database - ROWID (oracle) - any use for it?

Question

My understanding is that the ROWID is a unique value for each row in the result returned by a query.

Why do we need this ROWID? There is already the ROWNUM in ORACLE.

Have any one used ROWID in a SQL query?

Answer 1

ROWID is the physical location of a row. Consequently it is the fastest way of locating a row, faster even than a primary key lookup. So it can be useful in certain types of transaction where we select some rows, store their ROWIDs and then later on use the ROWIDs in where clauses for DML against those same rows.

The Oracle SELECT ... FOR UPDATE syntax implicitly uses ROWID, when we update the locked row using WHERE CURRENT OF. Also the EXCEPTIONS table (referenced when applying constraints with the EXCEPTIONS INTO clause) has a column ROW_ID. This allows us to quickly identify the rows which are breaking our constraint.

That latter example points to another general usage: when we are writing some generic piece of code and need a mechanism for storing UIDs without concerns regarding data type, composite keys, etc.

ROWNUM on the other hand is a pseudo-column which tags a row in a given result set. It has no permanent significance.

edit

The ROWID for a given record can change over the lifetime of a system, for instance through a table rebuild. Also if one record is deleted a new record could be given that ROWID. Consequently ROWIDs are not suitable for use as UIDs in the long term. But they are good enough for use within a transaction.