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## The Hash Function over different values gives same reesults.What is the logic behind hashing function and how to get unique results to unique values.

Explorer

select hash ('9SH305EJ5'); OK -339500666

select hash ('9SH305EIT'); OK -339500666

1 ACCEPTED SOLUTION
Rising Star

It is not supposed to generate unique values. The hash() function is working with ranges. It is supposed to index different ranges with integer values. Think about grouping similar ranges of values in a large data set into smaller subsets and have an index to find the respective subset.

A good explanation can be found there:

http://preshing.com/20110504/hash-collision-probabilities/

If you want to generate unique values, have a look at using UDF (reflect("java.util.UUID", "randomUUID"))

3 REPLIES 3
Rising Star

It is not supposed to generate unique values. The hash() function is working with ranges. It is supposed to index different ranges with integer values. Think about grouping similar ranges of values in a large data set into smaller subsets and have an index to find the respective subset.

A good explanation can be found there:

http://preshing.com/20110504/hash-collision-probabilities/

If you want to generate unique values, have a look at using UDF (reflect("java.util.UUID", "randomUUID"))

Rising Star

@Amol Kulkarni - does that answer your question? Solved?

Guru

Yes that's a known one. Hash function in hive functions similar to hash algorithm or hash sort logic in data structures.

Like modulo of odd number by 2 is always 1. The same way the two values provided by you results in having same hash value. In order to generate unique values make use of md5() function in hive to generate unique values. However I suggest not to this logic for generation of primary key for a table as the values out of md5() will be a total mess.

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