[GitHub] [arrow] wgtmac commented on a diff in pull request #35428: GH-35423: [C++][Parquet][BugFix] Parquet Encoding Fixing mismatched buffer size

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

wgtmac commented on code in PR #35428:
URL: https://github.com/apache/arrow/pull/35428#discussion_r1184663311


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cpp/src/parquet/column_reader.cc:
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@@ -857,7 +857,10 @@ class ColumnReaderImplBase {
   // first page with this encoding.
   void InitializeDataDecoder(const DataPage& page, int64_t levels_byte_size) {
     const uint8_t* buffer = page.data() + levels_byte_size;
-    const int64_t data_size = page.size() - levels_byte_size;
+    // PageReader may reuse the underlying buffer, so data_size
+    // should use `page.uncompressed_size() - levels_byte_size` rather
+    // than `page.size() - level_byte_size`.
+    const int64_t data_size = page.uncompressed_size() - levels_byte_size;

Review Comment:
   nit: it would be helpful to comment this is now a precise uncompressed data size, compared to an upper bound in the past.



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