From 4bbcec3d33cf2b5dcc9a54c7bac8e5c9294f5422 Mon Sep 17 00:00:00 2001
From: hcz <h.cz@qq.com>
Date: Wed, 19 Aug 2020 14:44:11 +0800
Subject: [PATCH] Add defaultValueOfTypeName

---
 src/Functions/defaultValueOfTypeName.cpp      | 69 +++++++++++++++++++
 .../registerFunctionsMiscellaneous.cpp        |  2 +
 2 files changed, 71 insertions(+)
 create mode 100644 src/Functions/defaultValueOfTypeName.cpp

diff --git a/src/Functions/defaultValueOfTypeName.cpp b/src/Functions/defaultValueOfTypeName.cpp
new file mode 100644
index 00000000000..5b57642aa0e
--- /dev/null
+++ b/src/Functions/defaultValueOfTypeName.cpp
@@ -0,0 +1,69 @@
+#include <Functions/IFunctionImpl.h>
+#include <Functions/FunctionFactory.h>
+#include <Core/Field.h>
+#include <Columns/ColumnConst.h>
+#include <DataTypes/DataTypeFactory.h>
+
+
+namespace DB
+{
+
+namespace ErrorCodes
+{
+    extern const int ILLEGAL_COLUMN;
+}
+
+/// Returns global default value for type name (example: 0 for numeric types, '' for String).
+class FunctionDefaultValueOfTypeName : public IFunction
+{
+public:
+    static constexpr auto name = "defaultValueOfTypeName";
+    static FunctionPtr create(const Context &)
+    {
+        return std::make_shared<FunctionDefaultValueOfTypeName>();
+    }
+
+    String getName() const override
+    {
+        return name;
+    }
+
+    bool useDefaultImplementationForNulls() const override { return false; }
+    bool useDefaultImplementationForLowCardinalityColumns() const override { return false; }
+
+    size_t getNumberOfArguments() const override
+    {
+        return 1;
+    }
+
+    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
+    {
+        auto col_type_const = typeid_cast<const ColumnConst *>(arguments.front().column.get());
+        if (!col_type_const || !isString(arguments.front().type))
+            throw Exception("The argument of function " + getName() + " must be a constant string describing type. "
+                    "Instead there is a column with the following structure: " + arguments.front().column->dumpStructure(),
+                    ErrorCodes::ILLEGAL_COLUMN);
+
+        return DataTypeFactory::instance().get(col_type_const->getValue<String>());
+    }
+
+    void executeImpl(Block & block, const ColumnNumbers & arguments, size_t result, size_t input_rows_count) const override
+    {
+        const IDataType & type = *block.getByPosition(arguments[0]).type;
+        block.getByPosition(result).column = type.createColumnConst(input_rows_count, type.getDefault());
+    }
+
+    ColumnPtr getResultIfAlwaysReturnsConstantAndHasArguments(const Block & block, const ColumnNumbers & arguments) const override
+    {
+        const IDataType & type = *block.getByPosition(arguments[0]).type;
+        return type.createColumnConst(1, type.getDefault());
+    }
+};
+
+
+void registerFunctionDefaultValueOfTypeName(FunctionFactory & factory)
+{
+    factory.registerFunction<FunctionDefaultValueOfTypeName>();
+}
+
+}
diff --git a/src/Functions/registerFunctionsMiscellaneous.cpp b/src/Functions/registerFunctionsMiscellaneous.cpp
index 11d3e129c28..25e64eba999 100644
--- a/src/Functions/registerFunctionsMiscellaneous.cpp
+++ b/src/Functions/registerFunctionsMiscellaneous.cpp
@@ -18,6 +18,7 @@ void registerFunctionBlockSerializedSize(FunctionFactory &);
 void registerFunctionToColumnTypeName(FunctionFactory &);
 void registerFunctionDumpColumnStructure(FunctionFactory &);
 void registerFunctionDefaultValueOfArgumentType(FunctionFactory &);
+void registerFunctionDefaultValueOfTypeName(FunctionFactory &);
 void registerFunctionBlockSize(FunctionFactory &);
 void registerFunctionBlockNumber(FunctionFactory &);
 void registerFunctionRowNumberInBlock(FunctionFactory &);
@@ -79,6 +80,7 @@ void registerFunctionsMiscellaneous(FunctionFactory & factory)
     registerFunctionToColumnTypeName(factory);
     registerFunctionDumpColumnStructure(factory);
     registerFunctionDefaultValueOfArgumentType(factory);
+    registerFunctionDefaultValueOfTypeName(factory);
     registerFunctionBlockSize(factory);
     registerFunctionBlockNumber(factory);
     registerFunctionRowNumberInBlock(factory);