From 754967bde6178ef7fd358b0fd7c37b92ce264b94 Mon Sep 17 00:00:00 2001
From: Alexey Milovidov <milovidov@yandex-team.ru>
Date: Sun, 12 Apr 2020 04:24:00 +0300
Subject: [PATCH] Add function "isConstant"

---
 src/Functions/isConstant.cpp                  | 52 +++++++++++++++++++
 .../registerFunctionsMiscellaneous.cpp        |  2 +
 .../0_stateless/01118_is_constant.reference   |  9 ++++
 .../queries/0_stateless/01118_is_constant.sql | 10 ++++
 4 files changed, 73 insertions(+)
 create mode 100644 src/Functions/isConstant.cpp
 create mode 100644 tests/queries/0_stateless/01118_is_constant.reference
 create mode 100644 tests/queries/0_stateless/01118_is_constant.sql

diff --git a/src/Functions/isConstant.cpp b/src/Functions/isConstant.cpp
new file mode 100644
index 00000000000..705b4eaac78
--- /dev/null
+++ b/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>();
+}
+
+}
+
diff --git a/src/Functions/registerFunctionsMiscellaneous.cpp b/src/Functions/registerFunctionsMiscellaneous.cpp
index 44e26542c7d..30cab4cc53a 100644
--- a/src/Functions/registerFunctionsMiscellaneous.cpp
+++ b/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);
diff --git a/tests/queries/0_stateless/01118_is_constant.reference b/tests/queries/0_stateless/01118_is_constant.reference
new file mode 100644
index 00000000000..aba2b912a08
--- /dev/null
+++ b/tests/queries/0_stateless/01118_is_constant.reference
@@ -0,0 +1,9 @@
+1
+1
+0
+1
+1
+---
+0
+0
+---
diff --git a/tests/queries/0_stateless/01118_is_constant.sql b/tests/queries/0_stateless/01118_is_constant.sql
new file mode 100644
index 00000000000..5cbff986dd2
--- /dev/null
+++ b/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 }