ORA_HASH by Example

What is ORA_HASH
Per Oracle Documentation:

ORA_HASH is a function that computes a hash value for a given expression. 
This function is useful for operations such as analyzing a subset of data and generating a random sample.


When to use ORA_HASH
Consider following scenario.
There is a table of 80,000,000 rows, and you need to return a result, which is say an AVG() of some column.
This would be a huge overhead to scan the whole 80,000,000 rows.

The solution would be to divide the table into buckets, say 20 buckets, each one stands for 5% on the rows, and run AVG() only on one of the buckets.

ORA_HASH Usage
ORA_HASH(expression, max_bucket, seed_value) = 0;

Where: 
expression Is any column, value, etc.

max_bucket Is the number of buckets. 
                        Numbering starts at zero.
                        So for example 99 - would mean 100 buckets.

seed_value: optional. 
                       Default value is zero. 
                       By providing a different seed values to same ORA_HASH function, 
                        the result returned by ORA_HASH would be different each time.

ORA_HASH Example
SELECT AVG(salary)
FROM SALARY_HISTORY
WHERE ORA_HASH(SALARY_HISTORY.seq_id,19)=0;

The ORA_HASH table works on SALARY_HISTORY.seq_id values.
This SQL returns AVG(salary) for entries in the first bucket (=0), out of total 20 buckets (,19).