Github user prasanthj commented on the issue:
https://github.com/apache/orc/pull/78
Hive auto-gathers statistics during write (INSERT, CTAS..). Just before
closing the file, the file sink operator gets the statistics from Writer,
publishes it for aggregation by the client. This just pushes the stats
collection part from processOp (row-by-row or vector batch processing) to
closeOp.
Similarly Reader side interface is used by ANALYZE queries to compute
statistics just by reading footer.
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