Merge pull request ClickHouse#89684 from ClickHouse/backport/25.8/89550

Backport ClickHouse#89550 to 25.8: Fix generic aggregation state for LowCardinality(String)

Author: Avogar

src/Columns/ColumnUnique.h

@@ -92,6 +92,7 @@ class ColumnUnique final : public COWHelper<IColumnUnique, ColumnUnique<ColumnTy
     StringRef serializeValueIntoArena(size_t n, Arena & arena, char const *& begin) const override;
     char * serializeValueIntoMemory(size_t n, char * memory) const override;
     const char * skipSerializedInArena(const char * pos) const override;
+    StringRef serializeAggregationStateValueIntoArena(size_t n, Arena & arena, char const *& begin) const override;
     void updateHashWithValue(size_t n, SipHash & hash_func) const override;
 
 #if !defined(DEBUG_OR_SANITIZER_BUILD)
@@ -495,6 +496,29 @@ char * ColumnUnique<ColumnType>::serializeValueIntoMemory(size_t n, char * memor
     return column_holder->serializeValueIntoMemory(n, memory);
 }
 
+template <typename ColumnType>
+StringRef ColumnUnique<ColumnType>::serializeAggregationStateValueIntoArena(size_t n, Arena & arena, char const *& begin) const
+{
+    if (is_nullable)
+    {
+        static constexpr auto s = sizeof(UInt8);
+
+        auto * pos = arena.allocContinue(s, begin);
+        UInt8 flag = (n == getNullValueIndex() ? 1 : 0);
+        unalignedStore<UInt8>(pos, flag);
+
+        if (n == getNullValueIndex())
+            return StringRef(pos, s);
+
+        auto nested_ref = column_holder->serializeAggregationStateValueIntoArena(n, arena, begin);
+
+        /// serializeAggregationStateValueIntoArena may reallocate memory. Have to use ptr from nested_ref.data and move it back.
+        return StringRef(nested_ref.data - s, nested_ref.size + s);
+    }
+
+    return column_holder->serializeAggregationStateValueIntoArena(n, arena, begin);
+}
+
 template <typename ColumnType>
 size_t ColumnUnique<ColumnType>::uniqueDeserializeAndInsertFromArena(const char * pos, const char *& new_pos)
 {

tests/queries/0_stateless/03708_low_cardinality_aggregate_state_compatibility.reference

@@ -0,0 +1,4 @@
+010D00040000000000000073747200
+010A00010000000000000000
+('str')
+('')

tests/queries/0_stateless/03708_low_cardinality_aggregate_state_compatibility.sql

@@ -0,0 +1,5 @@
+select hex(maxDistinctState(tuple('str'::Variant(LowCardinality(String)))));
+select hex(maxDistinctState(tuple(''::Variant(LowCardinality(String)))));
+select maxDistinctMerge(state) from (select maxDistinctState(tuple('str'::Variant(LowCardinality(String)))) as state);
+select maxDistinctMerge(state) from (select maxDistinctState(tuple(''::Variant(LowCardinality(String)))) as state);
+