrics(SECOND_REGION);
+    METRIC_SERVICE.stopService();
+  }
+
+  /**
+   * Verifies that flushing two DataRegions records each average series point sample in the
+   * histogram carrying that Region's tag, instead of routing every sample to the last-created
+   * histogram.
+   */
+  @Test
+  public void testRecordAverageSeriesPointNumByDataRegion() {
+    WRITING_METRICS.recordFlushingMemTableStatus("root.db-1", 100, 2, 20, 10);
+    WRITING_METRICS.recordFlushingMemTableStatus("root.db-2", 200, 4, 80, 20);
+
+    Histogram firstRegionHistogram = getAverageSeriesPointHistogram(FIRST_REGION);
+    Histogram secondRegionHistogram = getAverageSeriesPointHistogram(SECOND_REGION);
+
+    assertEquals(1, firstRegionHistogram.getCount());
+    assertEquals(10, firstRegionHistogram.takeSnapshot().getSum(), 0.001);
+    assertEquals(1, secondRegionHistogram.getCount());
+    assertEquals(20, secondRegionHistogram.takeSnapshot().getSum(), 0.001);
+  }
+
+  private Histogram getAverageSeriesPointHistogram(DataRegionId dataRegionId) {
+    return METRIC_SERVICE.getOrCreateHistogram(
+        Metric.FLUSHING_MEM_TABLE_STATUS.toString(),
+        MetricLevel.IMPORTANT,
+        Tag.NAME.toString(),
+        AVG_SERIES_POINT_NUM,
+        Tag.REGION.toString(),
+        dataRegionId.toString());
+  }
+}