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Comment: revise description of hive.fetch.task.conversion per HIVE-2925 discussion (thanks, Navis)

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Some select queries can be converted to a single FETCH task, minimizing latency. Currently the query should be single sourced not having any subquery and should not have any aggregations or distincts (which incur RS – ReduceSinkOperator, requiring a MapReduce task), lateral views and joins.

Supported values are minimal and more.

1. minimal: SELECT STAR SELECT *, FILTER on partition columns (WHERE and HAVING clauses), LIMIT only
2. more: SELECT SELECT, FILTER, LIMIT only (including TABLESAMPLE, virtual columns)

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