Equality delete tests

EQUALITY_DELETE_POC_SUMMARY.md

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+# Equality Delete Write Path - Proof of Concept
+
+## Summary
+
+This document demonstrates that **PyIceberg already supports the WRITE path for equality delete files**, even though the read path is not yet implemented.
+
+## What Works
+
+✅ Creating `DataFile` objects with `equality_ids` set
+✅ Adding equality delete files to tables via transactions
+✅ Correctly tracking equality deletes in snapshot metadata
+✅ Storing equality delete files in manifests
+✅ Multiple equality delete files with different `equality_ids`
+✅ Composite equality keys (multiple field IDs)
+
+## What Doesn't Work
+
+❌ Reading tables with equality delete files (raises `ValueError`)
+❌ Applying equality deletes during scans
+
+## Key Findings
+
+### 1. Infrastructure Already in Place
+
+The codebase has all the necessary infrastructure for equality deletes:
+
+- **`DataFileContent.EQUALITY_DELETES`** enum defined (`manifest.py:67`)
+- **`equality_ids`** field in DataFile schema (`manifest.py:506`)
+- **Snapshot tracking** for equality delete counts (`snapshots.py:134-154`)
+- **Manifest serialization** works correctly
+
+### 2. No Tests with Actual `equality_ids` Values
+
+My research found:
+- **0 tests** that set `equality_ids` to non-empty values like `[1, 2, 3]`
+- All existing tests either set it to `[]` or `None`
+- Snapshot tests only verify the accounting/metrics, not actual functionality
+
+### 3. The Write API
+
+To add pre-calculated equality delete files:
+
+```python
+# Create DataFile with equality_ids
+delete_file = DataFile.from_args(
+    content=DataFileContent.EQUALITY_DELETES,  # Key: mark as equality delete
+    file_path="s3://bucket/delete-file.parquet",
+    file_format=FileFormat.PARQUET,
+    partition=Record(),
+    record_count=num_rows,
+    file_size_in_bytes=file_size,
+    equality_ids=[1, 2],  # Key: field IDs for equality matching
+    column_sizes={...},
+    value_counts={...},
+    null_value_counts={...},
+    _table_format_version=2,
+)
+delete_file.spec_id = table.metadata.default_spec_id
+
+# Add via transaction
+with table.transaction() as txn:
+    update_snapshot = txn.update_snapshot()
+    with update_snapshot.fast_append() as append_files:
+        append_files.append_data_file(delete_file)  # Works for delete files!
+```
+
+### 4. Key Classes and Methods
+
+| Class/Method | Location | Purpose |
+|--------------|----------|---------|
+| `Transaction.update_snapshot()` | `table/__init__.py:448` | Create UpdateSnapshot |
+| `UpdateSnapshot.fast_append()` | `table/update/snapshot.py:697` | Fast append operation |
+| `_SnapshotProducer.append_data_file()` | `table/update/snapshot.py:153` | Add file (data or delete) |
+| `DataFile.from_args()` | `manifest.py:443` | Create DataFile object |
+| `ManifestWriter.add()` | `manifest.py:1088` | Write manifest entry |
+
+## Test Results
+
+Two proof-of-concept tests were created and pass successfully:
+
+### Test 1: Single Equality Delete File
+- Creates table with 5 rows
+- Writes equality delete file with 2 rows (delete by `id`)
+- Adds delete file via transaction with `equality_ids=[1]`
+- Verifies metadata tracking
+- **Result**: ✅ PASSED
+
+### Test 2: Multiple Equality Delete Files
+- Creates 3 different delete files:
+  - Delete by `id` only (`equality_ids=[1]`)
+  - Delete by `name` only (`equality_ids=[2]`)
+  - Delete by `id` AND `name` (`equality_ids=[1, 2]`)
+- Adds all in single transaction
+- Verifies all tracked correctly
+- **Result**: ✅ PASSED
+
+```bash
+$ pytest test_add_equality_delete.py -v
+test_add_equality_delete.py::test_add_equality_delete_file_via_transaction PASSED
+test_add_equality_delete.py::test_add_multiple_equality_delete_files_with_different_equality_ids PASSED
+====== 2 passed in 1.06s ======
+```
+
+## Understanding `equality_ids`
+
+The `equality_ids` field specifies which columns to use for row matching:
+
+| Example | Meaning |
+|---------|---------|
+| `equality_ids=[1]` | Match rows where field 1 equals |
+| `equality_ids=[2]` | Match rows where field 2 equals |
+| `equality_ids=[1, 2]` | Match rows where fields 1 AND 2 both equal (composite key) |
+
+The delete file's Parquet schema must contain the columns corresponding to these field IDs.
+
+## Implications
+
+### For Users Who Want to Write Equality Deletes
+
+**You can start using equality deletes TODAY** if you:
+1. Generate equality delete Parquet files externally
+2. Use the transaction API shown above to add them
+3. Don't need to read the table with PyIceberg (use Spark/etc for reads)
+
+### For Developers
+
+The write path is **complete and working**. The remaining work is the read path:
+1. Remove the error at `table/__init__.py:1996-1997`
+2. Implement equality delete matching in `plan_files()`
+3. Extend `_read_deletes()` to handle equality delete schemas
+4. Apply equality deletes in `_task_to_record_batches()`
+
+## Files Created
+
+- **`test_equality_delete_poc.py`** - Detailed standalone test with output
+- **`test_add_equality_delete.py`** - Clean pytest test suite (2 tests)
+- **`EQUALITY_DELETE_POC_SUMMARY.md`** - This document
+
+## Conclusion
+
+The PyIceberg codebase **already supports writing equality delete files** through the transaction API. The infrastructure is solid and works correctly. This POC demonstrates that users can start adding pre-calculated equality delete files to their tables today, though they'll need external tools (like Spark) to read the tables until the read path is implemented.
+
+The `equality_ids` field, despite never being tested with actual values in the existing test suite, works perfectly for its intended purpose.