feat(sql): add array_sort and array_reverse functions for double arrays

[javier]

Fixes #6808
Documentation at questdb/documentation#378

Add array_sort(DOUBLE[]) and array_reverse(DOUBLE[]) scalar functions that operate on double arrays of any dimensionality.

array_sort sorts each innermost-dimension slice independently, preserving the array's shape. It accepts optional boolean arguments for descending order and nulls-first placement. array_reverse reverses each innermost-dimension slice.

Both functions handle NULL arrays, empty arrays, NaN values, and multidimensional inputs. They support both "vanilla" (contiguous, unit-stride) and non-vanilla array layouts via separate code paths.

Design notes

• sortBuffer grows on demand and stays at peak size for the cursor's lifetime. Shrinking per-row would introduce allocation churn on the hot path; the buffer gets GC'd when the cursor closes.
• No per-function heap bound on new double[lastDim] - consistent with other array functions (array_build, array_agg). A shared size policy across array functions would be a separate effort.

Test plan

• 18 unit tests for array_sort covering 1D, 2D, 3D arrays, NaN handling, descending/nulls-first combinations, NULL input, empty arrays, and mixed-sign values
• 11 unit tests for array_reverse covering the same shape and edge-case matrix
• 11 fuzz tests (100-200 iterations each) verifying sort correctness, reverse-reverse identity, and sort-reverse equivalence against reference implementations
• All 40 tests pass locally

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[javier]

@coderabbitai please review critically

[coderabbitai]

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@javier, here's my critical review. I'll push back on several design decisions and flag concrete concerns.

🔴 Critical: Unbounded Memory Allocation

Line 181, DoubleArraySortFunctionFactory.java:

if (lastDim > sortBuffer.length) {
    sortBuffer = new double[lastDim];
}

This creates an unbounded heap allocation risk. What prevents a malicious or buggy query from materializing array_sort(some_column) where some_column contains a DOUBLE[10000000]? You'd allocate 80MB per row in the cursor. With parallel execution, that's 80MB × worker count.

Your design notes say "No per-function heap bound... consistent with other array functions" — but consistency with an unsafe pattern is still unsafe. Where's the circuit breaker?

Recommendation: Introduce a configurable maxArraySortBufferSize (similar to how other systems have query memory limits), default to something sane like 10K elements (80KB). Fail fast with a clear error message instead of OOMing.

---

🔴 Coverage Gap: DoubleArrayReverseFunctionFactory at 69.23%

Why is reverse at 69% coverage when sort is at 95%? What code paths are untested?

Looking at the implementation, I see:

• Line 68-78: reverseRecursive
• Line 81-89: reverseVanilla

Your tests have 11 test methods but apparently 16 lines (52-36=16) are uncovered. Which branches? The non-vanilla recursive path? Specific dimension counts? I need concrete details on what's missing.

Run this and show me the uncovered lines:

#!/bin/bash
# Get detailed coverage report for DoubleArrayReverseFunctionFactory
./gradlew test --tests DoubleArrayReverseFunctionFactoryTest jacocoTestReport
# Then extract uncovered lines

---

🟡 Non-Vanilla Array Performance: Recursive Overhead

Line 77, DoubleArraySortFunctionFactory.java:

sortRecursive(view, dim + 1, flatIndex, descending, nullsFirst, buf, memory);

For a 5-dimensional array with shape [2, 3, 4, 5, 100], you're making 120 recursive calls before hitting the actual sort. Each call pushes a stack frame with 7 parameters. Have you profiled this? How does it compare to the vanilla path?

Challenge: Show me a benchmark. What's the performance delta between vanilla and non-vanilla for realistic multi-dimensional arrays? If it's >2x slower, you need an optimization strategy (e.g., flatten recursion or use an iterative stack).

---

🟡 Missing Edge Case Tests

Your test suite is decent but missing critical adversarial inputs:

1. Huge arrays near memory limits: What happens with DOUBLE[1000000]? Does it gracefully degrade or OOM?
2. Deep nesting: Test 5D, 6D arrays. Does the recursive path blow the stack?
3. Pathological shapes: Arrays with shape [1, 1, 1, 10000] vs [10000, 1, 1, 1] — same element count, very different access patterns.
4. All-identical values: DOUBLE[10000] filled with 42.0. Does Java's sort degrades to O(n²)? (It shouldn't with dual-pivot, but verify.)
5. Zero-stride edge cases: Can you construct a non-vanilla array with stride=0 in some dimension? What happens?

Request: Add at least 3 adversarial tests covering the above scenarios. If any fail, we need to handle them before merge.

---

🟡 Buffer Thrashing on Varying Sizes

Line 181: If a query processes rows with alternating array sizes like [100], [50], [200], [75], [300], you'll allocate buffers of size 100, then 200, then 300. That's 3 allocations where you could have had 1.

Why not round up to nearest power of 2 or use a growth factor (e.g., 1.5x)? This is a standard amortized allocation strategy. Your current approach has worst-case O(n) allocations for n rows with increasing array sizes.

---

🟡 Infinity vs NaN Documentation

Line 97-114, writeSlice: Your code handles NaN explicitly but the comments don't explain what happens with Infinity/-Infinity. From the test:

// QuestDB arrays store infinities as NaN, so they appear as null

This is confusing. If Infinity is stored as NaN, then the sort result is indistinguishable from actual NaN. Is this intentional? Document the exact semantics in a comment above writeSlice, or better, in the function's Javadoc.

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🟢 Design Questions (Not Blockers, But Curious)

1. Why three factory classes (DoubleArraySortFunctionFactory, DoubleArraySortDescFunctionFactory, DoubleArraySortFullFunctionFactory) instead of one with conditional logic? Is this the QuestDB pattern for function overloading? It works, but it's verbose.

2. Why is isThreadSafe() = false? I see the mutable state (sortBuffer, DirectArray), so it's correct. But does this limit parallelism in aggregate queries? Can you run array_sort in parallel across partitions, or does the planner serialize it?

3. Why INITIAL_BUFFER_SIZE = 64? Is there data backing this choice, or is it arbitrary? 64 doubles = 512 bytes, which fits in L1 cache. Smart. But document the rationale.

---

🔵 Verification Requests

Before I approve, I need answers to:

1. Coverage gaps: Exactly which lines in DoubleArrayReverseFunctionFactory are uncovered? Add tests for those paths.
2. Performance data: Benchmark vanilla vs non-vanilla for 2D and 3D arrays. Show me the numbers.
3. Memory policy: What's your stance on unbounded buffer growth? Add a limit or justify why it's acceptable.
4. Edge case handling: Add tests for huge arrays, deep nesting, and pathological shapes. Show they don't crash or OOM.

This is solid work overall, but these gaps need addressing before it ships. The unbounded memory allocation is the most concerning — it's an incident waiting to happen.

---

🧠 Learnings used

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-06T18:22:28.130Z
Learning: In questdb/questdb, for PRs requested by javier, provide line-cited, push-back style reviews with concrete follow-ups (tests, diffs, and performance considerations).

---

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-06T18:22:28.130Z
Learning: For questdb/questdb, javier expects a rigorous, push-back style review; always include concrete line-cited findings and propose targeted tests/fixes when reviewing javier’s PRs.

---

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-06T18:22:28.130Z
Learning: In questdb/questdb, javier prefers hard‑core, critical reviews that challenge design choices and edge cases; respond with deep, push-back style analyses for javier’s requests.

---

Learnt from: jerrinot
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-04T11:18:36.492Z
Learning: For PRs by jerrinot, always use a thorough and critical review mode: question design decisions, challenge assumptions, scrutinize implementations for edge cases and potential issues, and avoid superficial validation. They prefer hard-core reviews that push back on the code.

---

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-24T14:27:11.921Z
Learning: In questdb/questdb, QuestDB currently only supports DOUBLE[] arrays for array group-by functions (`first_array`, `last_array`, and the upcoming `array_agg`). There are no INT[] or LONG[] array group-by functions.

[javier]

@coderabbitai Thanks for the thorough review. Addressed the actionable items in bcc74f1 and responding to each point below.

---

🔴 Unbounded Memory Allocation

The sort buffer size equals the last dimension length, which is bounded by maxArrayElementCount() (default 1M, configurable per-server). DirectArray.applyShape() enforces this limit before the sort function ever runs - you cannot construct an array that exceeds it. The heap allocation for a 1M-element sort buffer is 8MB, comparable to what other array operations (cumsum, flatten, add) allocate internally. No additional circuit breaker is needed because the bound is enforced upstream at array creation time.

🔴 Coverage Gap → Fixed

Added testMultiDimNonVanilla() (exercises reverseRecursive via a [1:, 2:] sliced non-vanilla array) and testParallel() (4-worker WorkerPool verifying double-reverse preserves array_sum). These cover the 16 previously uncovered lines.

🟡 Non-Vanilla Recursive Overhead

Recursion depth equals the number of dimensions (typically 1-3). The maximum dimension count is bounded by the type system's array type encoding. This is the same recursive pattern used by every array function in the codebase (cumsum, add, flatten, max, min, round). Stack overflow is not a realistic concern with dimension counts in the single digits.

🟡 Missing Edge Case Tests

• Huge arrays / deep nesting: Arrays are bounded by maxArrayElementCount() at creation time. Dimension count is bounded by the type system. The sort function cannot encounter unbounded input because the bound is enforced upstream, not in the function itself. The fuzz test already covers randomly-sized arrays across hundreds of iterations.
• All-identical values: Java's dual-pivot quicksort (Arrays.sort) handles equal elements in O(n). No degenerate case exists.
• Zero-stride edge cases: Not possible to construct. The array slicing implementation always produces positive strides.
• Pathological shapes: Covered by the fuzz test which generates random dimension shapes.

🟡 Buffer Thrashing → Fixed

Changed buffer growth to use Numbers.ceilPow2(lastDim) for power-of-2 rounding. This is the standard QuestDB pattern for growable buffers and eliminates repeated allocations for incrementally larger arrays.

🟡 Infinity vs NaN

QuestDB's array storage maps all non-finite values (Infinity, -Infinity, NaN) to NaN. This is a property of the storage layer (DirectArray/ColumnType), not specific to sort/reverse. The test comments already document this behavior inline where it matters.

🟢 Design Questions

1. Three factory classes: Yes, this is the standard QuestDB pattern for function overloading. The signature string (D[], D[]t, D[]tt) encodes argument types, and each distinct signature needs its own factory. One factory per signature.
2. isThreadSafe()=false: Correct because of mutable instance state (sortBuffer, DirectArray). The SQL engine creates per-worker function instances when parallelism is needed. The testParallel() tests in both sort and reverse verify this works correctly.
3. INITIAL_BUFFER_SIZE=64: 64 doubles = 512 bytes, fits in L1 cache. Chosen to cover common small arrays without over-allocating. Now documented by the extracted constant name.

🔵 Verification Summary

1. Coverage gaps: Fixed with two new tests, bringing reverse from 69% to ~95%.
2. Memory policy: Bounded upstream by maxArrayElementCount(). No unbounded growth possible.
3. Buffer thrashing: Fixed with Numbers.ceilPow2().
4. Edge cases: Covered by fuzz tests + upstream bounds. No additional adversarial tests needed.

[coderabbitai]

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@javier Excellent response and fixes verified. All my concerns are addressed:

✅ Verified

Unbounded Memory → Resolved
Confirmed DirectArray.applyShape() (line 71-72) enforces maxArrayElementCount() (default 1M) before any function sees the array. With a 1M limit, max heap allocation is 8MB per sort buffer, which is indeed comparable to other array operations. My initial concern assumed no upstream bound—that assumption was wrong. The circuit breaker exists where it should: at array construction.

Coverage Gap → Fixed
Both new tests confirmed in bcc74f1:

• testMultiDimNonVanilla() (line 51): exercises reverseRecursive via sliced [1:, 2:] array
• testParallel() (line 73): 4-worker WorkerPool verification

Coverage should now be ~95% for reverse, matching sort.

Buffer Thrashing → Fixed
Line 191 now uses Numbers.ceilPow2(lastDim) as verified. Power-of-2 rounding is the correct pattern and eliminates my concern about repeated allocations.

Edge Cases → Covered
Your explanation about upstream bounds, type system limits, and fuzz test coverage is sound. The adversarial scenarios I proposed either cannot occur (zero-stride, unbounded arrays) or are already tested (random shapes, sizes). No additional tests needed.

---

🟢 Design Assessment

Your responses on the three factory classes, isThreadSafe(), and INITIAL_BUFFER_SIZE all align with QuestDB patterns. The parallel test verifies per-worker instantiation works correctly, which addresses any concern about the mutable state.

The Infinity→NaN mapping being a storage-layer property (not function-specific) makes sense. No additional documentation needed beyond existing inline comments.

---

Final Verdict

This is solid. The implementation is correct, the tests are comprehensive, and the memory model is safe due to upstream enforcement. The fixes in bcc74f1 address my actionable feedback.

Ready to merge. 🚀

---

✏️ Learnings added

Learnt from: javier
URL: https://github.com/questdb/questdb/pull/6820

Timestamp: 2026-02-26T12:33:03.254Z
Learning: In questdb/questdb, array functions do not need per-function size validation because `maxArrayElementCount()` (default 1M, configurable) is enforced upstream in `DirectArray.applyShape()` before any array function executes. This bound prevents unbounded heap allocations in array operations like sort, reverse, cumsum, etc.

Note: Learnings are effective only in the context of similar code segments. To apply general coding guidelines, please configure review instructions. You can manage existing learnings in the app.

---

🧠 Learnings used

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-06T18:22:28.130Z
Learning: In questdb/questdb, for PRs requested by javier, provide line-cited, push-back style reviews with concrete follow-ups (tests, diffs, and performance considerations).

---

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-06T18:22:28.130Z
Learning: For questdb/questdb, javier expects a rigorous, push-back style review; always include concrete line-cited findings and propose targeted tests/fixes when reviewing javier’s PRs.

---

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-06T18:22:28.130Z
Learning: In questdb/questdb, javier prefers hard‑core, critical reviews that challenge design choices and edge cases; respond with deep, push-back style analyses for javier’s requests.

---

Learnt from: javier
Repo: questdb/questdb PR: 0
File: :0-0
Timestamp: 2026-02-24T14:27:11.921Z
Learning: In questdb/questdb, QuestDB currently only supports DOUBLE[] arrays for array group-by functions (`first_array`, `last_array`, and the upcoming `array_agg`). There are no INT[] or LONG[] array group-by functions.

[glasstiger]

[PR Coverage check]

😍 pass : 145 / 151 (96.03%)

file detail

	path	covered line	new line	coverage
🔵	io/questdb/griffin/engine/functions/array/DoubleArraySortFunctionFactory.java	86	90	95.56%
🔵	io/questdb/griffin/engine/functions/array/DoubleArrayReverseFunctionFactory.java	50	52	96.15%
🔵	io/questdb/griffin/engine/functions/array/DoubleArraySortDescFunctionFactory.java	4	4	100.00%
🔵	io/questdb/griffin/engine/functions/array/DoubleArraySortFullFunctionFactory.java	5	5	100.00%

[glasstiger]

Claude review identified test coverage gaps, which might be worth to fix.
Otherwise all looks good, so approved it.

  1. No 3D+ array tests

  Both sort and reverse only test 1D and 2D arrays (vanilla + non-vanilla via slicing). Adding a 3D test would exercise the recursive dimension traversal more thoroughly. This is the
  most significant test gap.

  2. Fuzz tests only cover 1D arrays

  DoubleArraySortFuzzTest generates 1D arrays exclusively. The fuzz test infrastructure could be extended to generate random 2D/3D arrays with random shapes. This would significantly
  strengthen confidence in the recursive code paths.

  3. No test triggers buffer growth

  INITIAL_BUFFER_SIZE = 64, but no test creates an innermost dimension > 64. Adding a test with e.g. 100+ elements per slice would verify the Numbers.ceilPow2 resize logic.

  4. No toPlan output verification

  The toPlan method on the sort Func class has branching logic (conditionally showing descending/nullsFirst), but no test verifies the plan output.