diff --git a/prisma/migrations/0415_foreign_key_and_prefix_indexes/migration.sql b/prisma/migrations/0415_foreign_key_and_prefix_indexes/migration.sql
new file mode 100644
index 000000000..1844f11a7
--- /dev/null
+++ b/prisma/migrations/0415_foreign_key_and_prefix_indexes/migration.sql
@@ -0,0 +1,101 @@
+-- Index hygiene: a leading index for every foreign key that cascades, and
+-- no index that is a prefix of another.
+--
+-- 1. Twenty-one foreign keys with ON DELETE CASCADE or SET NULL had no index
+--    leading with their column. Deleting the parent row (a medication, a
+--    schedule, a practitioner, an account) then scans the whole child table
+--    once per deleted parent: `medication_intake_events.medication_id` is the
+--    one that hurts, scanned in full for every medication an account delete
+--    removes. Each gets a plain btree on the column.
+--
+-- 2. Seventeen indexes repeated the leading columns of another index on the
+--    same table, in the same order and direction, mostly a `(user_id)` index
+--    beside a `(user_id, …)` unique key. The longer index answers every
+--    lookup the shorter one did, so the shorter one only cost writes and
+--    space. They go. `measurements` has its own migration (0416), because a
+--    lock on that table weighs differently.
+--
+-- `tests/integration/index-hygiene.test.ts` holds the schema to both rules
+-- from the catalog.
+--
+-- Lock wait. `CREATE INDEX` takes SHARE on its table (reads go on, writes
+-- wait while it builds; every table here is small, so the build is quick)
+-- and `DROP INDEX` takes ACCESS EXCLUSIVE for the instant the catalog
+-- changes. `CREATE INDEX CONCURRENTLY` cannot share a migration file with
+-- any other statement (0416), so the whole set runs in one `DO` block: one
+-- statement that commits or rolls back whole, with the lock wait bounded at
+-- five seconds so a long reader cannot queue the tables' writes behind the
+-- migration. Every statement is `IF [NOT] EXISTS`, so a lock timeout leaves
+-- the file safe to apply again, which the marker declares
+-- (docs/ops/deploy.md, "A migration that timed out on a lock").
+--
+-- @retry-on-lock-timeout
+
+DO $migration$
+BEGIN
+    PERFORM set_config('lock_timeout', '5s', true);
+
+    -- 1. Foreign keys without a leading index.
+    CREATE INDEX IF NOT EXISTS "passkeys_user_id_idx"
+        ON "passkeys" ("user_id");
+    CREATE INDEX IF NOT EXISTS "medication_schedules_medication_id_idx"
+        ON "medication_schedules" ("medication_id");
+    CREATE INDEX IF NOT EXISTS "medication_intake_events_medication_id_idx"
+        ON "medication_intake_events" ("medication_id");
+    CREATE INDEX IF NOT EXISTS "telegram_reminder_messages_schedule_id_idx"
+        ON "telegram_reminder_messages" ("schedule_id");
+    CREATE INDEX IF NOT EXISTS "medication_inventory_items_medication_id_idx"
+        ON "medication_inventory_items" ("medication_id");
+    CREATE INDEX IF NOT EXISTS "medication_side_effects_medication_id_idx"
+        ON "medication_side_effects" ("medication_id");
+    CREATE INDEX IF NOT EXISTS "withings_oauth_states_user_id_idx"
+        ON "withings_oauth_states" ("user_id");
+    CREATE INDEX IF NOT EXISTS "whoop_oauth_states_user_id_idx"
+        ON "whoop_oauth_states" ("user_id");
+    CREATE INDEX IF NOT EXISTS "fitbit_oauth_states_user_id_idx"
+        ON "fitbit_oauth_states" ("user_id");
+    CREATE INDEX IF NOT EXISTS "google_health_oauth_states_user_id_idx"
+        ON "google_health_oauth_states" ("user_id");
+    CREATE INDEX IF NOT EXISTS "whoop_connect_tickets_user_id_idx"
+        ON "whoop_connect_tickets" ("user_id");
+    CREATE INDEX IF NOT EXISTS "cycle_day_logs_cycle_id_idx"
+        ON "cycle_day_logs" ("cycle_id");
+    CREATE INDEX IF NOT EXISTS "invite_tokens_created_by_idx"
+        ON "invite_tokens" ("created_by");
+    CREATE INDEX IF NOT EXISTS "invite_tokens_used_by_idx"
+        ON "invite_tokens" ("used_by");
+    CREATE INDEX IF NOT EXISTS "invite_redemptions_user_id_idx"
+        ON "invite_redemptions" ("user_id");
+    CREATE INDEX IF NOT EXISTS "custom_metric_entries_custom_metric_id_idx"
+        ON "custom_metric_entries" ("custom_metric_id");
+    CREATE INDEX IF NOT EXISTS "encounters_reminder_id_idx"
+        ON "encounters" ("reminder_id");
+    CREATE INDEX IF NOT EXISTS "vaccination_records_encounter_id_idx"
+        ON "vaccination_records" ("encounter_id");
+    CREATE INDEX IF NOT EXISTS "vaccination_records_practitioner_id_idx"
+        ON "vaccination_records" ("practitioner_id");
+    CREATE INDEX IF NOT EXISTS "vaccination_records_reminder_id_idx"
+        ON "vaccination_records" ("reminder_id");
+    CREATE INDEX IF NOT EXISTS "managed_profile_handovers_created_by_id_idx"
+        ON "managed_profile_handovers" ("created_by_id");
+
+    -- 2. Indexes that are a prefix of another index on the same table.
+    DROP INDEX IF EXISTS "biomarkers_user_id_idx";
+    DROP INDEX IF EXISTS "clinician_share_link_documents_share_link_id_idx";
+    DROP INDEX IF EXISTS "coach_usage_user_id_date_key_idx";
+    DROP INDEX IF EXISTS "custom_metrics_user_id_idx";
+    DROP INDEX IF EXISTS "cycle_day_logs_user_id_date_idx";
+    DROP INDEX IF EXISTS "data_backups_user_id_idx";
+    DROP INDEX IF EXISTS "environment_contexts_user_id_date_idx";
+    DROP INDEX IF EXISTS "illness_day_logs_episode_id_idx";
+    DROP INDEX IF EXISTS "integration_statuses_user_id_idx";
+    DROP INDEX IF EXISTS "medications_user_id_idx";
+    DROP INDEX IF EXISTS "menstrual_cycles_user_id_start_date_idx";
+    DROP INDEX IF EXISTS "mood_entries_user_id_date_idx";
+    DROP INDEX IF EXISTS "notification_channels_user_id_idx";
+    DROP INDEX IF EXISTS "push_subscriptions_user_id_idx";
+    DROP INDEX IF EXISTS "refresh_tokens_user_id_idx";
+    DROP INDEX IF EXISTS "user_achievements_user_id_idx";
+    DROP INDEX IF EXISTS "user_known_devices_user_id_idx";
+END
+$migration$;
diff --git a/prisma/migrations/0416_drop_measurements_type_prefix_index/migration.sql b/prisma/migrations/0416_drop_measurements_type_prefix_index/migration.sql
new file mode 100644
index 000000000..ff86f6c2c
--- /dev/null
+++ b/prisma/migrations/0416_drop_measurements_type_prefix_index/migration.sql
@@ -0,0 +1,17 @@
+-- Drop `measurements_user_id_type_measured_at_idx`.
+--
+-- `(user_id, type, measured_at)` is the leading part of the unique key
+-- `(user_id, type, measured_at, source, sleep_stage)`, and live reads have
+-- `measurements_live_covering_idx` besides. Every query shape that used the
+-- shorter index plans the same way on the unique key, at a cost within a few
+-- percent (`tests/integration/index-hygiene.test.ts` compares the plans on a
+-- seeded table). The index cost every write to the densest table in the
+-- schema and, on a bloated instance, more space than the table itself.
+--
+-- `DROP INDEX CONCURRENTLY` takes no lock that blocks reads or writes: it
+-- waits for the transactions already using the index to finish and then
+-- removes it. It refuses to run inside a transaction block, and under
+-- `migrate deploy` it does so whenever the file holds any other statement
+-- (Prisma 7 on Postgres 16 reports 25001), so this file holds exactly this
+-- one statement. `IF EXISTS` makes a re-run a no-op.
+DROP INDEX CONCURRENTLY IF EXISTS "measurements_user_id_type_measured_at_idx";
diff --git a/prisma/schema.prisma b/prisma/schema.prisma
index e1975b2fc..63fa6d26a 100644
--- a/prisma/schema.prisma
+++ b/prisma/schema.prisma
@@ -1248,7 +1248,6 @@ model EnvironmentContext {
   updatedAt DateTime @updatedAt @map("updated_at")
 
   @@unique([userId, date])
-  @@index([userId, date])
   // `environment_contexts_aq_gap_idx` (user_id, date) WHERE aq_fetched_at IS
   // NULL is a partial index from migration 0381 (raw SQL only).
   @@map("environment_contexts")
@@ -1300,6 +1299,8 @@ model Passkey {
 
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("passkeys")
 }
 
@@ -1484,7 +1485,6 @@ model UserKnownDevice {
   // twice for one account, so the "first sighting → alert" decision is a
   // single unique-constrained upsert.
   @@unique([userId, deviceHash])
-  @@index([userId])
   @@map("user_known_devices")
 }
 
@@ -2088,7 +2088,6 @@ model Measurement {
   // v1.4.23 — Apple Health batch dedup key. NULL externalId is distinct
   // (Postgres semantics) so manual entries don't collide here.
   @@unique([userId, type, source, externalId])
-  @@index([userId, type, measuredAt])
   // v1.25 — type-less `(userId, measuredAt)` range scan. The bulk feature read
   // (`extractFeatures`) and consolidation scans walk a user's measurements by
   // time with no `type` predicate; `(userId, type, measuredAt)` can't serve
@@ -3003,7 +3002,6 @@ model Medication {
   /// re-mirror of the same concept id returns the existing row (the
   /// create route pre-queries the triple and catches the P2002 race).
   @@unique([userId, externalSource, externalId])
-  @@index([userId])
   @@index([treatmentClass])
   @@map("medications")
 }
@@ -3213,6 +3211,8 @@ model MedicationSchedule {
   medication       Medication                @relation(fields: [medicationId], references: [id], onDelete: Cascade)
   telegramMessages TelegramReminderMessage[]
 
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([medicationId])
   @@map("medication_schedules")
 }
 
@@ -3491,6 +3491,8 @@ model MedicationIntakeEvent {
   // without this one the purge planned a sequential scan and could not finish
   // inside statement_timeout. Raw SQL only (partial predicate not expressible
   // in Prisma).
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([medicationId])
   @@map("medication_intake_events")
 }
 
@@ -3655,6 +3657,8 @@ model MedicationInventoryItem {
   // `(user_id) WHERE notes IS NOT NULL AND notes_encrypted IS NULL` — see
   // migration 0243 (v1.28.46 H4) for the med-notes boot-discovery scan. Raw
   // SQL only (partial predicate not expressible in Prisma).
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([medicationId])
   @@map("medication_inventory_items")
 }
 
@@ -3718,6 +3722,8 @@ model MedicationSideEffect {
   // `(user_id) WHERE notes IS NOT NULL AND notes_encrypted IS NULL` — see
   // migration 0243 (v1.28.46 H4) for the med-notes boot-discovery scan. Raw
   // SQL only (partial predicate not expressible in Prisma).
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([medicationId])
   @@map("medication_side_effects")
 }
 
@@ -3809,6 +3815,8 @@ model TelegramReminderMessage {
 
   @@unique([recipientUserId, medicationId, scheduleId, date, phase, timeOfDay], map: "telegram_reminder_messages_recipient_slot_key")
   @@index([medicationId, date])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([scheduleId])
   @@map("telegram_reminder_messages")
 }
 
@@ -4004,6 +4012,9 @@ model InviteToken {
   redemptions InviteRedemption[]
 
   @@index([expiresAt])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([createdBy])
+  @@index([usedBy])
   @@map("invite_tokens")
 }
 
@@ -4047,6 +4058,8 @@ model ManagedProfileHandover {
   @@index([profileId])
   // `managed_profile_handovers_open_idx` UNIQUE (profile_id) WHERE used_at IS
   // NULL AND revoked_at IS NULL is a partial index from migration 0383.
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([createdById])
   @@map("managed_profile_handovers")
 }
 
@@ -4100,6 +4113,8 @@ model InviteRedemption {
   user   User?       @relation("InviteRedemptions", fields: [userId], references: [id], onDelete: SetNull)
 
   @@index([inviteId])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("invite_redemptions")
 }
 
@@ -4136,7 +4151,6 @@ model RefreshToken {
 
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
-  @@index([userId])
   /// Composite index — every reuse-detection / device-scoped lookup filters
   /// on (userId, deviceId), so a single-column device_id index would never
   /// be used while costing writes. Multiple live rows per (userId, deviceId)
@@ -4267,6 +4281,8 @@ model WithingsOAuthState {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([expiresAt])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("withings_oauth_states")
 }
 
@@ -4348,6 +4364,8 @@ model WhoopOAuthState {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([expiresAt])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("whoop_oauth_states")
 }
 
@@ -4380,6 +4398,8 @@ model WhoopConnectTicket {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([expiresAt])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("whoop_connect_tickets")
 }
 
@@ -4434,6 +4454,8 @@ model FitbitOAuthState {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([expiresAt])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("fitbit_oauth_states")
 }
 
@@ -4512,6 +4534,8 @@ model GoogleHealthOAuthState {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@index([expiresAt])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([userId])
   @@map("google_health_oauth_states")
 }
 
@@ -4642,7 +4666,6 @@ model MoodEntry {
   // that carry a source-stable id upsert on this; native/MANUAL entries
   // (NULL externalId) are distinct here and stay on the legacy key above.
   @@unique([userId, source, externalId])
-  @@index([userId, date])
   // v1.12.1 (P1-3) — mood-insights reads filter `{ userId, deletedAt:
   // null, moodLoggedAt: { gte } }` ordered by `moodLoggedAt desc`; this
   // index turns that into a range scan. See migration 0123.
@@ -5098,7 +5121,6 @@ model IntegrationStatus {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@unique([userId, integration])
-  @@index([userId])
   @@index([state])
   @@map("integration_statuses")
 }
@@ -5451,7 +5473,6 @@ model NotificationChannel {
   preferences NotificationPreference[]
 
   @@unique([userId, type])
-  @@index([userId])
   @@map("notification_channels")
 }
 
@@ -5479,7 +5500,6 @@ model PushSubscription {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@unique([userId, endpoint])
-  @@index([userId])
   @@map("push_subscriptions")
 }
 
@@ -5526,7 +5546,6 @@ model DataBackup {
   chunks DataBackupChunk[]
 
   @@unique([userId, type], map: "data_backups_user_id_type_key")
-  @@index([userId])
   @@map("data_backups")
 }
 
@@ -6369,7 +6388,6 @@ model Biomarker {
   mergesInto      BiomarkerMerge[]
 
   @@unique([userId, name])
-  @@index([userId])
   @@map("biomarkers")
 }
 
@@ -6412,7 +6430,6 @@ model CustomMetric {
   entries CustomMetricEntry[]
 
   @@unique([userId, name])
-  @@index([userId])
   @@map("custom_metrics")
 }
 
@@ -6450,6 +6467,8 @@ model CustomMetricEntry {
   // `(user_id) WHERE note IS NOT NULL` — see
   // migration 0357 for the free-text encryption boot-discovery scan. Raw SQL
   // only (partial predicate not expressible in Prisma).
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([customMetricId])
   @@map("custom_metric_entries")
 }
 
@@ -6497,7 +6516,6 @@ model UserAchievement {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@unique([userId, achievementId])
-  @@index([userId])
   @@map("user_achievements")
 }
 
@@ -7189,7 +7207,6 @@ model CoachUsage {
   user User @relation(fields: [userId], references: [id], onDelete: Cascade)
 
   @@unique([userId, dateKey])
-  @@index([userId, dateKey])
   @@map("coach_usage")
 }
 
@@ -7440,7 +7457,6 @@ model ClinicianShareLinkDocument {
   document  InboundDocument    @relation(fields: [documentId], references: [id], onDelete: Cascade)
 
   @@unique([shareLinkId, documentId])
-  @@index([shareLinkId])
   @@index([documentId])
   @@map("clinician_share_link_documents")
 }
@@ -8031,7 +8047,6 @@ model MenstrualCycle {
   dayLogs CycleDayLog[]
 
   @@unique([userId, startDate])
-  @@index([userId, startDate])
   // Keyset support for the `/api/sync/changes` cycle feed
   // (`where userId` ordered by `(updatedAt asc, id asc)`).
   @@index([userId, updatedAt, id])
@@ -8131,9 +8146,10 @@ model CycleDayLog {
   // NULL-distinct external dedup key for HealthKit re-ingest (the
   // MoodEntry dual-unique precedent).
   @@unique([userId, source, externalId])
-  @@index([userId, date])
   // Keyset support for the `/api/sync/changes` cycle-day feed.
   @@index([userId, updatedAt, id])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([cycleId])
   @@map("cycle_day_logs")
 }
 
@@ -8325,7 +8341,6 @@ model IllnessDayLog {
 
   @@unique([episodeId, date])
   @@index([userId, date])
-  @@index([episodeId])
   @@map("illness_day_logs")
 }
 
@@ -9332,6 +9347,8 @@ model Encounter {
   @@index([userId, deletedAt, occurredAt])
   @@index([userId, status, occurredAt])
   @@index([practitionerId])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([reminderId])
   @@map("encounters")
 }
 
@@ -9592,6 +9609,10 @@ model VaccinationRecord {
   @@index([userId, deletedAt, occurredAt])
   @@index([userId, antigenSlug, occurredAt])
   @@index([customVaccineId])
+  // Leads the foreign key, so a cascade from the parent row is an index lookup.
+  @@index([encounterId])
+  @@index([practitionerId])
+  @@index([reminderId])
   @@map("vaccination_records")
 }
 
diff --git a/src/__tests__/migration-lock-timeout-guard.test.ts b/src/__tests__/migration-lock-timeout-guard.test.ts
index 6afe05a6e..45c7595bf 100644
--- a/src/__tests__/migration-lock-timeout-guard.test.ts
+++ b/src/__tests__/migration-lock-timeout-guard.test.ts
@@ -89,6 +89,56 @@ function violations(name: string, sql: string): string[] {
   return found;
 }
 
+/**
+ * `CREATE INDEX CONCURRENTLY` / `DROP INDEX CONCURRENTLY` refuse to run in a
+ * transaction block, and under `migrate deploy` they do so whenever the file
+ * holds any other statement (SQLSTATE 25001, seen against Prisma 7 and
+ * Postgres 16). Such a statement therefore stands alone in its file.
+ */
+function concurrentViolations(name: string, sql: string): string[] {
+  const statements = topLevelStatements(sql);
+  const concurrent = statements.filter((statement) =>
+    /\bCONCURRENTLY\b/i.test(statement),
+  );
+  if (concurrent.length === 0 || statements.length === 1) return [];
+  return [
+    `${name}: a CONCURRENTLY statement must be the only statement in its file`,
+  ];
+}
+
+describe("concurrent index statements", () => {
+  it("stand alone in their migration file", () => {
+    const found = migrations().flatMap(({ name, sql }) =>
+      concurrentViolations(name, sql),
+    );
+    expect(found, found.join("\n")).toEqual([]);
+  });
+
+  it("sees the concurrent migrations there are", () => {
+    const concurrent = migrations()
+      .filter(({ sql }) =>
+        /\bCONCURRENTLY\b/i.test(topLevelStatements(sql).join(";")),
+      )
+      .map(({ name }) => name);
+    expect(concurrent).toContain("0416_drop_measurements_type_prefix_index");
+  });
+
+  it("refuses a concurrent statement beside another one", () => {
+    expect(
+      concurrentViolations(
+        "a",
+        'DROP INDEX CONCURRENTLY IF EXISTS "x";\nDROP INDEX CONCURRENTLY IF EXISTS "y";',
+      ),
+    ).toHaveLength(1);
+    expect(
+      concurrentViolations(
+        "b",
+        '-- CONCURRENTLY in a comment\nDROP INDEX CONCURRENTLY IF EXISTS "x";',
+      ),
+    ).toEqual([]);
+  });
+});
+
 describe("migration lock waits", () => {
   it("bounds lock waits where they take effect and marks only repeatable files", () => {
     const found = migrations().flatMap(({ name, sql }) =>
@@ -97,11 +147,13 @@ describe("migration lock waits", () => {
     expect(found, found.join("\n")).toEqual([]);
   });
 
-  it("sees the one migration that bounds its wait today", () => {
+  it("sees the migrations that bound their wait", () => {
     const marked = migrations()
       .filter(({ sql }) => RETRY_MARKER.test(sql))
       .map(({ name }) => name);
     expect(marked).toContain("0412_measurement_hot_updates");
+    expect(marked).toContain("0414_drop_medication_compliance_rollups");
+    expect(marked).toContain("0415_foreign_key_and_prefix_indexes");
   });
 
   it("catches each shape it exists to refuse", () => {
diff --git a/src/lib/personal-records/pr-detection-worker.ts b/src/lib/personal-records/pr-detection-worker.ts
index a6e468b77..e2839662a 100644
--- a/src/lib/personal-records/pr-detection-worker.ts
+++ b/src/lib/personal-records/pr-detection-worker.ts
@@ -19,8 +19,8 @@
  *   - All-time scan rather than a rolling window. The
  *     `personal_records_user_metric_idx` index makes
  *     `ORDER BY value LIMIT 1` constant-time; reading the worst-case
- *     ~30k measurement rows per metric is ~50 ms with the typed
- *     `measurements_user_id_type_measured_at_idx` index.
+ *     ~30k measurement rows per metric is ~50 ms over the
+ *     `(user_id, type, measured_at, …)` unique index.
  *   - Tie handling: an equal-value reading writes a row but the caller
  *     suppresses the push notification (the user matched, not beat
  *     their best). The notification suppression lives on the job
diff --git a/tests/integration/index-hygiene.test.ts b/tests/integration/index-hygiene.test.ts
new file mode 100644
index 000000000..370eb8df5
--- /dev/null
+++ b/tests/integration/index-hygiene.test.ts
@@ -0,0 +1,360 @@
+/**
+ * Index hygiene, read from the catalog of the migrated database.
+ *
+ * Two rules, each with its counter-check:
+ *
+ * 1. Every foreign key whose delete action does work in the child table
+ *    (CASCADE, SET NULL, SET DEFAULT) has an index leading with its columns.
+ *    Without one, deleting a parent row scans the child table once per
+ *    deleted parent; an account delete runs hundreds of them.
+ * 2. No btree index is a prefix of another btree index on the same table
+ *    (same leading columns, operator classes, collations and directions,
+ *    same predicate). The longer index answers every lookup the shorter one
+ *    did, so the shorter one only costs writes and space. A unique index is
+ *    never counted as the redundant one: it enforces something.
+ *
+ * And the evidence for migration 0416: on a seeded `measurements` table of
+ * production size, every query shape that used
+ * `measurements_user_id_type_measured_at_idx` plans the same way without it,
+ * on an index, at a cost within a few percent.
+ */
+import { Client } from "pg";
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
+
+import { getPrismaClient, truncateAllTables } from "./setup";
+
+/**
+ * Foreign keys allowed to go without a leading index, with the reason.
+ * Empty: every one has its index.
+ */
+const FK_EXCEPTIONS: Record<string, string> = {};
+
+/** Prefix indexes allowed to stay, with the reason. Empty. */
+const PREFIX_EXCEPTIONS: Record<string, string> = {};
+
+async function connect(): Promise<Client> {
+  const raw = process.env.DATABASE_URL;
+  if (!raw) throw new Error("DATABASE_URL is not set for the integration run");
+  const url = new URL(raw);
+  url.searchParams.delete("schema");
+  url.searchParams.delete("pgbouncer");
+  const client = new Client({ connectionString: url.toString() });
+  await client.connect();
+  return client;
+}
+
+const FK_WITHOUT_LEADING_INDEX = `
+  SELECT c.conrelid::regclass::text AS tbl, c.conname AS name
+    FROM pg_constraint c
+   WHERE c.contype = 'f'
+     AND c.connamespace = 'public'::regnamespace
+     AND c.confdeltype IN ('c', 'n', 'd')
+     AND NOT EXISTS (
+       SELECT 1 FROM pg_index i
+        WHERE i.indrelid = c.conrelid
+          AND i.indpred IS NULL
+          AND i.indexprs IS NULL
+          AND (string_to_array(i.indkey::text, ' ')::int2[])[1:array_length(c.conkey, 1)] @> c.conkey
+          AND (string_to_array(i.indkey::text, ' ')::int2[])[1:array_length(c.conkey, 1)] <@ c.conkey
+     )
+   ORDER BY 1, 2`;
+
+const ACTING_FK_COUNT = `
+  SELECT count(*)::int AS n FROM pg_constraint
+   WHERE contype = 'f' AND connamespace = 'public'::regnamespace
+     AND confdeltype IN ('c', 'n', 'd')`;
+
+/*
+ * `indkey`, `indclass`, `indcollation` and `indoption` are vectors whose array
+ * casts start at index 0; going through text gives ordinary 1-based arrays.
+ */
+const PREFIX_INDEXES = `
+  WITH idx AS (
+    SELECT i.indexrelid, i.indrelid, c.relname AS index_name,
+           t.relname AS table_name, i.indisunique, i.indisprimary,
+           string_to_array(i.indkey::text, ' ')::int[] AS cols,
+           string_to_array(i.indclass::text, ' ')::oid[] AS opc,
+           string_to_array(i.indcollation::text, ' ')::oid[] AS coll,
+           string_to_array(i.indoption::text, ' ')::int[] AS opt,
+           pg_get_expr(i.indpred, i.indrelid) AS pred,
+           i.indexprs IS NOT NULL AS has_exprs, am.amname
+      FROM pg_index i
+      JOIN pg_class c ON c.oid = i.indexrelid
+      JOIN pg_class t ON t.oid = i.indrelid
+      JOIN pg_am am ON am.oid = c.relam
+     WHERE t.relnamespace = 'public'::regnamespace
+  )
+  SELECT a.table_name AS tbl, a.index_name AS redundant, b.index_name AS covered_by
+    FROM idx a
+    JOIN idx b ON a.indrelid = b.indrelid AND a.indexrelid <> b.indexrelid
+   WHERE a.amname = 'btree' AND b.amname = 'btree'
+     AND NOT a.indisprimary AND NOT a.has_exprs AND NOT b.has_exprs
+     AND a.pred IS NOT DISTINCT FROM b.pred
+     AND cardinality(a.cols) <= cardinality(b.cols)
+     AND a.cols = b.cols[1:cardinality(a.cols)]
+     AND a.opc = b.opc[1:cardinality(a.cols)]
+     AND a.coll = b.coll[1:cardinality(a.cols)]
+     AND a.opt = b.opt[1:cardinality(a.cols)]
+     AND (NOT a.indisunique OR (b.indisunique AND a.cols = b.cols))
+     AND (cardinality(a.cols) < cardinality(b.cols)
+          OR (NOT a.indisunique AND b.indisunique)
+          OR (a.indisunique = b.indisunique AND a.indexrelid > b.indexrelid))
+   ORDER BY 1, 2`;
+
+const BTREE_INDEX_COUNT = `
+  SELECT count(*)::int AS n FROM pg_index i
+    JOIN pg_class c ON c.oid = i.indexrelid JOIN pg_am am ON am.oid = c.relam
+   WHERE c.relnamespace = 'public'::regnamespace AND am.amname = 'btree'`;
+
+describe("index hygiene", () => {
+  let db: Client;
+  beforeAll(async () => {
+    db = await connect();
+  });
+  afterAll(async () => {
+    await db.end();
+  });
+
+  it("gives every acting foreign key a leading index", async () => {
+    const examined = (await db.query(ACTING_FK_COUNT)).rows[0].n as number;
+    // The matcher has to have something to match: the schema has well over a
+    // hundred cascading foreign keys.
+    expect(examined).toBeGreaterThan(100);
+    const { rows } = await db.query(FK_WITHOUT_LEADING_INDEX);
+    const offenders = rows
+      .map((r) => `${r.tbl}.${r.name}`)
+      .filter((key) => !(key in FK_EXCEPTIONS));
+    expect(offenders, offenders.join("\n")).toEqual([]);
+  });
+
+  it("counter-check: flags a cascading foreign key without an index", async () => {
+    await db.query("BEGIN");
+    try {
+      await db.query(
+        `CREATE TABLE "zz_fk_probe" ("id" text PRIMARY KEY,
+           "user_id" text REFERENCES "users"("id") ON DELETE CASCADE)`,
+      );
+      const flagged = (await db.query(FK_WITHOUT_LEADING_INDEX)).rows.map(
+        (r) => r.tbl,
+      );
+      expect(flagged).toContain("zz_fk_probe");
+      await db.query(`CREATE INDEX ON "zz_fk_probe" ("user_id", "id")`);
+      const after = (await db.query(FK_WITHOUT_LEADING_INDEX)).rows.map(
+        (r) => r.tbl,
+      );
+      expect(after).not.toContain("zz_fk_probe");
+    } finally {
+      await db.query("ROLLBACK");
+    }
+  });
+
+  it("keeps no btree index that is a prefix of another", async () => {
+    const examined = (await db.query(BTREE_INDEX_COUNT)).rows[0].n as number;
+    expect(examined).toBeGreaterThan(300);
+    const { rows } = await db.query(PREFIX_INDEXES);
+    const offenders = rows
+      .filter((r) => !(r.redundant in PREFIX_EXCEPTIONS))
+      .map((r) => `${r.tbl}: ${r.redundant} is a prefix of ${r.covered_by}`);
+    expect(offenders, offenders.join("\n")).toEqual([]);
+  });
+
+  it("counter-check: flags a prefix and a duplicate, but not a reversed direction", async () => {
+    await db.query("BEGIN");
+    try {
+      await db.query(
+        `CREATE INDEX "zz_prefix_probe" ON "measurements" ("user_id", "type")`,
+      );
+      await db.query(
+        `CREATE INDEX "zz_duplicate_probe" ON "users" ("username")`,
+      );
+      await db.query(
+        `CREATE INDEX "zz_direction_probe" ON "measurements" ("user_id", "type", "measured_at" DESC, "source")`,
+      );
+      const flagged = (await db.query(PREFIX_INDEXES)).rows.map(
+        (r) => r.redundant,
+      );
+      expect(flagged).toContain("zz_prefix_probe");
+      expect(flagged).toContain("zz_duplicate_probe");
+      // `users_username_key` is unique: it enforces something, so it is
+      // never the one reported, even beside a plain twin.
+      expect(flagged).not.toContain("users_username_key");
+      // The live-covering index runs `measured_at DESC` too, but carries a
+      // predicate, so it is not a prefix of the probe either way.
+      expect(flagged).not.toContain("zz_direction_probe");
+    } finally {
+      await db.query("ROLLBACK");
+    }
+  });
+});
+
+describe("measurements without (user_id, type, measured_at)", () => {
+  const DROPPED = "measurements_user_id_type_measured_at_idx";
+  const USERS = 5;
+  const ROWS = 130_000;
+  let db: Client;
+  let userId: string;
+
+  beforeAll(async () => {
+    const prisma = getPrismaClient();
+    await truncateAllTables(prisma);
+    const ids: string[] = [];
+    for (let i = 0; i < USERS; i++) {
+      const user = await prisma.user.create({
+        data: {
+          username: `plan-probe-${i}`,
+          email: `plan-probe-${i}@example.test`,
+          role: "USER",
+        },
+      });
+      ids.push(user.id);
+    }
+    userId = ids[2];
+    db = await connect();
+    // Production's mix: dense pulse and steps, a long tail of manual types,
+    // three sources, one row in twenty a tombstone, a year of history.
+    await db.query(
+      `WITH t(type, unit, weight) AS (VALUES
+         ('PULSE'::measurement_type, 'bpm', 50), ('ACTIVITY_STEPS', 'count', 10),
+         ('HEART_RATE_VARIABILITY', 'ms', 10), ('SLEEP_DURATION', 'h', 8),
+         ('BLOOD_GLUCOSE', 'mg/dL', 5), ('OXYGEN_SATURATION', '%', 4),
+         ('WEIGHT', 'kg', 3), ('BLOOD_PRESSURE_SYS', 'mmHg', 3),
+         ('BLOOD_PRESSURE_DIA', 'mmHg', 3), ('RESTING_HEART_RATE', 'bpm', 2),
+         ('BODY_FAT', '%', 2)),
+       slots AS (SELECT row_number() OVER () AS n, type, unit
+                   FROM t, generate_series(1, t.weight))
+       INSERT INTO "measurements"
+         ("id", "user_id", "type", "value", "unit", "source", "measured_at",
+          "updated_at", "deleted_at")
+       SELECT 'plan-' || g, ($1::text[])[1 + g % $2], s.type, random() * 100, s.unit,
+              (ARRAY['APPLE_HEALTH', 'MANUAL', 'WITHINGS']::measurement_source[])[1 + g % 3],
+              timestamp '2026-10-20' - (g * interval '241 seconds'),
+              now(), CASE WHEN g % 20 = 0 THEN now() END
+         FROM generate_series(1, $3) g
+         JOIN slots s ON s.n = 1 + (g * 7 % 100)`,
+      [ids, USERS, ROWS],
+    );
+    await db.query(`ANALYZE "measurements"`);
+  }, 120_000);
+
+  afterAll(async () => {
+    await db.end();
+    await truncateAllTables(getPrismaClient());
+  });
+
+  /** The query shapes the read and write paths send with a user and a type. */
+  const QUERIES: Array<[string, string]> = [
+    [
+      "live range, newest first",
+      `SELECT * FROM measurements WHERE user_id = $1 AND type = 'WEIGHT'
+         AND measured_at >= '2026-07-20' AND deleted_at IS NULL
+       ORDER BY measured_at DESC`,
+    ],
+    [
+      "range including tombstones",
+      `SELECT * FROM measurements WHERE user_id = $1 AND type = 'WEIGHT'
+         AND measured_at >= '2026-07-20' ORDER BY measured_at DESC`,
+    ],
+    [
+      "latest of a type",
+      `SELECT max(measured_at) FROM measurements WHERE user_id = $1 AND type = 'PULSE'`,
+    ],
+    [
+      "count of a type",
+      `SELECT count(*) FROM measurements WHERE user_id = $1 AND type = 'BLOOD_GLUCOSE'`,
+    ],
+    [
+      "point lookup without source",
+      `SELECT id FROM measurements WHERE user_id = $1 AND type = 'PULSE'
+         AND measured_at = '2026-10-01 10:00'`,
+    ],
+    [
+      "day aggregate",
+      `SELECT date_trunc('day', measured_at), avg(value) FROM measurements
+        WHERE user_id = $1 AND type = 'PULSE' AND measured_at >= '2026-10-01'
+          AND measured_at < '2026-10-08' AND deleted_at IS NULL GROUP BY 1`,
+    ],
+    [
+      "two types over a month",
+      `SELECT * FROM measurements WHERE user_id = $1
+         AND type IN ('BLOOD_PRESSURE_SYS', 'BLOOD_PRESSURE_DIA')
+         AND measured_at BETWEEN '2026-09-01' AND '2026-10-01'`,
+    ],
+    [
+      "last before a date",
+      `SELECT * FROM measurements WHERE user_id = $1 AND type = 'SLEEP_DURATION'
+         AND measured_at < '2026-10-01' ORDER BY measured_at DESC LIMIT 1`,
+    ],
+    [
+      "tombstone a day",
+      `UPDATE measurements SET deleted_at = now() WHERE user_id = $1
+         AND type = 'HEART_RATE_VARIABILITY' AND measured_at >= '2026-10-10'
+         AND measured_at < '2026-10-11'`,
+    ],
+  ];
+
+  interface PlanNode {
+    "Node Type": string;
+    "Relation Name"?: string;
+    "Index Name"?: string;
+    "Total Cost": number;
+    Plans?: PlanNode[];
+  }
+
+  function walk(node: PlanNode, out: PlanNode[] = []): PlanNode[] {
+    out.push(node);
+    for (const child of node.Plans ?? []) walk(child, out);
+    return out;
+  }
+
+  async function plans(): Promise<Array<{ cost: number; nodes: PlanNode[] }>> {
+    const result = [];
+    for (const [, sql] of QUERIES) {
+      const { rows } = await db.query(`EXPLAIN (FORMAT JSON) ${sql}`, [userId]);
+      const root = rows[0]["QUERY PLAN"][0].Plan as PlanNode;
+      result.push({ cost: root["Total Cost"], nodes: walk(root) });
+    }
+    return result;
+  }
+
+  it("is gone after the migrations", async () => {
+    const { rowCount } = await db.query(
+      `SELECT 1 FROM pg_indexes WHERE indexname = $1`,
+      [DROPPED],
+    );
+    expect(rowCount).toBe(0);
+  });
+
+  it("plans every user-and-type query on an index, at the cost it had with it", async () => {
+    // The state before migration 0416, rebuilt inside a transaction.
+    await db.query("BEGIN");
+    let before;
+    try {
+      await db.query(
+        `CREATE INDEX "${DROPPED}" ON "measurements" ("user_id", "type", "measured_at")`,
+      );
+      before = await plans();
+    } finally {
+      await db.query("ROLLBACK");
+    }
+    const after = await plans();
+
+    // The comparison means something only if the old index was in use.
+    const usedBefore = before.filter((p) =>
+      p.nodes.some((n) => n["Index Name"] === DROPPED),
+    ).length;
+    expect(usedBefore).toBeGreaterThanOrEqual(3);
+
+    QUERIES.forEach(([label], i) => {
+      const seqScan = after[i].nodes.find(
+        (n) =>
+          n["Node Type"] === "Seq Scan" &&
+          n["Relation Name"] === "measurements",
+      );
+      expect(seqScan, `${label}: sequential scan`).toBeUndefined();
+      expect(
+        after[i].cost,
+        `${label}: cost ${after[i].cost} against ${before[i].cost}`,
+      ).toBeLessThanOrEqual(before[i].cost * 1.05 + 1);
+    });
+  }, 60_000);
+});