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2026-09-06 13:00:21 +01:00
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+# 061 — Sort helper indexes need to be purged after a period of no access
+
+## Summary
+
+Ticket 060 introduces per-sort-key helper `BTreeMap` indexes. Left
+unbounded, a user (or dashboard) that sorts by many distinct params over
+a session accumulates a helper index per param forever, each duplicating
+the full timeline's worth of keys. There's no existing time-since-access
+eviction pattern in `engine.rs` to model this on: the only eviction today
+is `evict_excess_logs` (`src/engine.rs:1356-1363`, called from ingest at
+`:181` and from `SetMaxLogs` at `:212`), which is a hard count-cap
+("drop oldest once over `max_logs`"), not an LRU/last-accessed scheme.
+
+## Scope
+
+- Track a last-accessed timestamp per helper index (e.g.
+  `HashMap<String, (BTreeMap<...>, Instant)>` or equivalent), updated
+  whenever a query actually reads from that index.
+- Add a sweep (triggered periodically, or opportunistically on
+  ingest/query — match whichever fits the existing engine actor's
+  message-loop structure) that drops any helper index whose
+  last-accessed time exceeds a fixed duration.
+- A dropped helper index must be transparently rebuildable: the next
+  query sorted by that param should lazily rebuild the index (a one-time
+  full scan) exactly as if it had never existed, rather than erroring or
+  falling back permanently to unsorted behavior.
+- Out of scope: making the purge duration user-configurable via
+  `Settings` — pick a reasonable fixed constant unless the ticket
+  implementer finds a strong reason to expose it.
+
+## Acceptance criteria
+
+- A helper index untouched for longer than the purge duration is
+  removed from engine memory.
+- Querying a sort key whose helper index was purged transparently
+  rebuilds it and returns correct results, with no user-visible error.
+- Actively-used helper indexes (queried more recently than the purge
+  duration) are never evicted.
+
+## Depends on
+
+Ticket 060 (timeline custom sort needs helper BTreeMap index) — this
+ticket assumes helper indexes already exist.