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https://issues.apache.org/jira/browse/HIVE-30064?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18130256#comment-18130256
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Denys Kuzmenko commented on HIVE-30064:
---------------------------------------
Merged
Thank you, [~ryu_kobayashi] for the fix!
> Report accurate affected-row counts for copy-on-write UPDATE and MERGE
> ----------------------------------------------------------------------
>
> Key: HIVE-30064
> URL: https://issues.apache.org/jira/browse/HIVE-30064
> Project: Hive
> Issue Type: Bug
> Components: Iceberg integration
> Reporter: Ryu Kobayashi
> Assignee: Ryu Kobayashi
> Priority: Major
> Labels: pull-request-available
>
> h2. Summary
> Copy-on-write UPDATE and MERGE operations can report an incorrect number of
> affected rows.
> The FileSink currently counts all rows written by the rewritten query.
> However, copy-on-write rewrites entire data files, so the output also
> contains unchanged survivor rows. MERGE additionally combines UPDATE, INSERT,
> DELETE, and survivor branches into a shared output.
> h2. Root cause
> The affected-row count is derived from the number of rows written by the
> FileSink rather than from the logical operation that produced each row.
> As a result:
> - CoW UPDATE can include unchanged rows from rewritten files.
> - CoW MERGE can include INSERT rows and unchanged survivor rows in the same
> output.
> - The reported `numModifiedRows` value does not necessarily represent the
> rows actually updated by the statement.
> h2. Actual behavior
> It inserted 3 items and updated only 2, but it incorrectly claims 4 items are
> affected. This display can confuse users.
>
> {code:java}
> 0: jdbc:hive2://localhost:10000/> CREATE TABLE cow_update_count (a INT, b
> STRING)
> > STORED BY ICEBERG TBLPROPERTIES (
> > 'write.update.mode' = 'copy-on-write'
> > );
> No rows affected (0.664 seconds)
> 0: jdbc:hive2://localhost:10000/> INSERT INTO cow_update_count VALUES (1,
> 'one'), (2, 'two'), (3, 'three');
> 3 rows affected (6.545 seconds)
> 0: jdbc:hive2://localhost:10000/> UPDATE cow_update_count SET b = 'changed'
> WHERE a IN (2, 3);
> 4 rows affected (13.451 seconds){code}
>
> h2. Suggested change
> Add a `cow_update_matched` marker to the projections generated by the CoW
> UPDATE and MERGE rewriters.
> - Set the marker to `true` for rows produced by the matched UPDATE branch.
> - Set the marker to `false` for unchanged survivor rows and other branches
> that should not contribute to the UPDATE count.
> - Make `FileSinkOperator` count only rows whose marker is `true` when the
> marker mode is enabled.
> - Keep the marker as an internal column so it is not written to the target
> table.
> This avoids relying on the physical Tez plan shape or operator layout to
> infer the affected-row count.
> h2. Confirmed behavior
> The change should be covered by regression tests for:
> - CoW UPDATE with matching rows
> - CoW UPDATE with no matching rows
> - CoW MERGE with UPDATE, DELETE, and INSERT branches
> - CoW MERGE with only INSERT or DELETE branches
> - Partitioned and unpartitioned tables
> The tests should verify both the reported affected-row count and the final
> table contents.
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