Fix constants in the result of MergingSortedAlgorithm.

src/Processors/Merges/Algorithms/MergedData.h

@@ -60,7 +60,19 @@ class MergedData
             throw Exception(ErrorCodes::LOGICAL_ERROR, "Cannot insert to MergedData from Chunk because MergedData is not empty.");
 
         UInt64 num_rows = chunk.getNumRows();
-        columns = chunk.mutateColumns();
+        UInt64 num_columns = chunk.getNumColumns();
+        auto chunk_columns = chunk.mutateColumns();
+
+        /// Here is a special code for constant columns.
+        /// Currently, 'columns' will contain constants, but 'chunk_columns' will not.
+        /// We want to keep constants in the result, so just re-create them carefully.
+        for (size_t i = 0; i < num_columns; ++i)
+        {
+            if (isColumnConst(*columns[i]))
+                columns[i] = columns[i]->cloneResized(num_rows);
+            else
+                columns[i] = std::move(chunk_columns[i]);
+        }
 
         if (rows_size < num_rows)
         {

tests/queries/0_stateless/02662_sorting_constants_and_distinct_crash.reference

@@ -0,0 +1,4 @@
+constant_1	250000
+constant_1	2000000
+constant_1	test_value_1
+constant_1	test_value_2

tests/queries/0_stateless/02662_sorting_constants_and_distinct_crash.sql

@@ -0,0 +1,27 @@
+drop table if exists test_table;
+CREATE TABLE test_table (string_value String) ENGINE = MergeTree ORDER BY string_value;
+system stop merges test_table;
+insert into test_table select * from (
+	select 'test_value_1'
+	from numbers_mt(250000)
+	union all
+	select 'test_value_2'
+	from numbers_mt(2000000)
+)
+order by rand();
+
+select distinct
+    'constant_1' as constant_value,
+    count(*) over(partition by constant_value, string_value) as value_cnt
+from (
+    select string_value
+    from test_table
+);
+
+select distinct
+ 'constant_1' as constant_value, *
+ from (select string_value from test_table)
+ ORDER BY constant_value, string_value settings max_threads=1;
+
+system start merges test_table;
+drop table test_table;