diff --git a/server/migrations/0008_memory_completions_event_index.sql b/server/migrations/0008_memory_completions_event_index.sql
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+++ b/server/migrations/0008_memory_completions_event_index.sql
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+-- Index on (event_id, completed_at DESC) for memory_completions.
+-- Speeds up the per-event "latest row" lookup used by the account-deletion
+-- reconcile in account.ts (the correlated subquery in
+-- INSERT INTO memory_events ... SELECT ... WHERE id = (SELECT id FROM mc2 ...)).
+-- Without this index that subquery is O(rows^2); with it, O(rows * log rows).
+CREATE INDEX IF NOT EXISTS idx_memory_completions_event_completed
+    ON memory_completions(event_id, completed_at DESC);