Implement supertype for LowCardinality

src/DataTypes/getLeastSupertype.cpp

@@ -161,62 +161,62 @@ DataTypePtr getLeastSupertype(const DataTypes & types)
         }
     }
 
-    /// For Nullable
+    /// For LowCardinality. This is above Nullable, because LowCardinality can contain Nullable but cannot be inside Nullable.
     {
-        bool have_nullable = false;
+        bool have_low_cardinality = false;
+        bool have_not_low_cardinality = false;
 
         DataTypes nested_types;
         nested_types.reserve(types.size());
 
         for (const auto & type : types)
         {
-            if (const DataTypeNullable * type_nullable = typeid_cast<const DataTypeNullable *>(type.get()))
+            if (const DataTypeLowCardinality * type_low_cardinality = typeid_cast<const DataTypeLowCardinality *>(type.get()))
             {
-                have_nullable = true;
-
-                if (!type_nullable->onlyNull())
-                    nested_types.emplace_back(type_nullable->getNestedType());
+                have_low_cardinality = true;
+                nested_types.emplace_back(type_low_cardinality->getDictionaryType());
             }
             else
+            {
+                have_not_low_cardinality = true;
                 nested_types.emplace_back(type);
             }
+        }
 
-        if (have_nullable)
+        /// All LowCardinality gives LowCardinality.
+        /// LowCardinality with high cardinality gives high cardinality.
+        if (have_low_cardinality)
         {
-            return std::make_shared<DataTypeNullable>(getLeastSupertype(nested_types));
+            if (have_not_low_cardinality)
+                return getLeastSupertype(nested_types);
+            else
+                return std::make_shared<DataTypeLowCardinality>(getLeastSupertype(nested_types));
         }
     }
 
-    /// For LowCardinality
+    /// For Nullable
     {
-        bool have_low_cardinality = false;
-        bool have_not_low_cardinality = false;
+        bool have_nullable = false;
 
         DataTypes nested_types;
         nested_types.reserve(types.size());
 
         for (const auto & type : types)
         {
-            if (const DataTypeLowCardinality * type_low_cardinality = typeid_cast<const DataTypeLowCardinality *>(type.get()))
+            if (const DataTypeNullable * type_nullable = typeid_cast<const DataTypeNullable *>(type.get()))
             {
-                have_low_cardinality = true;
-                nested_types.emplace_back(type_low_cardinality->getDictionaryType());
+                have_nullable = true;
+
+                if (!type_nullable->onlyNull())
+                    nested_types.emplace_back(type_nullable->getNestedType());
             }
             else
-            {
-                have_not_low_cardinality = true;
                 nested_types.emplace_back(type);
         }
-        }
 
-        /// All LowCardinality gives LowCardinality.
-        /// LowCardinality with high cardinality gives high cardinality.
-        if (have_low_cardinality)
+        if (have_nullable)
         {
-            if (have_not_low_cardinality)
-                return getLeastSupertype(nested_types);
-            else
-                return std::make_shared<DataTypeLowCardinality>(getLeastSupertype(nested_types));
+            return std::make_shared<DataTypeNullable>(getLeastSupertype(nested_types));
         }
     }
 

tests/queries/0_stateless/01377_supertype_low_cardinality.reference

+hello
+hello
+String
+String
+---
+---
+hello
+hello
+hello
+hello
+---
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+-
+hello
+hello
+---
+hello
+hello
+hello
+-
+hello
+hello
+hello
+-
+hello
+hello
+hello
+-
+hello
+hello
+hello
+---
+hello
+hello
+hello
+hello
+---
+['abc','def']
+['abc','def']
+['abc','def']

tests/queries/0_stateless/01377_supertype_low_cardinality.sql

+SELECT 'hello' UNION ALL SELECT toLowCardinality('hello');
+SELECT toTypeName(x) FROM (SELECT 'hello' AS x UNION ALL SELECT toLowCardinality('hello'));
+
+SELECT '---';
+
+create temporary table t1(a String);
+create temporary table t2(a LowCardinality(String));
+select a from t1 union all select a from t2;
+
+SELECT '---';
+
+CREATE TEMPORARY TABLE a (x String);
+CREATE TEMPORARY TABLE b (x LowCardinality(String));
+CREATE TEMPORARY TABLE c (x Nullable(String));
+CREATE TEMPORARY TABLE d (x LowCardinality(Nullable(String)));
+
+INSERT INTO a VALUES ('hello');
+INSERT INTO b VALUES ('hello');
+INSERT INTO c VALUES ('hello');
+INSERT INTO d VALUES ('hello');
+
+SELECT x FROM a;
+SELECT x FROM b;
+SELECT x FROM c;
+SELECT x FROM d;
+
+SELECT '---';
+
+SELECT x FROM a UNION ALL SELECT x FROM b;
+SELECT '-';
+SELECT x FROM a UNION ALL SELECT x FROM c;
+SELECT '-';
+SELECT x FROM a UNION ALL SELECT x FROM d;
+SELECT '-';
+SELECT x FROM b UNION ALL SELECT x FROM a;
+SELECT '-';
+SELECT x FROM b UNION ALL SELECT x FROM c;
+SELECT '-';
+SELECT x FROM b UNION ALL SELECT x FROM d;
+SELECT '-';
+SELECT x FROM c UNION ALL SELECT x FROM a;
+SELECT '-';
+SELECT x FROM c UNION ALL SELECT x FROM b;
+SELECT '-';
+SELECT x FROM c UNION ALL SELECT x FROM d;
+SELECT '-';
+SELECT x FROM d UNION ALL SELECT x FROM a;
+SELECT '-';
+SELECT x FROM d UNION ALL SELECT x FROM c;
+SELECT '-';
+SELECT x FROM d UNION ALL SELECT x FROM b;
+
+SELECT '---';
+
+SELECT x FROM b UNION ALL SELECT x FROM c UNION ALL SELECT x FROM d;
+SELECT '-';
+SELECT x FROM a UNION ALL SELECT x FROM c UNION ALL SELECT x FROM d;
+SELECT '-';
+SELECT x FROM a UNION ALL SELECT x FROM b UNION ALL SELECT x FROM d;
+SELECT '-';
+SELECT x FROM a UNION ALL SELECT x FROM b UNION ALL SELECT x FROM c;
+
+SELECT '---';
+
+SELECT x FROM a UNION ALL SELECT x FROM b UNION ALL SELECT x FROM c UNION ALL SELECT x FROM d;
+
+SELECT '---';
+
+SELECT [CAST('abc' AS LowCardinality(String)), CAST('def' AS Nullable(String))];
+SELECT [CAST('abc' AS LowCardinality(String)), CAST('def' AS FixedString(3))];
+SELECT [CAST('abc' AS LowCardinality(String)), CAST('def' AS LowCardinality(FixedString(3)))];