# HealthProbe Import Optimization Log

**Canonical path:** `HealthProbe/Doc/04-project/Import-Optimization-Log.md`  
**Created:** 2026-06-02  
**Purpose:** Track import performance work, measured results, regressions, and next experiments.

This is a living project log. Update it after each import optimization commit and after each real-device import report.

## Scope

The current optimization target is the initial / full-history HealthKit import into the SQLite archive.

Primary goals:
- complete large first-run imports without app freeze or watchdog-like stalls;
- keep memory bounded for low-end devices;
- reduce wall-clock duration enough to make future background / scheduled collection realistic;
- keep archive writes idempotent and differential;
- preserve SQLite as the source of truth.

Non-goals for this log:
- redesigning HealthKit background collection strategy;
- changing archive semantics;
- optimizing UI rendering after import, except when post-import work blocks the app.

## Measurement Fields

Use the HealthProbe diagnostic report fields below for comparisons:

| Field | Meaning |
|-------|---------|
| `WallClockDuration` / `Duration` | User-visible total operation time. |
| `SummedFetchElapsed` | Time spent fetching HealthKit samples. Per-metric sums may overlap. |
| `SummedProcessingElapsed` | Time spent converting HealthKit samples into archive rows. |
| `SummedInsertElapsed` | Time spent writing rows to SQLite. Current main bottleneck. |
| `SummedFinalizeElapsed` | Type verification, aggregate rebuild, and finalization cost. |
| Per-type `insertElapsed` | Most useful field for high-volume types such as Heart Rate and Active Energy. |

Important interpretation:
- per-metric timing sums can exceed wall-clock time because type fetches overlap;
- progress rates shown during import may overestimate throughput if overhead is not included;
- compare first snapshots only against first snapshots after a database reset.

## Real-Device Results

### 2026-06-02 Baseline Before Latest Batch/Chunk Work

Source: user-provided diagnostic report.

| Metric | Value |
|--------|-------|
| Wall clock | 20m 25s |
| Summed fetch | 1m 03s |
| Summed processing | 1m 31s |
| Summed insert | 17m 02s |
| Summed finalize | 9.2s |
| Heart Rate count | 923,466 |
| Heart Rate insert | 10m 45s |
| Active Energy insert | 4m 29s |
| Steps insert | 27.1s |
| Walking + Running Distance insert | 21.8s |

Conclusion: SQLite insert dominated the run. HealthKit fetch was not the limiting factor.

### 2026-06-02 After Batched Initial Archive Writes

Source: user-provided diagnostic report after commit `a026566`.

| Metric | Value |
|--------|-------|
| Wall clock | 18m 21s |
| Summed insert | 15m 44s |
| Heart Rate insert | 10m 03s |
| Active Energy insert | 3m 51s |
| Steps insert | 28.1s |
| Walking + Running Distance insert | 25.4s |

Conclusion: batching produced a useful improvement, but insert remained dominant.

### 2026-06-02 After Larger Initial Write Chunks

Source: user-provided diagnostic report after commit `c138b7b`.

| Metric | Value |
|--------|-------|
| Wall clock | 18m 30s |
| Summed metric total | 18m 02s |
| Summed fetch | 46.8s |
| Summed processing | 1m 37s |
| Summed insert | 15m 24s |
| Summed finalize | 10.5s |
| Heart Rate count | 922,404 |
| Heart Rate total | 11m 23s |
| Heart Rate fetch | 21.2s |
| Heart Rate processing | 56.1s |
| Heart Rate insert | 9m 58s |
| Active Energy count | 348,635 |
| Active Energy insert | 3m 48s |
| Steps insert | 24.2s |
| Walking + Running Distance insert | 20.0s |

Conclusion: larger chunks gave only marginal gains. Further optimization should reduce per-sample SQLite work rather than only increasing page/chunk size.

### 2026-06-02 After Direct Inserts For New Archive Samples

Commit: `44d9ebd` (`Use direct inserts for new archive samples`)  
Source: user-provided first-import diagnostic report after database reset.

Note: an earlier user-provided report looked significantly faster, but it was a
reimport rather than a fresh first snapshot and is not used as a direct
comparison against first-import runs.

| Metric | Value |
|--------|-------|
| Wall clock | 17m 13s |
| Summed metric total | 17m 13s |
| Summed fetch | 43.0s |
| Summed processing | 1m 40s |
| Summed insert | 14m 38s |
| Summed finalize | 9.5s |
| Heart Rate count | 922,431 |
| Heart Rate total | 10m 25s |
| Heart Rate fetch | 20.8s |
| Heart Rate processing | 57.0s |
| Heart Rate insert | 8m 59s |
| Active Energy count | 348,669 |
| Active Energy insert | 3m 54s |
| Steps insert | 24.2s |
| Walking + Running Distance insert | 20.7s |

Comparison against the previous comparable first-import run (`c138b7b`):

| Metric | Previous | Current | Change |
|--------|----------|---------|--------|
| Wall clock | 18m 30s | 17m 13s | -1m 17s / -7% |
| Summed insert | 15m 24s | 14m 38s | -46s / -5% |
| Heart Rate insert | 9m 58s | 8m 59s | -59s / -10% |
| Active Energy insert | 3m 48s | 3m 54s | +6s / +3% |
| Steps insert | 24.2s | 24.2s | flat |
| Walking + Running Distance insert | 20.0s | 20.7s | +0.7s / +4% |

Conclusion: direct inserts for brand-new dependent rows produced a valid but
modest first-import gain. The large reimport improvement was not representative
of a clean first snapshot. SQLite insert remains the dominant bottleneck.

### 2026-06-02 Non-Chain-Start Full Scan After Index Removal

Commit context: after `ff59257` (`Drop unused sample import indexes`)
Source: user-provided diagnostic report with `previousSnapshotID` present and
`isChainStart: false`.

This is not a comparable first-import benchmark for the unused-index removal,
but it is important because it shows that non-initial captures can be slower
than first imports when the app performs a full-history scan.

| Metric | Value |
|--------|-------|
| Wall clock | 22m 33s |
| Summed metric total | 22m 14s |
| Summed fetch | 52.0s |
| Summed processing | 2m 23s |
| Summed insert | 18m 44s |
| Summed finalize | 11.5s |
| Heart Rate count | 922,440 |
| Heart Rate total | 13m 30s |
| Heart Rate fetch | 24.3s |
| Heart Rate processing | 1m 29s |
| Heart Rate insert | 11m 25s |
| Active Energy count | 348,698 |
| Active Energy insert | 4m 44s |
| Steps insert | 40.4s |
| Walking + Running Distance insert | 36.0s |

Conclusion: this run should not be used to judge first-import index removal.
However, it indicates a separate bottleneck: subsequent full scans still spend
most of their time in SQLite writes, likely because unchanged samples are still
touching the archive write path. The next implementation target should reduce
per-sample work for unchanged existing samples during verification/full-scan
captures.

### 2026-06-02 First Import After Index Removal And Reset Fortification

Commit context: after `3dd5f48` (`Fortify scheduled test database reset`), with
the unused index removal from `ff59257` included. Source: user-provided
diagnostic report with `previousSnapshotID: none` and `isChainStart: true`.

This is a comparable first-import benchmark. The `a281c51` verified-event change
is included in the build, but it should not materially affect this run because a
clean first import creates brand-new samples rather than unchanged existing
samples.

| Metric | Value |
|--------|-------|
| Wall clock | 12m 43s |
| Summed metric total | 12m 42s |
| Summed fetch | 40.4s |
| Summed processing | 1m 37s |
| Summed insert | 10m 11s |
| Summed finalize | 10.8s |
| Total records | 1,579,168 |
| Heart Rate count | 922,450 |
| Heart Rate total | 8m 06s |
| Heart Rate fetch | 18.6s |
| Heart Rate processing | 56.1s |
| Heart Rate insert | 6m 41s |
| Active Energy count | 348,701 |
| Active Energy insert | 2m 09s |
| Steps insert | 21.6s |
| Walking + Running Distance insert | 19.2s |

Comparison against the previous comparable first-import run (`44d9ebd`):

| Metric | Previous | Current | Change |
|--------|----------|---------|--------|
| Wall clock | 17m 13s | 12m 43s | -4m 30s / -26% |
| Summed insert | 14m 38s | 10m 11s | -4m 27s / -30% |
| Heart Rate insert | 8m 59s | 6m 41s | -2m 18s / -26% |
| Active Energy insert | 3m 54s | 2m 09s | -1m 45s / -45% |
| Steps insert | 24.2s | 21.6s | -2.6s / -11% |
| Walking + Running Distance insert | 20.7s | 19.2s | -1.5s / -7% |

Conclusion: first-import reset is now clean and the unused-index removal produced
a large measurable gain. SQLite insert remains dominant, but the main bottleneck
has moved from about 14m38s to 10m11s.

### 2026-06-02 First Import After Additional Index Removal

Commit context: after `06ee6be` (`Drop additional import write indexes`). Source:
user-provided diagnostic report with `previousSnapshotID: none`,
`isChainStart: true`, and the same total record count as the previous clean run.

This is a comparable first-import benchmark for removing
`idx_sample_versions_time` and `idx_visibility_sample_open`.

| Metric | Value |
|--------|-------|
| Wall clock | 12m 39s |
| Summed metric total | 12m 39s |
| Summed fetch | 43.8s |
| Summed processing | 1m 35s |
| Summed insert | 10m 07s |
| Summed finalize | 10.1s |
| Total records | 1,579,168 |
| Heart Rate count | 922,450 |
| Heart Rate total | 8m 08s |
| Heart Rate fetch | 19.2s |
| Heart Rate processing | 55.0s |
| Heart Rate insert | 6m 44s |
| Active Energy count | 348,701 |
| Active Energy insert | 2m 07s |
| Steps insert | 20.7s |
| Walking + Running Distance insert | 18.2s |

Comparison against the previous comparable first-import run (`3dd5f48` context):

| Metric | Previous | Current | Change |
|--------|----------|---------|--------|
| Wall clock | 12m 43s | 12m 39s | -4s / -1% |
| Summed insert | 10m 11s | 10m 07s | -4s / -1% |
| Heart Rate insert | 6m 41s | 6m 44s | +3s / +1% |
| Active Energy insert | 2m 09s | 2m 07s | -2s / -2% |
| Steps insert | 21.6s | 20.7s | -0.9s / -4% |
| Walking + Running Distance insert | 19.2s | 18.2s | -1.0s / -5% |

Conclusion: removing these two extra indexes did not materially change first
import performance. The small differences are within expected run-to-run noise.
The larger first-import gain remains attributable to the earlier hot `samples`
index removal plus clean reset conditions.

### 2026-06-02 Incremental No-Delta Snapshot With Post-Import UI Freeze

Commit context: after `06ee6be`. Source: user-provided diagnostic report with
`previousSnapshotID` present, `isChainStart: false`, and the same total record
count as the preceding first snapshot.

| Metric | Value |
|--------|-------|
| Wall clock | 9.2s |
| Summed metric total | 8.9s |
| Summed fetch | 0.2s |
| Summed processing | 0.0s |
| Summed insert | 0.0s |
| Summed finalize | 8.5s |
| Total records | 1,579,168 |
| Heart Rate finalize | 4.8s |
| Active Energy finalize | 1.8s |

Conclusion: repeated no-delta capture is now fast and does not write unchanged
records. The user still observed roughly one minute of app unresponsiveness
after finalization, so the remaining issue is outside the measured import
phases. The most likely culprit is synchronous post-import Core Data cache
rebuild / dashboard refresh work. The dashboard cache refresh was moved to a
background task after this report; next reports should distinguish import time
from post-import UI recovery.

### 2026-06-03 Incremental No-Delta Snapshot After Visibility Fast-Path

Commit context: after `f60f09a`. Source: user-provided diagnostic report with
`previousSnapshotID` present, `isChainStart: false`, and total record count
unchanged at 1,579,168.

| Metric | Value |
|--------|-------|
| Wall clock | 13.6s |
| Summed metric total | 13.2s |
| Summed fetch | 0.2s |
| Summed processing | 0.0s |
| Summed insert | 0.0s |
| Summed finalize | 12.9s |
| Total records | 1,579,168 |
| Heart Rate finalize | 9.1s |
| Active Energy finalize | 1.8s |
| Steps finalize | 0.5s |
| Walking + Running Distance finalize | 0.4s |

Conclusion: the repeated no-delta write path is effectively eliminated. The
remaining measured import cost is finalization, especially Heart Rate daily
aggregate/materialized summary work. The next optimization should avoid
rescanning all visible records when a metric has no appeared, disappeared, or
representation-changed events in the current observation.

### 2026-06-03 Incremental Snapshot After Aggregate Copy Path

Commit context: after `19ba656`. Source: user-provided diagnostic report with
`previousSnapshotID` present and `isChainStart: false`.

This was not a pure no-delta run: total record count increased from 1,579,168 to
1,579,239 (+71 records). That means high-volume changed metrics still needed
normal processing/finalization paths, so the aggregate-copy optimization was not
fully exercised for Heart Rate or Active Energy.

| Metric | Value |
|--------|-------|
| Wall clock | 51.3s |
| Summed metric total | 34.3s |
| Summed fetch | 0.2s |
| Summed processing | 21.1s |
| Summed insert | 0.1s |
| Summed finalize | 12.1s |
| Total records | 1,579,239 |
| Heart Rate processing | 13.9s |
| Heart Rate finalize | 8.7s |
| Active Energy processing | 5.0s |
| Active Energy finalize | 1.9s |
| Steps processing | 1.1s |
| Walking + Running Distance processing | 0.9s |

Comparison against the previous repeated snapshot report:

| Metric | Previous | Current | Change |
|--------|----------|---------|--------|
| Total records | 1,579,168 | 1,579,239 | +71 |
| Wall clock | 13.6s | 51.3s | +37.7s |
| Summed processing | 0.0s | 21.1s | +21.1s |
| Summed insert | 0.0s | 0.1s | +0.1s |
| Summed finalize | 12.9s | 12.1s | -0.8s |
| Heart Rate finalize | 9.1s | 8.7s | -0.4s |

Conclusion: writes remain bounded (`SummedInsertElapsed` 0.1s), but this report
shows a new bottleneck for changed high-volume metrics: processing existing
records when a small number of records changed. A pure no-delta report is still
needed to validate the aggregate-copy path. The unexplained global gap between
wall clock and summed metric total also needs timing if it persists.

### 2026-06-03 Incremental Snapshot With Large Legacy Detail Cache Crash

Commit context: after `19ba656`. Source: user-provided diagnostic report plus
console crash log.

This was still not a pure no-delta run: total record count increased from
1,579,239 to 1,579,243 (+4 records). The report shows that archive inserts
remained effectively eliminated, while aggregate finalization improved versus
the previous changed incremental run.

| Metric | Previous | Current | Change |
|--------|----------|---------|--------|
| Total records | 1,579,239 | 1,579,243 | +4 |
| Wall clock | 51.3s | 44.7s | -6.6s |
| Summed metric total | 34.3s | 26.7s | -7.6s |
| Summed fetch | 0.2s | 0.2s | flat |
| Summed processing | 21.1s | 18.4s | -2.7s |
| Summed insert | 0.1s | 0.0s | -0.1s |
| Summed finalize | 12.1s | 7.4s | -4.7s |
| Heart Rate processing | 13.9s | 13.5s | -0.4s |
| Heart Rate finalize | 8.7s | 4.8s | -3.9s |
| Active Energy processing | 5.0s | 4.9s | -0.1s |
| Active Energy finalize | 1.9s | 1.8s | -0.1s |

The follow-up console log identifies the remaining post-import issue:
`healthKit.precomputeDetailCaches` built legacy SwiftData detail caches for
Heart Rate and Active Energy, scanning archives of about 137 MB / 922k records
and 52 MB / 349k records. Immediately after that work completed, Core Data
aborted on the main thread with a mutated-while-enumerated exception during
change processing.

Conclusion: the SQLite archive path is not the crash source. The expensive and
crash-prone work is the legacy `TypeCount.detailCacheData` precompute after the
snapshot save. Large type detail caches should be skipped and served from the
SQLite/Core Data archive/cache path instead.

### 2026-06-03 Incremental Snapshot After Large Detail Cache Skip

Commit context: after `e49a79d`. Source: user-provided diagnostic report. The
user still observed app unresponsiveness after the operation completed.

| Metric | Previous | Current | Change |
|--------|----------|---------|--------|
| Total records | 1,579,243 | 1,579,253 | +10 |
| Wall clock | 44.7s | 31.3s | -13.4s |
| Summed metric total | 26.7s | 30.6s | +3.9s |
| Summed fetch | 0.2s | 0.2s | flat |
| Summed processing | 18.4s | 18.1s | -0.3s |
| Summed insert | 0.0s | 0.0s | flat |
| Summed finalize | 7.4s | 11.5s | +4.1s |
| Heart Rate processing | 13.5s | 13.1s | -0.4s |
| Heart Rate finalize | 4.8s | 8.8s | +4.0s |
| Active Energy processing | 4.9s | 4.7s | -0.2s |
| Active Energy finalize | 1.8s | 1.9s | +0.1s |

Conclusion: the large legacy detail-cache skip removed the earlier crash signal
and reduced wall clock, but the app can still become unresponsive after the
snapshot reports success. The remaining suspect is the Dashboard Core Data cache
refresh because the view model still awaited cache rebuild before releasing the
post-snapshot UI path. That refresh should run fire-and-forget and publish its
result back to the main actor only when complete.

### 2026-06-03 Final Freeze Log: Small Detail Cache Still Crashes

Source: user-provided overnight freeze/crash log after the large-cache skip.

The log shows `healthKit.precomputeDetailCaches` still building two small
legacy SwiftData detail caches:

| Type | Current archive | Current count | Result |
|------|-----------------|---------------|--------|
| Stand Hours | 1.1 MB | 7,727 | built, 2 added |
| Environmental Sound Levels | 2 MB | 13,384 | built, 1 added |

Heart Rate and Active Energy were correctly skipped by the large-cache guard,
but Core Data still aborted immediately after `healthKit.precomputeDetailCaches.end`
with the same mutated-while-enumerated exception during change processing.

Conclusion: the issue is not only large archive size. Mutating legacy
`TypeCount.detailCacheData` during snapshot save is unsafe in this SwiftData /
Core Data context. Snapshot save must not precompute or clear this legacy cache.
The Snapshots list should also read observation timeline rows directly from
SQLite so it does not depend on a delayed Core Data cache rebuild to show the
freshly finished observation.

## Optimization Iterations

| Date | Commit | Change | Result / Status |
|------|--------|--------|-----------------|
| 2026-06-02 | `fd08ded` | Added explicit fetch / processing / insert / finalize timings to reports. | Made phase comparisons possible without inferring from UI progress. |
| 2026-06-02 | `87f1a85` | Cached repeated SQLite write-path lookups within grouped imports. | Reduced repeated id lookup pressure in hot path. |
| 2026-06-02 | `7294a01` | Fast-pathed visibility writes for new archive samples. | Removed redundant visibility close/existence work for brand-new samples. |
| 2026-06-02 | `585d77f` | Tightened archive verification aggregate queries. | Reduced finalization / verification rescans. |
| 2026-06-02 | `2dd279c` | Used rowid fast path for new archive sample rows. | Avoided follow-up id lookup queries when SQLite confirmed new inserts. |
| 2026-06-02 | `f569b6c` | Fixed scheduled test database reset. | Restored ability to compare fresh first-snapshot imports. |
| 2026-06-02 | `986f343` | Increased Heart Rate import write chunks. | Early attempt to reduce paging/write overhead for the largest metric. |
| 2026-06-02 | `c1ebd37` | Sped up archive verification finalization. | Reduced finalize pressure; insert remained dominant. |
| 2026-06-02 | `bcbf9a5` | Cleaned up import diagnostic timings. | Corrected date-fetch wall-clock measurement and report text. |
| 2026-06-02 | `a026566` | Batched initial import archive writes across several fetched pages. | Wall clock improved from about 20m25s to 18m21s on the measured first import. |
| 2026-06-02 | `c138b7b` | Increased initial import write chunk sizes. | Marginal improvement: summed insert from 15m44s to 15m24s on the next comparable run. |
| 2026-06-02 | `44d9ebd` | Used direct inserts for dependent rows when `samples` creates a new sample. | Confirmed modest first-import gain: wall clock 18m30s -> 17m13s, summed insert 15m24s -> 14m38s, Heart Rate insert 9m58s -> 8m59s. |
| 2026-06-02 | `ff59257` | Removed unused `samples` indexes on global UUID hash and semantic fingerprint. | Confirmed large first-import gain after clean reset: wall clock 17m13s -> 12m43s, summed insert 14m38s -> 10m11s, Heart Rate insert 8m59s -> 6m41s. Deleted-object lookup remains covered by `(sample_type_id, sample_uuid_hash)`. |
| 2026-06-02 | pending | Captured non-chain-start full-scan report after index removal. | Not comparable for first-import performance; reveals a separate full-scan/unchanged-sample write bottleneck. |
| 2026-06-02 | `a281c51` | Stopped writing `verified` observation events for unchanged existing samples. | Awaiting comparable non-chain-start/full-scan report. Expected signal is lower `SummedInsertElapsed` and especially lower Heart Rate insert time when most rows are unchanged. |
| 2026-06-02 | `3dd5f48` | Fortified scheduled test database reset with a disk marker and extra SQLite sidecar cleanup. | Real-device report confirmed reset produced `previousSnapshotID: none`, `isChainStart: true`, and a clean first-snapshot timeline. |
| 2026-06-02 | `06ee6be` | Removed unused `sample_versions(start_date, end_date)` and redundant `sample_visibility_ranges(sample_id, last_observation_id)` indexes. | Comparable first-import report was flat: wall clock 12m43s -> 12m39s and summed insert 10m11s -> 10m07s. Treat as no material performance change. |
| 2026-06-02 | pending | Moved Dashboard archive cache refresh/rebuild off the UI path after snapshot completion. | Awaiting real-device confirmation that the app no longer stays unresponsive for roughly one minute after a completed snapshot. |
| 2026-06-03 | `f60f09a` | Fast-path unchanged samples whose current version already has an open visibility range. | Confirmed on repeated no-delta capture: `SummedInsertElapsed` remained 0.0s; remaining cost is `SummedFinalizeElapsed` 12.9s, with Heart Rate finalize 9.1s. |
| 2026-06-03 | `19ba656` | Copy previous type summaries and daily aggregates for unchanged metric observations instead of rebuilding from visible ranges. | First follow-up was not no-delta (+71 records), so it is inconclusive for the unchanged path. It did show `SummedInsertElapsed` 0.1s and a new changed-metric processing bottleneck: Heart Rate processing 13.9s, Active Energy processing 5.0s. |
| 2026-06-03 | `e49a79d` | Skip legacy SwiftData detail-cache precompute for large type archives. | Triggered by a crash after building Heart Rate and Active Energy detail caches. Expected signal: no post-import `NSGenericException`, lower post-import wall-clock gap, and `healthKit.detailCache.skipLargeLegacyArchive` logs for high-volume types. |
| 2026-06-03 | `7d52262` | Start Dashboard archive cache refresh without awaiting it after snapshot completion. | Triggered by continued app unresponsiveness after a successful 31.3s incremental snapshot. Expected signal: progress sheet/result UI remains responsive while cache rows refresh later. |
| 2026-06-03 | `1229f19` | Disable legacy SwiftData detail-cache precompute completely and load Snapshots timeline from SQLite. | Triggered by overnight crash after two small detail caches were built. Expected signal: no `healthKit.detailCache.buildBegin` logs during snapshot save, no Core Data mutated-while-enumerated abort, and the new SQLite observation appears in Snapshots without waiting for cache rebuild. |

## Current Diagnosis

The import is no longer primarily a HealthKit fetch problem. On the latest comparable first-import measured run:

- total wall clock was 12m43s after the unused-index removal and clean reset;
- summed fetch was only 40.4s;
- summed insert was 10m11s;
- Heart Rate alone spent 6m41s inserting.

The likely bottleneck is per-row SQLite work:
- uniqueness checks on hot tables;
- index maintenance while importing high-volume rows;
- multiple dependent writes per sample;
- commit / transaction shape;
- no-op visibility range maintenance for unchanged existing samples;
- processing existing high-volume metrics when only a small number of records changed;
- legacy SwiftData detail-cache precompute after SQLite completes. This became
  the active crash/performance issue even for small caches; any snapshot-save
  mutation of `TypeCount.detailCacheData` is unsafe and should remain disabled.
- Dashboard Core Data archive cache refresh after snapshot completion, when the
  UI path awaits the rebuild instead of publishing results asynchronously.

## Open Issues / Observations

- Very small pages reduced freeze risk but introduced visible overhead.
- Some progress timing displayed in the UI did not include overhead, so elapsed time and rates looked better than the real operation.
- A previous Heart Rate import appeared to stall for long periods around roughly 900k records, but later progress resumed; avoid classifying this as a hard timeout without report evidence.
- After a completed import, the app may remain unresponsive or crash in legacy
  post-import cache work. A 2026-06-03 console log showed Heart Rate and Active
  Energy `TypeCount.detailCacheData` precompute immediately before a Core Data
  mutated-while-enumerated abort.
- A later 2026-06-03 overnight log showed the same abort after only Stand Hours
  and Environmental Sound Levels detail caches were built. Size limits are not
  enough; the whole snapshot-save detail-cache precompute path is disabled.
- Partial / old imported observations can pollute comparisons. Fresh first-snapshot performance comparisons should use a confirmed reset database.
- Non-chain-start full scans can be slower than first imports if unchanged existing samples still write per-sample archive evidence.

## Next Experiments

Prioritize experiments in this order:

1. Run an incremental snapshot after disabling legacy detail-cache precompute.
   Confirm there are no `healthKit.detailCache.buildBegin` logs, no Core Data
   mutated-while-enumerated abort, and the new observation appears in Snapshots.
2. Run a repeated no-delta benchmark after copying unchanged metric summaries and daily aggregates. Compare `SummedFinalizeElapsed`, `Heart Rate finalizeElapsed`, `Active Energy finalizeElapsed`, and wall clock.
3. Add or inspect timing around per-record processing for changed high-volume metrics, especially Heart Rate, to separate sample DTO/fingerprint work from SQLite idempotency checks.
4. Run a non-chain-start/full-scan benchmark after skipping unchanged `verified` events and fast-pathing already-open visibility ranges. Compare `SummedInsertElapsed`, `Heart Rate insertElapsed`, `Steps insertElapsed`, and `Walking + Running Distance insertElapsed`.
5. Reduce any remaining per-sample SQLite writes for unchanged existing samples during non-chain-start full scans.
6. Profile whether index maintenance dominates first-import insert cost.
7. Consider a guarded bulk-import mode for first observations:
   - keep archive semantics unchanged;
   - only relax work that can be safely reconstructed or validated;
   - re-enable normal idempotent paths for incremental observations.
8. Run a fresh first-import benchmark after the unused-index removal and compare `SummedInsertElapsed`, `Heart Rate insertElapsed`, and `Active Energy insertElapsed`.
9. Investigate whether first-import-only deferred index creation or temporary staging tables can reduce `samples` / `sample_versions` / `sample_observation_events` write cost without weakening final archive integrity.
10. Revisit adaptive page sizes only after SQLite write-path costs are reduced.
11. Revisit background / scheduled collection once initial import can finish reliably and post-import UI recovery is bounded.

## Verification Checklist For Each Optimization

- [ ] `git diff --check` passes.
- [ ] SQLite archive store tests pass.
- [ ] Import configuration tests pass if capture strategy changed.
- [ ] Repeated-page/idempotency behavior remains covered.
- [ ] A real-device report is attached or summarized in this log.
- [ ] The next experiment is recorded before moving on.
