Add function "isConstant"

src/Functions/isConstant.cpp

@@ -0,0 +1,52 @@
+#include <Functions/IFunctionImpl.h>
+#include <Functions/FunctionFactory.h>
+#include <DataTypes/DataTypesNumber.h>
+#include <Columns/ColumnsNumber.h>
+
+
+namespace DB
+{
+
+/// Returns 1 if and only if the argument is constant expression.
+/// This function is exists for development, debugging and demonstration purposes.
+class FunctionIsConstant : public IFunction
+{
+public:
+    static constexpr auto name = "isConstant";
+    static FunctionPtr create(const Context &)
+    {
+        return std::make_shared<FunctionIsConstant>();
+    }
+
+    String getName() const override
+    {
+        return name;
+    }
+
+    bool useDefaultImplementationForNulls() const override { return false; }
+
+    size_t getNumberOfArguments() const override
+    {
+        return 1;
+    }
+
+    DataTypePtr getReturnTypeImpl(const DataTypes & /*arguments*/) const override
+    {
+        return std::make_shared<DataTypeUInt8>();
+    }
+
+    void executeImpl(Block & block, const ColumnNumbers & arguments, size_t result, size_t input_rows_count) override
+    {
+        const auto & elem = block.getByPosition(arguments[0]);
+        block.getByPosition(result).column = ColumnUInt8::create(input_rows_count, isColumnConst(*elem.column));
+    }
+};
+
+
+void registerFunctionIsConstant(FunctionFactory & factory)
+{
+    factory.registerFunction<FunctionIsConstant>();
+}
+
+}
+

src/Functions/registerFunctionsMiscellaneous.cpp

@@ -56,6 +56,7 @@ void registerFunctionBasename(FunctionFactory &);
 void registerFunctionTransform(FunctionFactory &);
 void registerFunctionGetMacro(FunctionFactory &);
 void registerFunctionGetScalar(FunctionFactory &);
+void registerFunctionIsConstant(FunctionFactory &);
 
 #if USE_ICU
 void registerFunctionConvertCharset(FunctionFactory &);
@@ -114,6 +115,7 @@ void registerFunctionsMiscellaneous(FunctionFactory & factory)
     registerFunctionTransform(factory);
     registerFunctionGetMacro(factory);
     registerFunctionGetScalar(factory);
+    registerFunctionIsConstant(factory);
 
 #if USE_ICU
     registerFunctionConvertCharset(factory);

tests/queries/0_stateless/01118_is_constant.reference

@@ -0,0 +1,9 @@
+1
+1
+0
+1
+1
+---
+0
+0
+---

tests/queries/0_stateless/01118_is_constant.sql

@@ -0,0 +1,10 @@
+select isConstant(1);
+select isConstant([1]);
+select isConstant(arrayJoin([1]));
+SELECT isConstant((SELECT 1));
+SELECT isConstant(x) FROM (SELECT 1 x);
+SELECT '---';
+SELECT isConstant(x) FROM (SELECT 1 x UNION ALL SELECT 2);
+SELECT '---';
+select isConstant(); -- { serverError 42 }
+select isConstant(1, 2); -- { serverError 42 }