Determine number of buckets for implicitly bucketed ACIDv2 tables

Hive tries to set number of reducers equal to number of buckets here: 
https://github.com/apache/hive/blob/9857c4e584384f7b0a49c34bc2bdf876c2ea1503/ql/src/java/org/apache/hadoop/hive/ql/parse/SemanticAnalyzer.java#L6958 

The numberOfBuckets for implicitly bucketed tables is set to -1 by default. When this is the case, it is left to hive to estimate the number of reducers required the job, based on job input, and configuration parameters.

https://github.com/apache/hive/blob/9857c4e584384f7b0a49c34bc2bdf876c2ea1503/ql/src/java/org/apache/hadoop/hive/ql/exec/Utilities.java#L3369

This estimate is not optimal in all cases. In the worst case, it case result in a single reducer being launched , which can lead to a significant bottleneck in performance .

Ideally,  the number of reducers launched should equal to number of buckets, which is the case for explicitly bucketed tables.