fix: dedup chunks at 10-minute date_bin granularity

[tonyalaribe]

Follow-up to #56/#61. The hour-chunked dedup still fails on the largest project: one hour holds >2.1GB of string data (Offset overflow error: 2222394106 — Arrow i32 offsets) and its materialization OOM-killed the 19:30Z container from a 34GB baseline. Binning by date_bin(INTERVAL '10 minutes', …) — the flush-bucket granularity — caps a chunk at ~110k rows / 1-2GB strings for that project, under both the offset limit and a sane memory peak. The probe lists only duplicate-containing bins, so steady-state sweeps remain probe-only. dedup_compaction_test 2/2 green.

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[claude]

Code Review

Overview

This is a focused, well-motivated hotfix that changes dedup chunking from 1-hour to 10-minute bins to address two distinct production failure modes:

1. Arrow i32 offset overflow — one hour of the largest project exceeds 2.1 GB of string data (observed: 2,222,394,106 bytes > INT32_MAX).
2. OOM kill — materializing a full hour during the rewrite was allocating tens of GB transiently.

The change is minimal (+6/-2) and directly targets the root cause. The logic is correct: 10-minute bins align with the flush-bucket granularity, so no duplicate group can ever straddle a bin boundary.

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Correctness

• date_bin alignment is correct. date_bin(INTERVAL '10 minutes', ts, TIMESTAMP '1970-01-01T00:00:00') bins to :00/:10/:20/:30/:40/:50 of every hour because 10 minutes divides evenly into 60. This is consistent with the flush-bucket granularity described in the PR.
• Sealed-chunk guard remains correct. sealed_before = now - 2h with the check if end > sealed_before { skip } — a 10-minute bin is only rewritten once it has been closed for at least 2 hours. The 2-hour margin was designed around a ~70-minute buffer window, which still holds here.
• Probe and rewrite are consistent. The probe groups by 10-minute bins, and the rewrite filter uses the same 10-minute [start, start+10min) window. No alignment mismatch.

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Issues

Minor — stale variable name hours (pre-existing, but newly misleading after this change):

let mut hours = Vec::new();  // now holds 10-minute bin timestamps

Renaming to bins would prevent future confusion when someone reads the loop.

Minor — stale prose comment (pre-existing, right time to update it):

// Rewriting an hour that late data may still flush into races
// replace_where against the append...

This block comment (around line 3195) still says "hour" after the bin granularity changed. Should read "bin" or "10-minute bin".

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Test Coverage

The existing dedup_compaction_collapses_cross_flush_duplicates test pins the timestamp 3 hours in the past — well outside the 2-hour sealed window — so the 10-minute vs. 1-hour distinction doesn't exercise any new code path. The test validates that dedup works at all with the new SQL, which is valuable, but it doesn't cover:

• Bin boundary alignment: rows at :09:59 and :10:00 landing in different bins and being rewritten independently.
• The sealed-chunk filter at the new granularity: a bin from 1h55m ago should be skipped; a bin from 2h05m ago should be rewritten.

These are lower-priority given the prod failure is deterministic and well-understood, but worth a follow-up.

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Performance

• The probe aggregation cost is unchanged (same GROUP BY, different bin expression).
• The rewrite now processes up to 6× more bins, but each is 6× smaller — net materialization cost is similar, and peak transient allocation per chunk drops from ~1–2 GB to ~110–200 MB, well within both the Arrow i32 limit and any reasonable container memory budget.
• Probe-only bins (no duplicates) are still skipped, so steady-state overhead is unaffected.

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Summary

The fix is correct and safe to merge. Two small follow-ups worth tracking:

1. Rename hours → bins in dedup_partition to match the new semantics.
2. Update the "Rewriting an hour..." prose comment to say "bin".
3. (Optional) Add a targeted test for bin boundary behavior and the sealed-chunk filter at 10-minute granularity.

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