Re: [PR] PARQUET-2423: [C++][Parquet] Avoid allocating buffer obeject in RecordReader's SkipRecords [arrow]

["pitrou (via GitHub)" <gi...@apache.org>]

pitrou commented on code in PR #39818:
URL: https://github.com/apache/arrow/pull/39818#discussion_r1474502179


##########
cpp/src/parquet/column_reader.cc:
##########
@@ -1478,16 +1480,29 @@ class TypedRecordReader : public TypedColumnReaderImpl<DType>,
     // We skipped the levels by incrementing 'levels_position_'. For values
     // we do not have a buffer, so we need to read them and throw them away.
     // First we need to figure out how many present/not-null values there are.
-    std::shared_ptr<::arrow::ResizableBuffer> valid_bits;
-    valid_bits = AllocateBuffer(this->pool_);
-    PARQUET_THROW_NOT_OK(valid_bits->Resize(bit_util::BytesForBits(skipped_records),
-                                            /*shrink_to_fit=*/true));
+    int64_t buffer_size = bit_util::BytesForBits(skipped_records);
+    if (valid_bits_for_skip_ == nullptr) {
+      valid_bits_for_skip_ = AllocateBuffer(this->pool_);
+    }
+    if (buffer_size > valid_bits_for_skip_->size()) {

Review Comment:
   > One question is how can the [skipped_records](https://github.com/apache/arrow/blob/main/cpp/src/parquet/column_reader.cc#L1473) for `SkipRecordsInBufferNonRepeated` ever exceed [kMinLevelBatchSize](https://github.com/apache/arrow/blob/main/cpp/src/parquet/column_reader.cc#L1369) (i.e., 1024)?
   
   Why not? You can add an assertion to check this with the test suite.



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