Optimize timeline for limit=1 (#1971)

* optimize timeline skeleton w/ limit=1

* build

.github/workflows/build-and-push-bsky-aws.yaml

@@ -3,6 +3,7 @@ on:
   push:
     branches:
       - main
+      - timeline-limit-1-opt
 env:
   REGISTRY: ${{ secrets.AWS_ECR_REGISTRY_USEAST2_PACKAGES_REGISTRY }}
   USERNAME: ${{ secrets.AWS_ECR_REGISTRY_USEAST2_PACKAGES_USERNAME }}

packages/bsky/src/api/app/bsky/feed/getTimeline.ts

@@ -1,3 +1,4 @@
+import { sql } from 'kysely'
 import { InvalidRequestError } from '@atproto/xrpc-server'
 import { Server } from '../../../../lexicon'
 import { FeedAlgorithm, FeedKeyset, getFeedDateThreshold } from '../util/feed'
@@ -55,6 +56,11 @@ export const skeleton = async (
     throw new InvalidRequestError(`Unsupported algorithm: ${algorithm}`)
   }
 
+  if (limit === 1 && !cursor) {
+    // special case for limit=1, which is often used to check if there are new items at the top of the timeline.
+    return skeletonLimit1(params, ctx)
+  }
+
   const keyset = new FeedKeyset(ref('feed_item.sortAt'), ref('feed_item.cid'))
   const sortFrom = keyset.unpack(cursor)?.primary
 
@@ -117,6 +123,54 @@ export const skeleton = async (
   }
 }
 
+// The limit=1 case is used commonly to check if there are new items at the top of the timeline.
+// Since it's so common, it's optimized here.  The most common strategy that postgres takes to
+// build a timeline is to grab all recent content from each of the user's follow, then paginate it.
+// The downside here is that it requires grabbing all recent content from all follows, even if you
+// only want a single result.  The approach here instead takes the single most recent post from
+// each of the user's follows, then sorts only those and takes the top item.
+const skeletonLimit1 = async (params: Params, ctx: Context) => {
+  const { viewer } = params
+  const { db } = ctx
+  const { ref } = db.db.dynamic
+  const creatorsQb = db.db
+    .selectFrom('follow')
+    .where('creator', '=', viewer)
+    .select('subjectDid as did')
+    .unionAll(sql`select ${viewer} as did`)
+  const feedItemsQb = db.db
+    .selectFrom(creatorsQb.as('creator'))
+    .innerJoinLateral(
+      (eb) => {
+        const keyset = new FeedKeyset(
+          ref('feed_item.sortAt'),
+          ref('feed_item.cid'),
+        )
+        const creatorFeedItemQb = eb
+          .selectFrom('feed_item')
+          .innerJoin('post', 'post.uri', 'feed_item.postUri')
+          .whereRef('feed_item.originatorDid', '=', 'creator.did')
+          .where('feed_item.sortAt', '>', getFeedDateThreshold(undefined, 2))
+          .selectAll('feed_item')
+          .select([
+            'post.replyRoot',
+            'post.replyParent',
+            'post.creator as postAuthorDid',
+          ])
+        return paginate(creatorFeedItemQb, { limit: 1, keyset }).as('result')
+      },
+      (join) => join.onTrue(),
+    )
+    .selectAll('result')
+  const keyset = new FeedKeyset(ref('result.sortAt'), ref('result.cid'))
+  const feedItems = await paginate(feedItemsQb, { limit: 1, keyset }).execute()
+  return {
+    params,
+    feedItems,
+    cursor: keyset.packFromResult(feedItems),
+  }
+}
+
 const hydration = async (
   state: SkeletonState,
   ctx: Context,

packages/bsky/tests/views/timeline.test.ts

@@ -154,6 +154,20 @@ describe('timeline views', () => {
     expect(results(paginatedAll)).toEqual(results([full.data]))
   })
 
+  it('agrees what the first item is for limit=1 and other limits', async () => {
+    const { data: timeline } = await agent.api.app.bsky.feed.getTimeline(
+      { limit: 10 },
+      { headers: await network.serviceHeaders(alice) },
+    )
+    const { data: timelineLimit1 } = await agent.api.app.bsky.feed.getTimeline(
+      { limit: 1 },
+      { headers: await network.serviceHeaders(alice) },
+    )
+    expect(timeline.feed.length).toBeGreaterThan(1)
+    expect(timelineLimit1.feed.length).toEqual(1)
+    expect(timelineLimit1.feed[0].post.uri).toBe(timeline.feed[0].post.uri)
+  })
+
   it('reflects self-labels', async () => {
     const carolTL = await agent.api.app.bsky.feed.getTimeline(
       {},