Prober: the claim's random starting points drawn in code

Postgres evaluated random() once for all 20 slices: every claim still
came from one place in the queue (214 of 214 taken from a.b.zogg).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/services/prober.ts

@@ -126,17 +126,18 @@ export class Prober {
     for (const kind of KINDS) {
       const want = coordinatorConfig.probe.batch - this.queue.length
       if (want <= 0) return
+      // The starting points chosen here: in the query Postgres drew random() once for all slices.
+      const [{ lo, hi }] = (await db.rawQuery(`SELECT min(id) AS lo, max(id) AS hi FROM probe_candidates`)).rows
+      const starts = Array.from({ length: SLICES }, () => Number(lo) + Math.floor(Math.random() * (Number(hi) - Number(lo) + 1)))
       const { rows } = await db.rawQuery(
         `UPDATE probe_candidates SET status = 'taken' WHERE id IN (
-           SELECT pc.id FROM (SELECT min(id) AS lo, max(id) AS hi FROM probe_candidates) b
+           SELECT pc.id FROM unnest(?::bigint[]) AS o(start)
-           CROSS JOIN LATERAL generate_series(1, ?) AS s
            CROSS JOIN LATERAL (
              SELECT pc.id FROM probe_candidates pc JOIN releases r ON r.id = pc.release_id
-             WHERE pc.status = 'waiting' AND pc.kind = ? AND r.settled_at IS NOT NULL
+             WHERE pc.status = 'waiting' AND pc.kind = ? AND r.settled_at IS NOT NULL AND pc.id >= o.start
-               AND pc.id >= b.lo + floor(random() * (b.hi - b.lo + 1))::bigint + s * 0
              ORDER BY pc.id LIMIT ? FOR UPDATE OF pc SKIP LOCKED) pc)
          RETURNING id, release_id, kind, message_id`,
-        [SLICES, kind, Math.ceil(want / SLICES)]
+        [starts, kind, Math.ceil(want / SLICES)]
       )
       for (const r of rows) this.queue.push({ id: Number(r.id), releaseId: Number(r.release_id), kind: r.kind, messageId: r.message_id })
     }