[MINOR] Lazy set the numBytes of ColumnarBatch (#11759)

Author: jinchengchenghh

cpp/core/memory/ColumnarBatch.cc

@@ -106,11 +106,14 @@ std::string ArrowCStructColumnarBatch::getType() const {
 }
 
 int64_t ArrowCStructColumnarBatch::numBytes() {
-  int64_t bytes = cArray_->n_buffers;
-  for (int64_t i = 0; i < cArray_->n_children; ++i) {
-    bytes += cArray_->children[i]->n_buffers;
+  if (!numBytes_.has_value()) {
+    int64_t bytes = cArray_->n_buffers;
+    for (int64_t i = 0; i < cArray_->n_children; ++i) {
+      bytes += cArray_->children[i]->n_buffers;
+    }
+    numBytes_ = bytes;
   }
-  return bytes;
+  return numBytes_.value();
 }
 
 std::shared_ptr<ArrowSchema> ArrowCStructColumnarBatch::exportArrowSchema() {

cpp/core/memory/ColumnarBatch.h

@@ -18,6 +18,7 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 
 #include "arrow/c/bridge.h"
 #include "arrow/c/helpers.h"
@@ -59,6 +60,8 @@ class ColumnarBatch {
 
  protected:
   int64_t exportNanos_;
+  // If the batch is immutable batch, the numBytes_ should be a fixed value, lazy set it.
+  std::optional<int64_t> numBytes_;
 };
 
 class ArrowColumnarBatch final : public ColumnarBatch {

cpp/velox/memory/GpuBufferColumnarBatch.cc

@@ -51,11 +51,14 @@ std::vector<char> GpuBufferColumnarBatch::toUnsafeRow(int32_t rowId) const {
 }
 
 int64_t GpuBufferColumnarBatch::numBytes() {
-  int64_t numBytes = 0;
-  for (const auto& buffer : buffers_) {
-    numBytes += buffer->size();
+  if (!numBytes_.has_value()) {
+    int64_t bytes = 0;
+    for (const auto& buffer : buffers_) {
+      bytes += buffer->size();
+    }
+    numBytes_ = bytes;
   }
-  return numBytes;
+  return numBytes_.value();
 }
 
 // Optimize to release the previous buffer after merge it.