Backport #17397 to 20.12: Fix crash while reading from JOIN table with LowCardinality types

src/Storages/StorageJoin.cpp

@@ -10,6 +10,7 @@
 #include <DataTypes/NestedUtils.h>
 #include <Interpreters/joinDispatch.h>
 #include <Interpreters/TableJoin.h>
+#include <Interpreters/castColumn.h>
 #include <Common/assert_cast.h>
 #include <Common/quoteString.h>
 
@@ -321,7 +322,7 @@ private:
     template <ASTTableJoin::Kind KIND, ASTTableJoin::Strictness STRICTNESS, typename Maps>
     Chunk createChunk(const Maps & maps)
     {
-        MutableColumns columns = restored_block.cloneEmpty().mutateColumns();
+        MutableColumns mut_columns = restored_block.cloneEmpty().mutateColumns();
 
         size_t rows_added = 0;
 
@@ -329,7 +330,7 @@ private:
         {
 #define M(TYPE)                                           \
     case HashJoin::Type::TYPE:                                \
-        rows_added = fillColumns<KIND, STRICTNESS>(*maps.TYPE, columns); \
+        rows_added = fillColumns<KIND, STRICTNESS>(*maps.TYPE, mut_columns); \
         break;
             APPLY_FOR_JOIN_VARIANTS_LIMITED(M)
 #undef M
@@ -342,19 +343,23 @@ private:
         if (!rows_added)
             return {};
 
-        /// Correct nullability
+        Columns columns;
+        columns.reserve(mut_columns.size());
+        for (auto & col : mut_columns)
+            columns.emplace_back(std::move(col));
+
+        /// Correct nullability and LowCardinality types
         for (size_t i = 0; i < columns.size(); ++i)
         {
-            bool src_nullable = restored_block.getByPosition(i).type->isNullable();
-            bool dst_nullable = sample_block.getByPosition(i).type->isNullable();
+            const auto & src = restored_block.getByPosition(i);
+            const auto & dst = sample_block.getByPosition(i);
 
-            if (src_nullable && !dst_nullable)
+            if (!src.type->equals(*dst.type))
             {
-                auto & nullable_column = assert_cast<ColumnNullable &>(*columns[i]);
-                columns[i] = nullable_column.getNestedColumnPtr()->assumeMutable();
+                auto arg = src;
+                arg.column = std::move(columns[i]);
+                columns[i] = castColumn(arg, dst.type);
             }
-            else if (!src_nullable && dst_nullable)
-                columns[i] = makeNullable(std::move(columns[i]))->assumeMutable();
         }
 
         UInt64 num_rows = columns.at(0)->size();

tests/queries/0_stateless/01586_storage_join_low_cardinality_key.reference

+1
+1
+1

tests/queries/0_stateless/01586_storage_join_low_cardinality_key.sql

+CREATE TABLE low_card
+(
+    `lc` LowCardinality(String)
+)
+ENGINE = Join(ANY, LEFT, lc);
+
+INSERT INTO low_card VALUES ( '1' );
+
+SELECT * FROM low_card;
+SELECT * FROM low_card WHERE lc = '1';
+SELECT CAST(lc AS String) FROM low_card;