[GitHub] [iceberg] powerzhangquan commented on a change in pull request #3432: Doc: add a page to explain row-level deletes

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powerzhangquan commented on a change in pull request #3432:
URL: https://github.com/apache/iceberg/pull/3432#discussion_r781784823



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File path: site/docs/row-level-deletes.md
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+
+# Row-Level Deletes
+
+Iceberg supports metadata-based deletion through the `DeleteFiles` interface.
+It allows you to quickly delete a specific file or any file that might match a given expression without the need to read or write any data in the table.
+
+Row-level deletes target more complicated use cases such as general data protection regulation (GDPR).
+Copy-on-write and merge-on-read are two different approaches to handle row-level delete operations. Here are their definitions in Iceberg:
+
+- **copy-on-write**: a delete directly rewrites all the affected data files.
+- **merge-on-read**: delete information is encoded in the form of _delete files_. The table reader can apply all delete information at read time.
+
+Overall, copy-on-write is more efficient in reading data, whereas merge-on-read is more efficient in writing deletes, but requires more maintenance and tuning to be performant in reading data with deletes.
+Users can choose to use **both** copy-on-write and merge-on-read for the same Iceberg table based on different situations. 
+For example, a time-partitioned table can have newer partitions maintained with the merge-on-read approach through a streaming pipeline,
+and older partitions maintained with the copy-on-write approach to apply less frequent GDPR deletes from batch ETL jobs.
+
+There are use cases that could only be supported by one approach such as change data capture (CDC).
+There are also limitations for different compute engines that lead them to prefer one approach over another.
+Please check out the documentation of the specific compute engines to see the details of their capabilities related to row-level deletes.
+This article will focus on explaining Iceberg's core design of copy-on-write and merge-on-read.
+
+!!!Note
+    Update is modeled as a delete with an insert within the same transaction in Iceberg, so this article only explains delete-related concepts. 
+
+## Copy-on-write
+
+In the copy-on-write approach, given a user's delete requirement, the write process would search for all the affected data files and perform a rewrite operation.
+
+For example, consider an unpartitioned table with schema `(id bigint, category string, data string)` that has the following files:
+
+```
+file A: (1, 'c1', 'data1'), (2, 'c1', 'data2')
+file B: (3, 'c2', 'data1'), (4, 'c2', 'data2')
+file C: (5, 'c3', 'data3'), (6, 'c3', 'data2')
+```
+
+A copy-on-write deletion of `data='data1'` can rewrite files A and B into a new file D. D contains the rows that were not deleted from files A and B. The table after the deletion looks like:
+
+```
+file C: (5, 'c3', 'data3'), (6, 'c3', 'data2')
+file D: (2, 'c1', 'data2'), (4, 'c2', 'data2')
+```
+
+There is no effect on read side in the copy-on-write approach.
+
+## Merge-on-read
+
+### Definitions
+
+Iceberg supports 2 different types of row-level delete files: **position deletes** and **equality deletes**.
+The **sequence number** concept is also needed to describe the relative age of data and delete files.
+If you are unfamiliar with these concepts, please read the [row-level deletes](../spec/#row-level-deletes) and [sequence numbers](../spec/#sequence-numbers) sections in the Iceberg spec for more information before proceeding.

Review comment:
       I tried to open two links 'row-level deletes' and 'sequence numbers', but I got 404, the path configuration should be '../spec.md#row-level-deletes' and '../spec.md# sequence-numbers'




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