Merge pull request #12275 from ClickHouse/supertype-low-cardinality

Supertype for LowCardinality

src/DataTypes/getLeastSupertype.cpp

@@ -9,6 +9,7 @@
 #include <DataTypes/DataTypeArray.h>
 #include <DataTypes/DataTypeTuple.h>
 #include <DataTypes/DataTypeNullable.h>
+#include <DataTypes/DataTypeLowCardinality.h>
 #include <DataTypes/DataTypeNothing.h>
 #include <DataTypes/DataTypeString.h>
 #include <DataTypes/DataTypeDateTime.h>
@@ -160,6 +161,39 @@ DataTypePtr getLeastSupertype(const DataTypes & types)
         }
     }
 
+    /// For LowCardinality. This is above Nullable, because LowCardinality can contain Nullable but cannot be inside Nullable.
+    {
+        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 DataTypeLowCardinality * type_low_cardinality = typeid_cast<const DataTypeLowCardinality *>(type.get()))
+            {
+                have_low_cardinality = true;
+                nested_types.emplace_back(type_low_cardinality->getDictionaryType());
+            }
+            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_not_low_cardinality)
+                return getLeastSupertype(nested_types);
+            else
+                return std::make_shared<DataTypeLowCardinality>(getLeastSupertype(nested_types));
+        }
+    }
+
     /// For Nullable
     {
         bool have_nullable = false;

tests/queries/0_stateless/01377_supertype_low_cardinality.reference

@@ -0,0 +1,71 @@
+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

@@ -0,0 +1,71 @@
+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)))];