feat(schedule-engine): stop persisting per-tick schedule state

.server-changes/stop-persisting-schedule-tick-state.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: improvement
+---
+
+Stop writing per-tick state (`lastScheduledTimestamp`, `nextScheduledTimestamp`, `lastRunTriggeredAt`) on `TaskSchedule` and `TaskScheduleInstance`. The schedule engine now carries the previous fire time forward via the worker queue payload, eliminating ~270K dead-tuple-driven autovacuums per year on these hot tables and the associated `IO:XactSync` mini-spikes on the writer. Customer-facing `payload.lastTimestamp` semantics are unchanged.

apps/webapp/app/presenters/v3/ScheduleListPresenter.server.ts

@@ -6,7 +6,10 @@ import { getLimit } from "~/services/platform.v3.server";
 import { findCurrentWorkerFromEnvironment } from "~/v3/models/workerDeployment.server";
 import { ServiceValidationError } from "~/v3/services/baseService.server";
 import { CheckScheduleService } from "~/v3/services/checkSchedule.server";
-import { calculateNextScheduledTimestampFromNow } from "~/v3/utils/calculateNextSchedule.server";
+import {
+  calculateNextScheduledTimestampFromNow,
+  previousScheduledTimestamp,
+} from "~/v3/utils/calculateNextSchedule.server";
 import { BasePresenter } from "./basePresenter.server";
 
 type ScheduleListOptions = {
@@ -193,8 +196,8 @@ export class ScheduleListPresenter extends BasePresenter {
           },
         },
         active: true,
-        lastRunTriggeredAt: true,
         createdAt: true,
+        updatedAt: true,
       },
       where: {
         projectId: project.id,
@@ -244,6 +247,29 @@ export class ScheduleListPresenter extends BasePresenter {
     });
 
     const schedules: ScheduleListItem[] = rawSchedules.map((schedule) => {
+      // Approximate "last run" from the cron's previous slot. Skip inactive
+      // schedules — the cron's previous slot reflects what *would* have
+      // fired, but a deactivated schedule didn't actually fire there. Skip
+      // when the cron's previous slot predates `updatedAt`: any config
+      // change (cron edited, timezone changed, deactivate/reactivate)
+      // bumps updatedAt, and a slot from before the most recent change
+      // didn't fire under the current configuration. cron-parser throws
+      // on malformed expressions, so degrade to undefined per-row rather
+      // than failing the whole list. UI is best-effort; the runs page is
+      // the source of truth.
+      let lastRun: Date | undefined;
+      if (schedule.active) {
+        try {
+          const cronPrev = previousScheduledTimestamp(
+            schedule.generatorExpression,
+            schedule.timezone
+          );
+          lastRun = cronPrev.getTime() > schedule.updatedAt.getTime() ? cronPrev : undefined;
+        } catch {
+          lastRun = undefined;
+        }
+      }
+
       return {
         id: schedule.id,
         type: schedule.type,
@@ -256,7 +282,7 @@ export class ScheduleListPresenter extends BasePresenter {
         timezone: schedule.timezone,
         active: schedule.active,
         externalId: schedule.externalId,
-        lastRun: schedule.lastRunTriggeredAt ?? undefined,
+        lastRun,
         nextRun: calculateNextScheduledTimestampFromNow(
           schedule.generatorExpression,
           schedule.timezone

apps/webapp/app/v3/utils/calculateNextSchedule.server.ts

@@ -22,6 +22,20 @@ function calculateNextStep(schedule: string, timezone: string | null, currentDat
     .toDate();
 }
 
+export function previousScheduledTimestamp(
+  schedule: string,
+  timezone: string | null,
+  fromTimestamp: Date = new Date()
+) {
+  return parseExpression(schedule, {
+    currentDate: fromTimestamp,
+    utc: timezone === null,
+    tz: timezone ?? undefined,
+  })
+    .prev()
+    .toDate();
+}
+
 export function nextScheduledTimestamps(
   cron: string,
   timezone: string | null,

internal-packages/database/prisma/schema.prisma

@@ -2128,6 +2128,7 @@ model TaskSchedule {
   ///Instances of the schedule that are active
   instances TaskScheduleInstance[]
 
+  /// @deprecated stop writing 2026-04-30; reads moved out of code (UI now derives from cron's previous slot). Drop in follow-up.
   lastRunTriggeredAt DateTime?
 
   project   Project @relation(fields: [projectId], references: [id], onDelete: Cascade, onUpdate: Cascade)
@@ -2173,7 +2174,9 @@ model TaskScheduleInstance {
 
   active Boolean @default(true)
 
+  /// @deprecated stop writing 2026-04-30; engine derives from cron + exactScheduleTime. Drop in follow-up.
   lastScheduledTimestamp DateTime?
+  /// @deprecated stop writing 2026-04-30; engine derives from cron + now(). Drop in follow-up.
   nextScheduledTimestamp DateTime?
 
   //you can only have a schedule attached to each environment once