We are working with a table in a 3rd party database that does not have a primary key but does have a unique index.
I have therefore been looking at using the ORA_HASH function to produce a de facto unique Id by passing in the values of the columns in the unique index.
Unfortunately, I can already see that we have a few collisions, which means that we can't derive a unique id using this method.
Is there an alternative to ORA_HASH that would provide a unique id for a unique input?
I suppose I could generate an Id using DBMS_CRYPTO.Hash but I'd ideally like to get a numeric value.
Edit
The added complication is that I then need to store these records in another (SQL Server) database and then compare the records from the original and the replica tables. So rank doesn't help me here since records can be added or deleted in the original table.
DBMS_CRYPTO.HASH could be used to generate a high-bit hash (high enough to give you a very low, but not zero, chance of collisions), but it returns 'RAW' not 'NUMBER'.
To guarantee no collisions ever, you need a one-to-one hash function. As far as I know, Oracle does not provide one.
A practical approach would be to create a new table to map unique keys to a newly generated primary key. E.g., unique value ("ABC",123, 888) maps to 838491 (where you generated 838491 using a sequence).
You'd have to update the mapping table periodically, to account for inserted rows, and that would be a pain, but it would let you generate your own PKs and keep track of them without a lot of complication.