ROWID

In Oracle, the ROWID is a unique identifier for a row in a database table. It is an internal pointer that allows quick access to a specific row without the need for indexing or performing complex searches. Here’s a detailed explanation of ROWID in Oracle:

1. What is a ROWID?

ROWID is a pseudo column in Oracle that contains the physical address of a row in a table. It’s unique to each row and reflects the location of the row on disk, making it the fastest way to access a row in Oracle. It’s often used internally by Oracle to optimize performance.

2. Structure of ROWID

A ROWID is a string of 18 characters, divided into parts that represent the following:

• Block number: The location of the data block within the datafile. This is typically a 6-digit number.
• Row number: A number representing the row's position within the data block.
• Datafile number: A reference to the file in which the block resides.
• Object ID: The ID of the table or cluster in which the row exists.

The ROWID can be broken down into these 4 parts (formatted for understanding, but the actual representation may look different in hexadecimal):

AAAA BBBB CCCC DDDD EEEE FFFF

Where:

• AAAA: Datafile number
• BBBB: Block number
• CCCC: Row number within the block
• DDDD EEEE FFFF: Object ID of the table or cluster.

3. Usage of ROWID

3.1. Efficient Row Access

Since ROWID uniquely identifies a row by its physical location, it’s the fastest way to access data. Oracle uses ROWID internally for operations like SELECT, UPDATE, and DELETE to quickly locate and process rows.

3.2. ROWID in Queries

You can select the ROWID of rows directly in queries. For example:

SELECT ROWID, first_name, last_name

FROM employees

WHERE department_id = 10;

This will return the ROWID along with the employee names for employees in department 10.

3.3. ROWID for Update and Delete Operations

If you want to modify or delete specific rows, using ROWID ensures accuracy and efficiency. For instance, when updating a row, you can target a specific row directly using ROWID:

UPDATE employees

SET salary = salary * 1.1

WHERE ROWID = 'AAACz7AAEAAAAHnAAA';

Similarly, deleting a row based on ROWID:

DELETE FROM employees

WHERE ROWID = 'AAACz7AAEAAAAHnAAA';

3.4. ROWID for Performance

Because accessing data through ROWID is so fast (since it directly maps to a physical location on disk), it's sometimes used in performance optimization scenarios, especially for large tables or complex queries where other methods may take more time.

3.5. ROWID with Joins

In some cases, ROWID can be used in joins to quickly access data:

SELECT e1.first_name, e2.first_name

FROM employees e1, employees e2

WHERE e1.ROWID < e2.ROWID;

This query compares the ROWID values of two rows in the same table.

4. Types of ROWID

Oracle uses different types of ROWID values depending on the storage architecture of the database:

4.1. Traditional ROWID

The traditional ROWID is the physical identifier used before Oracle 9i. It’s based on the file, block, and row structure.

4.2. Extended ROWID (ROWID with the ROWID pseudo-column)

With Oracle 9i and above, Oracle introduced "extended ROWID" that supports larger databases, and ROWID is 18 characters long.

4.3. UROWID (Universal ROWID)

In some cases, a UROWID may be used, particularly in the context of LOB (Large Object) data types or when working with clusters or tablespaces that store data in different formats.

SELECT UROWID

FROM some_table;

5. Important Characteristics of ROWID

• Uniqueness: Each ROWID is unique within a table, meaning no two rows can share the same ROWID.
• Immutability: The ROWID does not change unless the row is moved to a different location in the database (for example, due to a table rebuild or partitioning).
• Non-Portability: ROWID is not portable across different databases. Moving data between databases may change the ROWID because it is tied to physical storage.
• Not Always Reliable for Data Comparison: While ROWID identifies the physical location, it doesn’t provide information about the row’s logical identity. Thus, you should not rely solely on ROWID for application-level data integrity.

6. When Not to Use ROWID

• Portable Applications: If your database is going to be migrated between systems, relying on ROWID can lead to issues, as the physical storage layout might change, invalidating the ROWID.
• For Logical Keying: Don’t use ROWID for logical identification of a record. For example, using ROWID as a substitute for a primary key or unique constraint is not a good practice because it only reflects the physical location, not the logical identity.

7. Alternatives to ROWID

• Primary Key: A logical identifier for rows that remains consistent even when the physical storage changes.
• Unique Constraints: Ensure data integrity without depending on physical location.

8. ROWID and Performance

• Faster than Indexes: Accessing a row using ROWID is generally faster than accessing it via an index, because it directly refers to the row's location in storage.
• Indexed Queries: When using indexed queries, Oracle can still use ROWID to directly fetch the rows once the index has identified the candidate rows.

9. ROWID in Conjunction with Partitioning

In a partitioned table, ROWID will still reference the row’s physical location within a specific partition. However, for partitions, Oracle may generate a different partition-specific ROWID structure.

10. Using ROWID for Duplicate Handling

You can use ROWID to remove duplicate rows. For example, if you want to delete duplicate rows from a table, you can use a query like this:

DELETE FROM employees

WHERE ROWID NOT IN (

    SELECT MIN(ROWID)

    FROM employees

    GROUP BY first_name, last_name);

This query removes duplicates, leaving only one instance of each combination of first_name and last_name.

Conclusion

ROWID is a powerful tool in Oracle for fast row access and manipulation, but it should be used with caution. It is an internal identifier and not a replacement for logical row identification mechanisms like primary keys. Understanding its usage and limitations helps ensure better data management and more efficient database operations.