branch-4.1: [fix](decimalv2) fix wrong result of decimalv2 avg #62346 (#62413)

Cherry-picked from #62346

Co-authored-by: TengJianPing <tengjianping@selectdb.com>

Author: github-actions[bot]

be/src/exprs/aggregate/aggregate_function_avg.h

@@ -67,11 +67,11 @@ struct AggregateFunctionAvgData {
             // null is handled in AggregationNode::_get_without_key_result
             return static_cast<ResultT>(sum);
         }
-        // to keep the same result with row vesion; see AggregateFunctions::decimalv2_avg_get_value
+        // DecimalV2 has fixed internal scale=9; its arithmetic operators already
+        // handle scale correctly, so no external multiplier is needed.
         if constexpr (T == TYPE_DECIMALV2 && IsDecimalV2<ResultT>) {
             DecimalV2Value decimal_val_count(count, 0);
-            DecimalV2Value decimal_val_sum(sum * multiplier);
-            DecimalV2Value cal_ret = decimal_val_sum / decimal_val_count;
+            DecimalV2Value cal_ret = sum / decimal_val_count;
             return cal_ret;
         } else {
             if constexpr (T == TYPE_DECIMAL256) {

regression-test/data/datatype_p0/decimalv2/test_decimalv2_agg.out

@@ -17,3 +17,19 @@
 -- !decimalv2_sum2 --
 1000000000000000000.000000000
 
+-- !decimalv2_avg_scale0_groupby --
+-1.000000000
+-1000000000.000000000
+-1111111111.000000000
+-1234567890.000000000
+-9999999999.000000000
+0E-9
+1.000000000
+1000000000.000000000
+1111111111.000000000
+1234567890.000000000
+9999999999.000000000
+
+-- !decimalv2_avg_scale0_all --
+0E-9
+

regression-test/suites/datatype_p0/decimalv2/test_decimalv2_agg.groovy

@@ -61,4 +61,36 @@ suite("test_decimalv2_agg", "nonConcurrent") {
     """
     qt_decimalv2_sum2 " select sum(k1) from test_decimalv2_agg; "
 
+    // test avg on decimalv2(10,0) with group by - scale 0 regression
+    sql """
+        drop table if exists test_decimalv2_avg_scale0;
+    """
+    sql """
+        create table test_decimalv2_avg_scale0 (
+            id int,
+            data decimalv2(10,0) not null
+        ) distributed by hash(id) buckets 1
+        properties("replication_num"="1");
+    """
+    sql """
+        insert into test_decimalv2_avg_scale0 values
+            (0, 1234567890),
+            (1, 9999999999),
+            (2, 1000000000),
+            (3, 1111111111),
+            (4, -1234567890),
+            (5, -9999999999),
+            (6, -1000000000),
+            (7, -1111111111),
+            (8, 1),
+            (9, 0),
+            (10, -1);
+    """
+    order_qt_decimalv2_avg_scale0_groupby """
+        select avg(data) from test_decimalv2_avg_scale0 group by data;
+    """
+    qt_decimalv2_avg_scale0_all """
+        select avg(data) from test_decimalv2_avg_scale0;
+    """
+
 }