[Feature][Vector] Sub-Shard Granularity for Primary-Key Vector Index Segments

[Xoln]

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Motivation

## Motivation

The current primary-key vector index maintains one ANN segment per `(partition, bucket, level)`. When any file in a level changes due to compaction, the **entire level's segment must be fully rebuilt**, even if only a small subset of files was affected.

For a level with 10 files totaling 10M rows, replacing just 2 files (2M rows) forces a rebuild of all 10M rows. This creates:
- Excessive rebuild cost for DiskANN/HNSW (minutes for large levels)
- Low `canAccept()` success rate when compact frequency > build time
- Delayed index availability under `waitCompaction=false`

## Proposal

Introduce a **Sub-Shard** mechanism that splits a level's index into finer-grained segments, so that only affected shards are rebuilt when compaction changes files.

### Three Granularity Options

| Scope | Segment Covers | Rebuild Unit | Segments per Level (10 files) |
|-------|---------------|--------------|-------------------------------|
| **LEVEL** (current) | All files in level | Entire level | 1 |
| **FILE_GROUP** | 2-4 adjacent files by key range | Affected group only | 3-5 |
| **PER_FILE** | Single compact-output file | Single file only | 10 |

### Example: 10 files, compact replaces 2

| Metric | LEVEL (current) | FILE_GROUP | PER_FILE |
|--------|----------------|-----------|---------|
| Rebuild data volume | 10M rows | 3M rows | **2M rows** |
| IVF-Flat rebuild time | ~50s | ~15s | **~10s** |
| DiskANN rebuild time | ~300s | ~90s | **~60s** |
| Unchanged segments | 0 | 2 groups | 8 files |
| canAccept success rate | Low | Medium | **High** |

### Algorithm Compatibility

| Algorithm | Precision Impact (PER_FILE) | Recommendation |
|-----------|---------------------------|----------------|
| IVF-Flat / IVF-PQ | ≈0% (IVF is partition-based) | PER_FILE |
| DiskANN / HNSW | -3~8% (graph connectivity severed) | FILE_GROUP |

**IVF algorithms are naturally suited for sub-sharding** since they search each partition independently. Graph-based indexes (DiskANN/HNSW) lose cross-shard neighbor connectivity, so FILE_GROUP with larger shards (2-5M rows) is recommended for them.

## Design Overview

### Core Changes

1. **`ShardPartitioner`** (new): Partitions level files into shards based on configured scope
2. **`PrimaryKeyIndexLevels.pick()`**: Returns plans for individual shards instead of entire levels
3. **`PkVectorBucketIndexState`**: Validates segments per-shard (partial match) instead of exact full-level match
4. **`BucketedVectorIndexMaintainer`**: Supports multiple parallel `PendingBuild` tasks for different shards
5. **`PrimaryKeyVectorBucketSearch`**: Searches multiple sub-shard segments and merges results
6. **`PrimaryKeyIndexSourceMeta`**: Extended with `shardIndex` field (version 2, backward compatible)

### Configuration

```sql
'pk-vector.shard.scope' = 'per-file' -- level | file-group | per-file
'pk-vector.shard.target-row-count' = '5000000' -- file-group mode: target rows per group
'pk-vector.shard.max-files-per-group' = '4' -- file-group mode: max files per group
'pk-vector.shard.max-parallel-builds' = '3' -- max concurrent shard builds

Search Path

Per-File mode, 10 segments in a level:
 → Parallel ANN search on 10 small segments (each ~1M rows, ~0.8ms)
 → Merge results → global topK
 → Total latency (parallel): ~1.8ms vs current 5ms (faster due to smaller graphs)

Backward Compatibility

• PrimaryKeyIndexSourceMeta version upgrade (V1 → V2), V1 still readable
• Default scope=level preserves existing behavior
• Online upgrade: change scope → next compact cycle gradually replaces segments
• Downgrade: set scope=level → next compact rebuilds full-level segment

Performance Summary

Dimension	LEVEL	FILE_GROUP	PER_FILE
Rebuild cost	High (full)	Medium (1/3-1/5)	Low (1/N)
Search precision (IVF)	Baseline	≈Same	≈Same
Search precision (graph)	Highest	-2~3%	-3~8%
Search latency (parallel)	Baseline	Faster	Fastest
canAccept success	Low	Medium	High
Storage overhead	Baseline	+5%	+10%
Best for IVF	✓	✓✓	✓✓✓
Best for DiskANN	✓✓✓	✓✓	✓

Key Code References

• PrimaryKeyIndexLevels.java - current single-segment-per-level enforcement
• PkVectorBucketIndexState.java - current exact full-level match validation
• BucketedVectorIndexMaintainer.java - current single PendingBuild model
• PrimaryKeyVectorBucketSearch.java - current single-segment search path
• PR [core] Align primary-key index maintenance with data levels #8672 "Align primary-key index maintenance with data levels"

Solution

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Anything else?

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Are you willing to submit a PR?

• I'm willing to submit a PR!