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would pick out half of the 3rd cluster as each bucket would be composed of (32/64)=1/2 of a cluster.

For information about creating bucketed tables with the CLUSTERED BY clause, see Create Table (especially Bucketed Sorted Tables) and Bucketed Tables.

Block Sampling

Block sampling is available starting with Hive 0.8. Addressed under JIRA - https://issues.apache.org/jira/browse/HIVE-2121

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Or user can specify total length to be read, but it has same limitation with PERCENT sampling. (As of Hive 0.10.0 - https://issues.apache.org/jira/browse/HIVE-3401)

block_sample: TABLESAMPLE (ByteLengthLiteral)

ByteLengthLiteral : (Digit)+ ('b' | 'B' | 'k' | 'K' | 'm' | 'M' | 'g' | 'G')

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Hive also supports limiting input by row count basis, but it acts differently with above two.
First, it does not need CombineHiveInputFormat which means this can be used with non-native tables. Second, the row count given by user is applied to each split. So total row count can be vary by number of input splits. (As of Hive 0.10.0 - https://issues.apache.org/jira/browse/HIVE-3401)

block_sample: TABLESAMPLE (n ROWS)

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