Merge pull request ClickHouse#86949 from ClickHouse/backport/25.8/86365

Backport ClickHouse#86365 to 25.8: Fix cast from LowCardinality(Nullable(T)) to Dynamic

Author: clickhouse-gh[bot]

src/Functions/FunctionsConversion.h

@@ -5559,16 +5559,16 @@ class FunctionCast final : public IFunctionBase
                     }
                 }
 
-                MutableColumnPtr variant_column;
+                ColumnPtr variant_column;
                 /// If there were no NULLs, we can just clone the column.
+                /// We use cloneWithDefaultOnNull to make the dictionary not-nullable in the result column.
                 if (variant_size_hint == col_lc.size())
-                    variant_column = IColumn::mutate(column);
+                    variant_column = col_lc.cloneWithDefaultOnNull();
                 /// Otherwise we should filter column.
                 else
-                    variant_column = IColumn::mutate(column->filter(filter, variant_size_hint));
+                    variant_column = assert_cast<const ColumnLowCardinality &>(*column->filter(filter, variant_size_hint)).cloneWithDefaultOnNull();
 
-                assert_cast<ColumnLowCardinality &>(*variant_column).nestedRemoveNullable();
-                return createVariantFromDescriptorsAndOneNonEmptyVariant(variant_types, std::move(discriminators), std::move(variant_column), variant_discr);
+                return createVariantFromDescriptorsAndOneNonEmptyVariant(variant_types, std::move(discriminators), variant_column, variant_discr);
             }
             else
             {

tests/queries/0_stateless/03605_dynamic_to_nullable_low_cardinality_bug.reference

@@ -0,0 +1 @@
+1

tests/queries/0_stateless/03605_dynamic_to_nullable_low_cardinality_bug.sql

@@ -0,0 +1,15 @@
+SET allow_suspicious_low_cardinality_types = 1, allow_experimental_dynamic_type = 1;
+
+DROP TABLE IF EXISTS t0;
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t0 (c0 Dynamic) ENGINE = MergeTree() ORDER BY tuple();
+CREATE TABLE t1 (c0 LowCardinality(Nullable(Int))) ENGINE = MergeTree() ORDER BY tuple() SETTINGS min_bytes_for_wide_part = 0;
+
+INSERT INTO TABLE t0 (c0) VALUES (1::LowCardinality(Int));
+INSERT INTO TABLE t1 (c0) VALUES (1);
+
+SELECT 1 FROM t0 JOIN t1 ON t0.c0 = t1.c0;
+
+DROP TABLE t0;
+DROP TABLE t1;
+