November 6, 2016 Leave a comment
These four clauses provided by HIVE are very important in context of Big Data as they provide a way to achieve sorting on big datasets.
- ORDER BY x: guarantees global ordering, but does this by pushing all data through just one reducer. This is basically unacceptable for large datasets. You end up one sorted file as output.
- SORT BY x: orders data at each of N reducers, but each reducer can receive overlapping ranges of data. You end up with N or more sorted files with overlapping ranges.
- DISTRIBUTE BY x: ensures each of N reducers gets non-overlapping ranges of x, but doesn’t sort the output of each reducer. You end up with N or unsorted files with non-overlapping ranges.
- CLUSTER BY x: ensures each of N reducers gets non-overlapping ranges, then sorts by those ranges at the reducers. This gives you global ordering, and is the same as doing (DISTRIBUTE BY x and SORT BY x). You end up with N or more sorted files with non-overlapping ranges.
More detailed explanation is given here