---
phase: 25-lazy-evaluation-cache-efficiency
plan: "01"
subsystem: storage, sidecar
tags: [polars, parquet, lazy-evaluation, memory-efficiency]

requires:
  - phase: none
    provides: independent phase
provides:
  - Column-projected Parquet dedup in TickStorage.write_ticks
  - In-place sort for Stathera diagnostic in sync_flush
affects: [storage, sidecar-midnight]

tech-stack:
  added: []
  patterns: [column-projection-for-dedup, in-place-sort-over-copy]

key-files:
  created: []
  modified:
    - python/opendeviationbar/storage/parquet.py
    - python/opendeviationbar/sidecar/midnight.py

key-decisions:
  - "Used pl.read_parquet(columns=) instead of scan_parquet().select().collect() for type-checker compatibility"

patterns-established:
  - "Column projection: use columns= parameter on read_parquet for ID-only dedup checks"
  - "In-place sort: prefer list.sort() over sorted() when the original order is not needed"

requirements-completed: [LAZY-01, LAZY-02]

duration: 3min
completed: 2026-03-27
---

# Phase 25 Plan 01: Lazy Evaluation + Cache Efficiency Summary

**Column-projected Parquet dedup reads only agg_trade_id (1/7 columns) and in-place list sort eliminates batch copy in Stathera diagnostic**

## Performance

- **Duration:** 3 min
- **Started:** 2026-03-27T22:04:02Z
- **Completed:** 2026-03-27T22:07:00Z
- **Tasks:** 2
- **Files modified:** 2

## Accomplishments

- write_ticks dedup now reads only the agg_trade_id column instead of all 7 columns for anti-join comparison, deferring full file load to the merge branch only
- Stathera diagnostic in \_sync_flush_rest_bars uses list.sort() in-place instead of sorted() which allocated a full copy

## Task Commits

Each task was committed atomically:

1. **Task 1: scan_parquet for dedup in write_ticks** - `79fa9c8` (perf)
2. **Task 2: In-place sort for Stathera diagnostic** - `7740f06` (perf)

## Files Created/Modified

- `python/opendeviationbar/storage/parquet.py` - Column-projected dedup with pl.read_parquet(columns=["agg_trade_id"])
- `python/opendeviationbar/sidecar/midnight.py` - In-place list.sort() for Stathera diagnostic batch

## Decisions Made

- Used `pl.read_parquet(path, columns=["agg_trade_id"])` instead of `pl.scan_parquet().select().collect()` because Polars type stubs return `InProcessQuery | DataFrame` from `.collect()`, causing ty type-checker errors. The `columns=` parameter achieves identical column projection with clean types.

## Deviations from Plan

None - plan executed exactly as written.

## Known Stubs

None.

## Issues Encountered

None.

## User Setup Required

None - no external service configuration required.

## Next Phase Readiness

- Phase 25 is the final phase of v7.2 Memory Efficiency milestone
- All three v7.2 phases (23, 24, 25) can now be marked complete

---

_Phase: 25-lazy-evaluation-cache-efficiency_
_Completed: 2026-03-27_
