Add NewColumn() to ColumnArray

Returns a new column of the proper type for inserting new values. Useful
for contexts in which the item type isn't immediately known.

Author: theory

clickhouse/columns/array.cpp

@@ -59,6 +59,10 @@ ColumnRef ColumnArray::GetAsColumn(size_t n) const {
     return data_->Slice(GetOffset(n), GetSize(n));
 }
 
+ColumnRef ColumnArray::NewColumn() const {
+    return data_->CloneEmpty();
+}
+
 ColumnRef ColumnArray::Slice(size_t begin, size_t size) const {
     if (size && begin + size > Size())
         throw ValidationError("Slice indexes are out of bounds");

clickhouse/columns/array.h

@@ -46,6 +46,15 @@ class ColumnArray : public Column {
         return GetAsColumn(n)->AsStrict<T>();
     }
 
+    /// Create a new, empty column of the same type as the array element.
+    ColumnRef NewColumn() const;
+
+    /// Shorthand to create a new column casted to a proper type.
+    template <typename T>
+    auto NewColumnAsType() const {
+        return NewColumn()->AsStrict<T>();
+    }
+
 public:
     /// Increase the capacity of the column for large block insertion.
     void Reserve(size_t new_cap) override;

ut/column_array_ut.cpp

@@ -53,7 +53,7 @@ TEST(ColumnArray, Append) {
     auto arr1 = std::make_shared<ColumnArray>(std::make_shared<ColumnUInt64>());
     auto arr2 = std::make_shared<ColumnArray>(std::make_shared<ColumnUInt64>());
 
-    auto id = std::make_shared<ColumnUInt64>();
+    auto id = arr1->NewColumnAsType<ColumnUInt64>();
     id->Append(1);
     arr1->AppendAsColumn(id);