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Oracle Hash Tips

Oracle Database Tips by Donald BurlesonJune 14, 2015

Question:  What is the ora_hash function and how do I use it?

Answer:  ora_hash is a function that computes a hash value for a given expression.  This function has 3 parameters: expr; max_bucket; and seed_value.   The expr parameter determines the data that you want Oracle to compute a hash for, and it can be any type or length of data.  max_bucket and seed_value are both optional.  The first can be any number between 0 and 4294967295 and defaults to the latter.  seed_value enables Oracle to produce different results for the same set of data, and while it has the same upper and lower bounds as max_bucket, the default for seed_value is 0.  The ora_hash function returns a NUMBER value.

The following example creates a hash for each combination of customer ID and product ID from the sh.sales tables, divides the hash values into 100 different buckets, and then returns the sum of the amount_sold values in the first bucket.  By adding or changing a seed_value, you can get different hash results from the same query. 

SELECT SUM(amount_sold) FROM sales
    WHERE ORA_HASH(CONCAT(cust_id, prod_id), 99) = 0;  

SUM(AMOUNT_SOLD)
----------------
            7315


 
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