Understanding Apache Iceberg Delete Files

August 29, 2024

Apache Iceberg is a powerful open-source table format for large-scale, distributed data storage. It enables complex data management tasks like schema evolution, time travel, and efficient query execution on massive datasets. An important feature of Iceberg is its ability to handle data deletions efficiently without requiring expensive rewrites of entire datasets when a table is “merge-on-read”. This capability is made possible by delete files—specialized files that track row-level deletions in an Iceberg table.

We’ll dive deep into the role of delete files in Apache Iceberg. We’ll explore delete files, how they work, and why they are essential for maintaining data consistency and optimizing query performance in a data lakehouse environment. By the end of this post, you’ll have a solid understanding of how delete files function within Iceberg and how they can be leveraged to enhance your data management strategies.

What Are Delete Files in Apache Iceberg?

Delete files in Apache Iceberg are specialized metadata files that store information about rows deleted from a table. Unlike traditional data deletion methods that require entire files or partitions to be rewritten, delete files allow for granular, row-level deletions without altering the original data files. This makes delete operations in Iceberg both efficient and scalable.

Types of Delete Files

Iceberg supports two types of delete files:

Position Deletes: These delete files specify the exact position of rows within a data file that should be considered deleted. They are used when the physical location of the data (i.e., the row’s position in the file) is known.

+--------------------------------------------+-----------------+------------------------------------+
| file_path                                  | pos             | row                                |
+--------------------------------------------+-----------------+------------------------------------+
| s3://bucket/path/to/data-file-1.parquet    | 0               | { "id": 1, "category": "marsupial",|
|                                            |                 |   "name": "Koala" }               |
| s3://bucket/path/to/data-file-1.parquet    | 102             | { "id": 2, "category": "toy",      |
|                                            |                 |   "name": "Teddy" }               |
+--------------------------------------------+-----------------+------------------------------------+

Equality Deletes: These delete files mark rows for deletion based on specific column values rather than their position. For example, suppose a record with a particular ID needs to be deleted. In that case, an equality delete file can specify that any row matching this ID should be excluded from query results.

+-------------------+-----------------+------------------------------------+
| equality_ids      | id              | category        | name             |
+-------------------+-----------------+-----------------+------------------+
| equality_ids=[1]  | 3               | NULL            | Grizzly          |
+-------------------+-----------------+-----------------+------------------+
| equality_ids=[1,2]| 4               | NULL            | Polar            |
+-------------------+-----------------+-----------------+------------------+

Importance of Delete Files

By separating deletion metadata from the data files, Iceberg ensures that data deletions are handled efficiently when fast writes with many row-level changes are needed. This separation also allows Iceberg to maintain the ACID properties—Atomicity, Consistency, Isolation, and Durability—essential for reliable data management in distributed systems.

The Role of Delete Files

Delete files allow Iceberg to handle row-level deletions with precision and efficiency, a feature that is particularly valuable in environments where datasets are large, complex, and continuously evolving. By using delete files, Iceberg can apply deletions without physically altering the data files, which leads to several key advantages.

Row-Level Deletions Without Rewrites

One of the most significant benefits of delete files is their ability to perform row-level deletions without rewriting the original data files. In traditional data management systems, deleting data often involves costly operations where entire files or partitions must be rewritten to exclude the deleted records. This can be both time-consuming and resource-intensive, especially in large-scale datasets.

In contrast, Apache Iceberg leverages delete files to mark specific rows as deleted while the original data files remain unchanged. This approach significantly reduces the overhead of deletions and ensures that data modifications can be handled quickly and efficiently.

Contents Inside a Delete File

To understand how delete files achieve their role in Apache Iceberg, it’s important to look at the specific metadata they contain. Each delete file is carefully structured to provide all the information necessary to identify and apply deletions to the relevant data files.

Key Fields in a Delete File

Here are some of the critical fields you’ll find inside a delete file:

file_path: This field indicates the path of the data file to which the delete file applies. It’s essential for mapping the delete operations to the correct data file in the dataset.
pos: Present in position delete files, this field specifies the exact position of the row within the data file that should be marked as deleted. This allows for precise, row-level deletions based on the physical layout of the data.
row: In equality delete files, the row field contains the values that identify which rows should be deleted. For instance, if a particular ID needs to be deleted across multiple data files, this field will hold that ID value.
partition: This field contains the partition information of the data that is subject to deletion. It helps ensure that the delete file is applied only to the relevant partitions, further optimizing the deletion process.
sequence_number: Iceberg uses sequence numbers to track the order of changes made to the data. The sequence_number in a delete file indicates when the deletion was committed relative to other changes in the dataset.

Conclusion

Delete files in Apache Iceberg are a powerful tool that enables efficient, performant, and precise row-level updates, especially in large-scale, distributed environments. By allowing row-level deletions without the need to rewrite entire data files, delete files optimize the performance of data lakehouse operations while maintaining the integrity and consistency of the dataset.

Understanding how to leverage both position and equality delete files is crucial for data engineers looking to implement scalable, performant data architectures.

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