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Since HDFS does not support changing the data in a file after it is
writtenis written, ORC stores the top level index at the end of the file. The
overall The overall structure of the file is given in the figure above.
The file's tail consists of 3 parts- ; the file metadata, file footer,
and footer and postscript. 

The metadata for ORC is stored using Protocol Buffers, which provides the ability to add new fields without breaking readers. This document incorporates the Protobuf definition from the ORC source code and the readeris reader are encouraged to review the Protobuf encoding if they need to understandunderstand the byte-level encoding

Postscript

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