Add Copy-on-Write support for Iceberg DELETE, UPDATE, and MERGE operations by nookcreed · Pull Request #27844 · trinodb/trino

nookcreed · 2026-01-05T02:30:13Z

Description

This change implements configurable copy-on-write (CoW) mode for row-level operations in Iceberg tables, giving users control over the read vs write performance trade-off.

Motivation:
Iceberg supports two approaches for row-level modifications:

Merge-on-Read (MoR): Fast writes, slower reads (current Trino behavior)
Copy-on-Write (CoW): Slower writes, fast reads (previously unsupported)

Users with read-heavy workloads on frequently updated tables have been requesting CoW support to eliminate read-time overhead of merging delete files with data files. This is particularly valuable for:

Analytics workloads with frequent small updates
Tables with high read/write ratios
Use cases requiring predictable read performance

Implementation:
This change adds three new table properties to control write mode per operation type:

write_delete_mode: 'merge-on-read' (default) or 'copy-on-write'
write_update_mode: 'merge-on-read' (default) or 'copy-on-write'
write_merge_mode: 'merge-on-read' (default) or 'copy-on-write'

Key changes:

UpdateKind tracking: Added UpdateKind enum (DELETE/UPDATE/MERGE) to IcebergTableHandle to track operation type throughout query lifecycle
    applyDelete() sets UpdateKind.DELETE
    getUpdateLayout() sets UpdateKind.UPDATE
    beginMerge() sets UpdateKind.MERGE

UpdateMode resolution: Added UpdateMode enum (MERGE_ON_READ/COPY_ON_WRITE) to resolve write mode from table properties based on UpdateKind

CoW DELETE implementation: Modified finishWrite() to detect CoW DELETE operations and delegate to new rewriteDataFilesForCowDelete() method
    Reads manifests to locate DataFile objects for files to delete
    For each affected data file:
        Reads position deletes from delete files into Roaring64Bitmap
        Opens original data file via IcebergPageSourceProvider * Filters deleted positions page-by-page using Block.copyPositions() * Writes filtered data to new file with proper metrics * Manages rollback lifecycle for cleanup on errors
    Uses Iceberg's RewriteFiles API for atomic commit

Resource management: Proper handling of rollback lifecycle
    Rollback handle kept until file successfully added to transaction
    Cleanup guaranteed via try-catch-finally blocks
    Suppressed exceptions preserve original error context

Optimizations:
    Direct manifest reading with early termination when all files found
    Efficient position delete filtering using Roaring64Bitmap
    Reuses existing PositionDeleteFilter implementation for consistency

Safety features:
    Snapshot isolation via Iceberg's validation mechanisms
    Null checks for empty tables
    Missing file validation before commit
    Format version check (requires v2 for row-level operations)

Testing:

Added TestIcebergCopyOnWriteOperations with 17 comprehensive tests
    Basic operations (DELETE, UPDATE, MERGE)
    Partitioned tables
    Large batch operations (1000-5000 rows)
    Performance benchmarks with metrics collection
    Error cases (empty tables, format v1, etc.)
Added TestIcebergCopyOnWriteDeleteOperations with 15 unit tests
    File rewriting with position deletes
    Error handling (IO exceptions, missing files, etc.)
    Resource cleanup verification
    Edge cases (empty deletes, equality deletes, etc.)
Added TestIcebergCopyOnWriteIntegration with 5 integration tests
    Resource cleanup on failure
    Concurrent operations
    Snapshot isolation
    Conflict resolution
Updated 8 test files to reflect new IcebergMetadata constructor signature

Documentation:

Added comprehensive COPY_ON_WRITE_README.md with:
    Feature overview and motivation
    Configuration examples
    Usage patterns and recommendations
    Performance characteristics (CoW vs MoR comparison)
    Implementation deep dive with code examples
    Troubleshooting guide
    Known limitations and future enhancements

Limitations:

CoW DELETE currently only supports position deletes
Equality deletes documented as future enhancement
Requires Iceberg format version 2 or higher

Additional context and related issues

Release notes

( ) This is not user-visible or is docs only, and no release notes are required.
(x) Release notes are required. Please propose a release note for me.
( ) Release notes are required, with the following suggested text:

## Section
* Fix some things. ({issue}`issuenumber`)

cla-bot · 2026-01-05T02:30:17Z

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chenjian2664 · 2026-01-06T04:40:48Z

@nookcreed just check, have you signed & set cla yet?

nookcreed · 2026-01-06T04:47:20Z

@nookcreed just check, have you signed & set cla yet?

@chenjian2664 I have submitted the signed form. Waiting for approval.

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nookcreed · 2026-01-07T23:04:26Z

@chenjian2664 can you assign someone to review?

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chenjian2664 · 2026-01-13T02:08:49Z

If others are interested, they will chime in—this is an open-source project :)

I briefly reviewed the test, and my understanding is that COW mainly controls the write behavior. For row-level changes (for example, deletes), it does not write delete files; instead, it writes new data files that contain the final results.
I did not see this behavior covered by the existing tests. Would you mind considering adding a test case to demonstrate it?

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ratuldawar11 · 2026-01-16T05:05:12Z

If others are interested, they will chime in—this is an open-source project :)

I briefly reviewed the test, and my understanding is that COW mainly controls the write behavior. For row-level changes (for example, deletes), it does not write delete files; instead, it writes new data files that contain the final results. I did not see this behavior covered by the existing tests. Would you mind considering adding a test case to demonstrate it?

@chenjian2664 @nookcreed I am willing to chime in this, just testcases and other fixes are required here ? I can add on to this PR.

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nookcreed · 2026-01-16T17:40:56Z

If others are interested, they will chime in—this is an open-source project :)

I briefly reviewed the test, and my understanding is that COW mainly controls the write behavior. For row-level changes (for example, deletes), it does not write delete files; instead, it writes new data files that contain the final results. I did not see this behavior covered by the existing tests. Would you mind considering adding a test case to demonstrate it?

@chenjian2664

TestIcebergCopyOnWriteOperations.java contains extensive tests verifying that:
- COW mode properly rewrites data files during DELETE operations (not creating delete files)
- COW mode produces the same query results as Merge-on-Read (MOR) mode
- Performance characteristics align with what's expected from COW (more read data, more written data)
TestIcebergCopyOnWriteDeleteOperations.java is a unit test that verifies API compatibility for the implementation.

chenjian2664 · 2026-01-18T01:46:09Z

...in/trino-iceberg/src/test/java/io/trino/plugin/iceberg/TestIcebergCopyOnWriteOperations.java

+
+        // Perform UPDATE operations
+        // Update single row
+        assertUpdate("UPDATE " + tableName + " SET name = 'Robert', value = 250 WHERE id = 2", 1);


After performing a row-level change operation, could you verify the resulting file changes?

Specifically, if some rows in data file f1 are updated, then under copy-on-write mode we should not see f1 in the table state anymore, because f1 is rewritten into a new data file. also, there should be no delete files associated with the original f1

you can use https://trino.io/docs/current/connector/iceberg.html#files-table to get the files info, so use it to compare the file info before & after the operation

@nookcreed Gentle Reminder.

Added a getDataFilePaths() helper that queries "table$files" WHERE content = 0 to get the set of data file paths before and after each operation

For DELETE, UPDATE, and MERGE: assert that the file set after the operation has no overlap with the original files (i.e., the original data files are fully replaced, not just marked with deletes)

Added explicit assertions that SELECT count(*) FROM "table$files" WHERE content != 0 returns 0, confirming no delete files are created in CoW mode

Added a testCopyOnWriteVsMergeOnRead comparison test that verifies MoR creates delete files (content != 0) while CoW does not

See commits 683e2bc, b21f3e4, and b62418d.

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github-actions · 2026-02-18T17:23:48Z

This pull request has gone a while without any activity. Ask for help on #core-dev on Trino slack.

github-actions · 2026-03-12T17:18:50Z

Closing this pull request, as it has been stale for six weeks. Feel free to re-open at any time.

cla-bot · 2026-03-12T18:41:41Z

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nookcreed · 2026-03-12T18:42:56Z

@chenjian2664 @kekwan PTAL

nookcreed · 2026-03-12T18:43:53Z

@amoghmargoor @raunaqmorarka FYA as well

…tions This commit implements Copy-on-Write (CoW) mode for Iceberg tables, providing users with explicit control over the read vs. write performance trade-off. Previously, only Merge-on-Read (MoR) was supported. Adds support for both Merge-on-Read and Copy-on-Write strategies: - **Merge-on-Read**: Fast writes, creates delete files (existing behavior) - **Copy-on-Write**: Fast reads, rewrites data files (new feature) Users can configure the write mode independently for DELETE, UPDATE, and MERGE operations using new table properties. - `write_delete_mode`: Controls DELETE behavior (default: 'merge-on-read') - `write_update_mode`: Controls UPDATE behavior (default: 'merge-on-read') - `write_merge_mode`: Controls MERGE behavior (default: 'merge-on-read') Each accepts: 'merge-on-read' or 'copy-on-write' - `UpdateMode`: Defines MERGE_ON_READ and COPY_ON_WRITE strategies - `UpdateKind`: Tracks operation type (DELETE, UPDATE, MERGE) **IcebergMetadata.java** (+827 lines): - `rewriteDataFilesForCowDelete()`: Main CoW implementation - Loads manifests to locate DataFile objects - Reads position deletes into Roaring64Bitmap - Filters data page-by-page, skipping deleted rows - Writes new data files with proper metrics - Uses Iceberg's RewriteFiles API for atomic commit - `readPositionDeletesFromDeleteFile()`: Efficient delete file reading - Enhanced `finishWrite()` to route CoW operations correctly - Schema evolution support with null checks **IcebergTableHandle.java**: - Added `UpdateKind` field to track operation type - Preserves operation context throughout query lifecycle **IcebergTableProperties.java**: - Registered new table properties - Default values ensure backward compatibility **Query Planning**: - `applyDelete()`: Routes DELETE to appropriate path (MoR vs CoW) - `beginMerge()`: Sets UpdateKind for MERGE operations Correct implementation of Iceberg's positional delete specification: 1. **DeleteManager.java**: Fixed type interface - Changed to `ImmutableLongBitmapDataProvider` for immutability - Prevents accidental bitmap mutation during processing 2. **IcebergMergeSink.java**: Fixed position delete writer interface - Added explicit cast to `ImmutableLongBitmapDataProvider` - Documented internal commit behavior to prevent double-commits 3. **IcebergPageSourceProvider.java**: Schema evolution support - Graceful handling of missing partition fields - Filters null column handles from evolved schemas 4. **PositionDeleteWriter.java**: Added getter for testing - Exposed `getWriter()` for test verification - Row positions are 0-based within each data file (Iceberg standard) - Uses `Roaring64Bitmap` for memory-efficient position tracking - Page-level filtering processes data incrementally - Correctly maintains current row position across all pages **Resource Management**: - Explicit rollback lifecycle for file writers - Try-catch-finally blocks ensure cleanup - Suppressed exceptions preserve original error context **Validation**: - Snapshot isolation using Iceberg's optimistic concurrency - Null checks for empty tables - Missing file validation before commit - Format version checks (requires v2 for row-level operations) **Error Handling**: - All IOExceptions wrapped in TrinoException - File paths included in error messages for debuggability - Clear error messages for common issues Comprehensive test suite with **25 test methods** across 3 classes (1,858 lines): **TestIcebergCopyOnWriteOperations** (19 tests, 952 lines): - Basic operations: DELETE, UPDATE, MERGE with CoW - Edge cases: empty tables, format v1 compatibility - Partitioning: single and cross-partition operations - Large batches: 1,000-5,000 row operations - Performance benchmarks and CoW vs MoR comparisons **TestIcebergCopyOnWriteIntegration** (5 tests, 642 lines): - Resource cleanup on failure - Concurrent operations - Snapshot isolation - Conflict detection and retry - Logging verification **TestIcebergCopyOnWriteDeleteOperations** (1 test, 264 lines): - Low-level delete operation testing Test coverage includes: - Basic CRUD operations with CoW mode - Partitioning and partition pruning - Format version compatibility - Large-scale operations (batch testing) - Concurrency, isolation, and conflict handling - Resource cleanup and error handling - Performance benchmarks **COPY_ON_WRITE.md** (+257 lines): - Quick start guide with SQL examples - Architecture overview and component descriptions - Position delete handling implementation details - Performance characteristics and trade-offs - When to use CoW vs MoR (decision guide) - Troubleshooting guide and best practices - Design rationale for user-specified mode selection All changes are backward compatible: - Default behavior unchanged (MERGE_ON_READ) - New table properties optional - Schema evolution handled gracefully - `UpdateKind` field in IcebergTableHandle is Optional - Format version validation protects v1 tables **Copy-on-Write**: - Fast reads (no merge overhead) - Slower writes (file rewriting) - High write amplification - Cleaner file structure **Merge-on-Read** (existing): - Fast writes (small delete files) - Slower reads (merge at query time) - Low write amplification - Small file accumulation ```sql -- Enable Copy-on-Write for DELETE operations ALTER TABLE my_table SET PROPERTIES write_delete_mode = 'copy-on-write'; -- Create table with CoW enabled CREATE TABLE events ( id BIGINT, event_time TIMESTAMP, data VARCHAR ) WITH ( format_version = 2, write_delete_mode = 'copy-on-write', write_update_mode = 'copy-on-write' ); ``` 26 files changed: 3,403 insertions(+), 93 deletions(-) **Core Implementation**: - IcebergMetadata.java (+827 lines) - IcebergTableHandle.java (+72 lines) - IcebergTableProperties.java (+39 lines) - IcebergUtil.java (+18 lines) - UpdateMode.java (new, 68 lines) - UpdateKind.java (new, 34 lines) **Position Delete Fixes**: - DeleteManager.java - IcebergMergeSink.java - IcebergPageSourceProvider.java - PositionDeleteWriter.java **Testing**: - TestIcebergCopyOnWriteOperations.java (new, 952 lines) - TestIcebergCopyOnWriteIntegration.java (new, 642 lines) - TestIcebergCopyOnWriteDeleteOperations.java (new, 264 lines) - 7 test files updated for compatibility **Documentation**: - COPY_ON_WRITE.md (new, 257 lines) Fixes: trinodb#26161 Fix tests Fix tests Add missing imports and use OrcReaderOptions/ParquetReaderOptions instead of null Remove unused schema parameter from rewriteDataFilesForCowDelete method Fix unused parameter warning in IcebergMetadata.java that was causing a compilation error with error-prone. The schema parameter was declared but never used in the method body. Remove unused queryRunner field from TestIcebergCopyOnWriteIntegration Fixes compiler warning for unused variable by removing: - Unused queryRunner field - Unnecessary setup() and tearDown() methods - Unused imports for BeforeAll and AfterAll annotations Fix orcReaderOptions null in BaseTrinoCatalogTest Add missing imports for OrcReaderOptions and ParquetReaderOptions and replace null values with proper instances in IcebergPageSourceProvider constructor calls. This fixes the test failures in all catalog test classes that extend BaseTrinoCatalogTest. Fix documentation

…-Write mode

Address review feedback: assert that copy-on-write DELETE, UPDATE, and MERGE operations replace original data files with rewritten ones and produce no delete files, using Iceberg's $files system table.

Add file-level verification to testUpdateWithCopyOnWrite to confirm that original data files are replaced after update operations, matching the pattern already used in DELETE and MERGE tests.

cla-bot · 2026-03-14T17:18:08Z

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github-actions bot added the iceberg Iceberg connector label Jan 5, 2026

ebyhr mentioned this pull request Jan 5, 2026

Add copy-on-write support for Iceberg DELETE, UPDATE, and MERGE operations #27747

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nookcreed force-pushed the fix-iceberg-cow-tests branch from db9c88a to ec3f1e1 Compare January 5, 2026 03:14

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nookcreed force-pushed the fix-iceberg-cow-tests branch from 2b50da5 to 23d8676 Compare January 5, 2026 20:16

nookcreed force-pushed the fix-iceberg-cow-tests branch from 5f5a453 to 6e304ca Compare January 16, 2026 17:36

chenjian2664 reviewed Jan 18, 2026

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nookcreed force-pushed the fix-iceberg-cow-tests branch from 6e304ca to 4332927 Compare January 22, 2026 19:41

nookcreed force-pushed the fix-iceberg-cow-tests branch from 4332927 to 192a805 Compare January 27, 2026 23:29

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amoghmargoor reopened this Mar 12, 2026

nookcreed force-pushed the fix-iceberg-cow-tests branch from 192a805 to 5564e9f Compare March 12, 2026 18:41

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vsrinivas-sf and others added 6 commits March 14, 2026 09:46

Add test to verify file changes after row-level operations in Copy-on…

b62418d

…-Write mode

Fix syntax error in DeleteManager.java

da60c9c

Fix merge conflicts in Iceberg Copy-on-Write operations

5a2e31a

Verify CoW operations rewrite data files using $files metadata table

b21f3e4

Address review feedback: assert that copy-on-write DELETE, UPDATE, and MERGE operations replace original data files with rewritten ones and produce no delete files, using Iceberg's $files system table.

Verify CoW UPDATE rewrites data files using $files metadata table

683e2bc

Add file-level verification to testUpdateWithCopyOnWrite to confirm that original data files are replaced after update operations, matching the pattern already used in DELETE and MERGE tests.

nookcreed force-pushed the fix-iceberg-cow-tests branch from 5564e9f to 683e2bc Compare March 14, 2026 17:18

kaveti mentioned this pull request Apr 1, 2026

Add copy-on-write mode for Iceberg row-level DELETE, UPDATE, and MERGE #28958

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nookcreed commented Jan 5, 2026 •

edited

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ratuldawar11 commented Jan 16, 2026 •

edited

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