Every ORDER BY id query on the messages table sorted or scanned the
whole session: get_messages_around's window seek, latest_message_row_id
(LIMIT 1), and get_messages' full-load ordering all paid O(session
history) per call — hot mid-turn via session_search and reactions.
messages.id is an original column (INTEGER PRIMARY KEY AUTOINCREMENT),
so the index lives in SCHEMA_SQL next to idx_messages_session — no
legacy-column migration hazard (the kanban lesson from #28776 does not
apply).
Measured (real schema, one 20k-message session, median of 30):
get_messages_around 7.08 -> 0.22 ms (32x), latest_message_row_id
3.37 -> 0.011 ms (307x), get_messages full load 111.6 -> 98.6 ms
(1.13x — remaining cost is row deserialization, not the sort).
Window results byte-identical at probe points across the session.
Tests: VM-step pin (get_messages_around bounded work, calibrated
~12 vs ~855 handler calls, threshold 300 — fails without the index)
and window parity with/without the index. No EXPLAIN/plan text
(behavior contracts, AGENTS.md).
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