Add test.

src/Processors/Merges/Algorithms/SummingSortedAlgorithm.cpp

@@ -45,8 +45,8 @@ struct SummingSortedAlgorithm::AggregateDescription
     AlignedBuffer state;
     bool created = false;
 
-    /// For LowCardinality, convert is converted to nested type. nested_type is nullptr if no conversion needed.
+    /// Those types are used only for simple aggregate functions.
-    /// This is used only for simple aggregate functions.
+    /// For LowCardinality, convert to nested type. nested_type is nullptr if no conversion needed.
     DataTypePtr nested_type; /// Nested type for LowCardinality, if it is.
     DataTypePtr real_type; /// Type in header.
 
@@ -275,7 +275,7 @@ static SummingSortedAlgorithm::ColumnsDefinition defineColumns(
 
                     desc.real_type = column.type;
                     desc.nested_type = recursiveRemoveLowCardinality(desc.real_type);
-                    if (desc.real_type->equals(*desc.nested_type))
+                    if (desc.real_type.get() == desc.nested_type.get())
                         desc.nested_type = nullptr;
                 }
                 else if (!is_agg_func)

tests/queries/0_stateless/01913_summing_mt_and_simple_agg_function_with_lc.reference

@@ -0,0 +1,2 @@
+1	x
+1	y

tests/queries/0_stateless/01913_summing_mt_and_simple_agg_function_with_lc.sql

@@ -0,0 +1,21 @@
+drop table if exists smta;
+
+CREATE TABLE smta
+(
+    `k` Int64,
+    `a` AggregateFunction(max, Int64),
+    `city` SimpleAggregateFunction(max, LowCardinality(String))
+)
+ENGINE = SummingMergeTree
+ORDER BY k;
+
+insert into smta(k, city) values (1, 'x');
+
+select k, city from smta;
+
+insert into smta(k, city) values (1, 'y');
+optimize table smta;
+
+select k, city from smta;
+
+drop table if exists smta;