CAST keep nullable (#11733)

src/Core/Settings.h

@@ -379,6 +379,7 @@ struct Settings : public SettingsCollection<Settings>
     \
     M(SettingBool, allow_experimental_geo_types, false, "Allow geo data types such as Point, Ring, Polygon, MultiPolygon", 0) \
     M(SettingBool, data_type_default_nullable, false, "Data types without NULL or NOT NULL will make Nullable", 0) \
+    M(SettingBool, cast_keep_nullable, false, "CAST operator keep Nullable for result data type", 0) \
     \
     /** Obsolete settings that do nothing but left for compatibility reasons. Remove each one after half a year of obsolescence. */ \
     \

src/Functions/FunctionsConversion.cpp

@@ -1,10 +1,16 @@
 #include <Functions/FunctionFactory.h>
 #include <Functions/FunctionsConversion.h>
+#include <Interpreters/Context.h>
 
 
 namespace DB
 {
 
+FunctionOverloadResolverImplPtr CastOverloadResolver::create(const Context & context)
+{
+    return createImpl(context.getSettingsRef().cast_keep_nullable);
+}
+
 void registerFunctionsConversion(FunctionFactory & factory)
 {
     factory.registerFunction<FunctionToUInt8>();

src/Functions/FunctionsConversion.h

@@ -2377,10 +2377,13 @@ public:
     using MonotonicityForRange = FunctionCast::MonotonicityForRange;
 
     static constexpr auto name = "CAST";
-    static FunctionOverloadResolverImplPtr create(const Context &) { return createImpl(); }
-    static FunctionOverloadResolverImplPtr createImpl() { return std::make_unique<CastOverloadResolver>(); }
 
-    CastOverloadResolver() {}
+    static FunctionOverloadResolverImplPtr create(const Context & context);
+    static FunctionOverloadResolverImplPtr createImpl(bool keep_nullable) { return std::make_unique<CastOverloadResolver>(keep_nullable); }
+
+    CastOverloadResolver(bool keep_nullable_)
+        : keep_nullable(keep_nullable_)
+    {}
 
     String getName() const override { return name; }
 
@@ -2415,13 +2418,18 @@ protected:
                 " Instead there is a column with the following structure: " + column->dumpStructure(),
                 ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
 
-        return DataTypeFactory::instance().get(type_col->getValue<String>());
+        DataTypePtr type = DataTypeFactory::instance().get(type_col->getValue<String>());
+        if (keep_nullable && arguments.front().type->isNullable())
+            return makeNullable(type);
+        return type;
     }
 
     bool useDefaultImplementationForNulls() const override { return false; }
     bool useDefaultImplementationForLowCardinalityColumns() const override { return false; }
 
 private:
+    bool keep_nullable;
+
     template <typename DataType>
     static auto monotonicityForType(const DataType * const)
     {

src/Functions/if.cpp

@@ -693,11 +693,12 @@ private:
 
     static ColumnPtr makeNullableColumnIfNot(const ColumnPtr & column)
     {
-        if (isColumnNullable(*column))
-            return column;
+        auto materialized = materializeColumnIfConst(column);
 
-        return ColumnNullable::create(
-            materializeColumnIfConst(column), ColumnUInt8::create(column->size(), 0));
+        if (isColumnNullable(*materialized))
+            return materialized;
+
+        return ColumnNullable::create(materialized, ColumnUInt8::create(column->size(), 0));
     }
 
     static ColumnPtr getNestedColumn(const ColumnPtr & column)

src/Interpreters/castColumn.cpp

@@ -29,7 +29,8 @@ ColumnPtr castColumn(const ColumnWithTypeAndName & arg, const DataTypePtr & type
         }
     };
 
-    FunctionOverloadResolverPtr func_builder_cast = std::make_shared<FunctionOverloadResolverAdaptor>(CastOverloadResolver::createImpl());
+    FunctionOverloadResolverPtr func_builder_cast =
+        std::make_shared<FunctionOverloadResolverAdaptor>(CastOverloadResolver::createImpl(false));
 
     ColumnsWithTypeAndName arguments{ temporary_block.getByPosition(0), temporary_block.getByPosition(1) };
     auto func_cast = func_builder_cast->build(arguments);

tests/queries/0_stateless/01322_cast_keep_nullable.reference

@@ -0,0 +1,10 @@
+0	Int32
+0	Int32
+1	Nullable(Int32)
+1	Nullable(Int32)
+2	Nullable(Float32)
+2	Nullable(UInt8)
+3	Nullable(Int32)
+\N	Nullable(Int32)
+42	Nullable(Int32)
+\N	Nullable(Int32)

tests/queries/0_stateless/01322_cast_keep_nullable.sql

@@ -0,0 +1,19 @@
+SET cast_keep_nullable = 0;
+
+SELECT CAST(toNullable(toInt32(0)) AS Int32) as x, toTypeName(x);
+SELECT CAST(toNullable(toInt8(0)) AS Int32) as x, toTypeName(x);
+
+SET cast_keep_nullable = 1;
+
+SELECT CAST(toNullable(toInt32(1)) AS Int32) as x, toTypeName(x);
+SELECT CAST(toNullable(toInt8(1)) AS Int32) as x, toTypeName(x);
+
+SELECT CAST(toNullable(toFloat32(2)), 'Float32') as x, toTypeName(x);
+SELECT CAST(toNullable(toFloat32(2)), 'UInt8') as x, toTypeName(x);
+SELECT CAST(toNullable(toFloat32(2)), 'UUID') as x, toTypeName(x); -- { serverError 70 }
+
+SELECT CAST(if(1 = 1, toNullable(toInt8(3)), NULL) AS Int32) as x, toTypeName(x);
+SELECT CAST(if(1 = 0, toNullable(toInt8(3)), NULL) AS Int32) as x, toTypeName(x);
+
+SELECT CAST(a, 'Int32') as x, toTypeName(x) FROM (SELECT materialize(CAST(42, 'Nullable(UInt8)')) AS a);
+SELECT CAST(a, 'Int32') as x, toTypeName(x) FROM (SELECT materialize(CAST(NULL, 'Nullable(UInt8)')) AS a);