Made ColumnVector::replicate() implementation 3x faster

dbms/src/Columns/ColumnVector.cpp

@@ -346,18 +346,14 @@ ColumnPtr ColumnVector<T>::replicate(const IColumn::Offsets & offsets) const
     if (0 == size)
         return this->create();
 
-    auto res = this->create();
-    typename Self::Container & res_data = res->getData();
-    res_data.reserve(offsets.back());
+    auto res = this->create(offsets.back());
 
-    IColumn::Offset prev_offset = 0;
+    auto it = res->getData().begin();
     for (size_t i = 0; i < size; ++i)
     {
-        size_t size_to_replicate = offsets[i] - prev_offset;
-        prev_offset = offsets[i];
-
-        for (size_t j = 0; j < size_to_replicate; ++j)
-            res_data.push_back(data[i]);
+        const auto span_end = res->getData().begin() + offsets[i];
+        for (; it < span_end; ++it)
+            *it = data[i];
     }
 
     return res;

dbms/src/Columns/tests/gtest_column_unique.cpp

@@ -186,3 +186,28 @@ TEST(column_unique, column_unique_unique_deserialize_from_arena_Nullable_String_
     auto column = ColumnNullable::create(std::move(column_string), std::move(null_mask));
     column_unique_unique_deserialize_from_arena_impl(*column, *data_type);
 }
+
+TEST(ColumnVector, correctness_of_replicate)
+{
+    const auto column = ColumnUInt8::create();
+
+    column->insertValue(3);
+    column->insertValue(2);
+    column->insertValue(1);
+
+    const auto empty_column = column->replicate({0, 0, 0});
+    const auto empty_column_ptr = typeid_cast<const ColumnUInt8 *>(empty_column.get());
+    EXPECT_NE(empty_column_ptr, nullptr);
+    EXPECT_EQ(empty_column_ptr->size(), 0);
+
+    const auto new_column = column->replicate({1, 1, 5});
+    const auto new_column_ptr = typeid_cast<const ColumnUInt8 *>(new_column.get());
+    EXPECT_NE(new_column_ptr, nullptr);
+    EXPECT_EQ(new_column_ptr->size(), 5);
+    auto it = new_column_ptr->getData().cbegin();
+    for (const auto num : {3, 1, 1, 1, 1})
+    {
+        EXPECT_EQ(*it, num);
+        ++it;
+    }
+}