fix(core): fix missing partition dir on table reader open when writing identical data with dedup by ideoma · Pull Request #6479 · questdb/questdb

ideoma · 2025-12-01T13:15:33Z

Fixes storage error that can result in transient query failures withan error message similar to:

2025-11-28T12:05:43.150214Z C i.q.c.h.p.JsonQueryProcessorState [5526041371967] error [msg=`Partition '2025-11-28T075000-000001' does not exist in table 'crypto_price_1m' directory. Run [ALTER TABLE table_name FORCE DROP PARTITION LIST '2025-11-28T075000-000001'] to repair the table or the database from the backup.`, errno=0, q=`

The bug is that PartitionPurgeJob can see a partition version while scanning the directories that will not be committed by the TableWriter and removed before the commit. However, PartitionPurgeJob scans partitions before reading _txn file, and when it reads the _txn file, the transaction can already be committed, so it counts those "phantom" deleted partitions as valid versions and assumes that they can be used by TableReaders.

The fix is to check that the next partition exists before deleting the overwritten partition version.

Step by step:

The race condition occurs when:

O3PartitionPurgeJob scans partition directories and finds versions .0, .1, .2
Meanwhile, TableWriter creates partition .2 temporarily for a potential copy-on-write operation
TableWriter determines copy-on-write isn't needed (e.g., identical data with dedup), removes .2, and commits
O3PartitionPurgeJob reads _txn file (now updated), sees the commit succeeded
O3PartitionPurgeJob calculates that .0 and .1 are safe to delete because .2 exists
But .2 was already removed - it was a "phantom" partition
TableReader later tries to open the partition but finds no valid version

…g identical data with dedup

coderabbitai · 2025-12-01T13:15:39Z

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Walkthrough

Changes add a validation guard in the partition purge job to detect phantom partitions (those created and removed before transaction commit) and introduces an error counter in PartitionOverwriteControl to track partition overwrite errors. Tests are enhanced with assertions verifying the counter remains zero and include a new phantom partition scenario test.

Changes

Cohort / File(s)	Summary
Phantom partition validation and error tracking `core/src/main/java/io/questdb/cairo/O3PartitionPurgeJob.java`, `core/src/main/java/io/questdb/cairo/PartitionOverwriteControl.java`	Added existence check for partition directory in purge processing to skip phantom partitions with informational logging. Introduced `AtomicInteger errorCount` field and public `getErrorCount()` getter in PartitionOverwriteControl; increments counter before throwing partition overwrite exceptions.
Test validation and scenarios `core/src/test/java/io/questdb/test/cairo/o3/O3PartitionPurgeTest.java`	Added assertions across multiple test methods verifying error counter equals zero post-purge. Introduced new `testPhantomPartition()` test method simulating partition disappearance via FilesFacade override to validate purge proceeds without errors.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Addition of straightforward existence check guard in purge job
Basic atomic counter initialization and getter in control class
Repetitive test assertion additions across multiple methods

Suggested labels

Bug, storage

Suggested reviewers

bluestreak01

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run `@coderabbitai generate docstrings` to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: preventing errors from phantom partitions when deduplicating identical data.
Description check	✅ Passed	The description comprehensively explains the race condition, its consequences, and the implemented fix with clear step-by-step context.

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ideoma · 2025-12-01T14:47:50Z

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coderabbitai · 2025-12-01T14:47:57Z

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coderabbitai · 2025-12-01T15:17:57Z

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📥 Commits

Reviewing files that changed from the base of the PR and between 2131fbe and c7b6842.

📒 Files selected for processing (3)

core/src/main/java/io/questdb/cairo/O3PartitionPurgeJob.java (1 hunks)
core/src/main/java/io/questdb/cairo/PartitionOverwriteControl.java (3 hunks)
core/src/test/java/io/questdb/test/cairo/o3/O3PartitionPurgeTest.java (21 hunks)

🧰 Additional context used

🧬 Code graph analysis (1)

core/src/main/java/io/questdb/cairo/O3PartitionPurgeJob.java (1)

core/src/main/java/io/questdb/cairo/TableUtils.java (1)

TableUtils (82-2127)

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🔇 Additional comments (5)

core/src/main/java/io/questdb/cairo/PartitionOverwriteControl.java (3)

33-33: LGTM!

Appropriate import for the error counter field.

40-40: LGTM!

The error counter field is correctly declared as final AtomicInteger, ensuring thread-safe access in the concurrent environment where partition purge operations occur.

81-83: LGTM!

The error counter implementation correctly increments before throwing the exception and provides a public getter for test verification. The AtomicInteger ensures thread-safe operations.

Also applies to: 104-104

core/src/test/java/io/questdb/test/cairo/o3/O3PartitionPurgeTest.java (2)

137-137: LGTM!

The error count assertions are correctly added to all test methods to verify that purge operations complete without triggering partition overwrite errors. This ensures the phantom partition fix doesn't introduce false positives.

Also applies to: 210-210, 248-248, 276-276, 311-311, 402-402, 439-439, 480-480, 532-532, 669-669, 716-716, 750-750, 810-810, 830-830, 881-881, 933-933, 975-975, 1032-1032, 1094-1094, 1156-1156

573-635: Excellent test coverage for the phantom partition scenario.

The test effectively simulates the race condition described in the PR objectives:

Creates partition versions and a phantom directory that will be scanned

Mocks FilesFacade.exists() to return false for the phantom partition, simulating its removal between directory scanning and the existence check

Verifies the purge job aborts gracefully without recording errors

This validates the fix in O3PartitionPurgeJob.processPartition0 handles phantom partitions correctly.

core/src/main/java/io/questdb/cairo/O3PartitionPurgeJob.java

…tErrorCount() is 0

glasstiger · 2025-12-01T16:17:27Z

[PR Coverage check]

😍 pass : 10 / 11 (90.91%)

file detail

	path	covered line	new line	coverage
🔵	io/questdb/cairo/PartitionOverwriteControl.java	2	3	66.67%
🔵	io/questdb/cairo/O3PartitionPurgeJob.java	8	8	100.00%

fix(core): fix missing partition dir on table reader open when writin…

c7b6842

…g identical data with dedup

ideoma changed the title ~~fix(core): fix missing partition dir on table reader open when writing identical data with dedupg identical data with dedupg identical data with dedup~~ fix(core): fix missing partition dir on table reader open when writing identical data with dedup Dec 1, 2025

coderabbitai bot reviewed Dec 1, 2025

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core/src/main/java/io/questdb/cairo/O3PartitionPurgeJob.java Outdated Show resolved Hide resolved

bluestreak01 and others added 3 commits December 1, 2025 15:24

fix typos

1fc5d03

Merge branch 'master' into fix-missing-partition

1448191

add fuzz test assertion that engine.getPartitionOverwriteControl().ge…

4e3adb7

…tErrorCount() is 0

bluestreak01 approved these changes Dec 2, 2025

View reviewed changes

bluestreak01 merged commit 33a3e6d into master Dec 2, 2025
43 checks passed

bluestreak01 deleted the fix-missing-partition branch December 2, 2025 01:36

coderabbitai bot mentioned this pull request Dec 24, 2025

Fix HAVING clause in Cube.js driver #6578

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coderabbitai bot mentioned this pull request Jan 12, 2026

fix(core): fix transient file does not exist error in queries #6629

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fix(core): fix missing partition dir on table reader open when writing identical data with dedup#6479

fix(core): fix missing partition dir on table reader open when writing identical data with dedup#6479
bluestreak01 merged 4 commits intomasterfrom
fix-missing-partition

ideoma commented Dec 1, 2025 •

edited by bluestreak01

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ideoma commented Dec 1, 2025 •

edited by bluestreak01

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