[GitHub] [orc] wgtmac commented on a change in pull request #1008: ORC-1087: [C++] Handle unloaded seek positions when seeking in an uncompressed chunk

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wgtmac commented on a change in pull request #1008:
URL: https://github.com/apache/orc/pull/1008#discussion_r783179137



##########
File path: c++/src/Compression.cc
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@@ -533,24 +545,37 @@ DIAGNOSTIC_PUSH
   }
 
   /** There are three possible scenarios when seeking a position:
-   * 1. The seeked position is already read and decompressed into
-   *    the output stream.
-   * 2. It is already read from the input stream, but has not been
-   *    decompressed yet, ie. it's not in the output stream.
-   * 3. It is not read yet from the inputstream.
+   * 1. The chunk of the seeked position is already read and decompressed into the output
+   *    stream, ie. chunk header is read and chunk contents are in the output stream.
+   * 2. The chunk of the seeked position is partially read. This only happens for

Review comment:
       The root cause is in line 495 where DecompressionStream::Next only reads availableSize but not the full uncompressed chunk. So another fix is to enforce read full uncompressed chunk there.
   

##########
File path: c++/src/Compression.cc
##########
@@ -533,24 +545,37 @@ DIAGNOSTIC_PUSH
   }
 
   /** There are three possible scenarios when seeking a position:

Review comment:
       **four** possible scenarios




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