diff --git a/src/lib/measurements/daily-series-read.ts b/src/lib/measurements/daily-series-read.ts
index 8c20993ab..0ce67d26f 100644
--- a/src/lib/measurements/daily-series-read.ts
+++ b/src/lib/measurements/daily-series-read.ts
@@ -121,8 +121,8 @@ export async function readDailySeries(opts: {
   // every bucket's UTC-midnight start read as the previous local day west of
   // UTC. The 7-day chart buckets raw rows by the local day, so switching
   // ranges moved daily totals between neighbouring days. The live fold is
-  // one indexed pass per window (about 0.1 s for 90 days of per-minute
-  // samples) and returns at most one row per day.
+  // one pass per window (about 0.3 s for a year of per-minute samples,
+  // measured against Postgres 16) and returns at most one row per day.
   const timeZone = opts.timeZone ?? (await resolveUserTimezone(userId));
   const rows = await readLiveBuckets({
     userId,
@@ -185,24 +185,31 @@ export async function readLiveBuckets(opts: {
   // `TRUNC_UNIT` map keyed by the Zod-constrained grain, never from input.
   const unit = Prisma.raw(`'${TRUNC_UNIT[grain]}'`);
   // Cumulative types SUM per bucket, every other type averages. The type list
-  // is the closed code constant, spliced as literals; `s."type"` is a GROUP BY
+  // is the closed code constant, spliced as literals; `m."type"` is a GROUP BY
   // column, so choosing the aggregate per group is legal.
   const cumulativeList = [...CUMULATIVE_HK_TYPES]
     .map((t) => `'${t}'`)
     .join(",");
   const aggregator = Prisma.raw(
-    `(CASE WHEN s."type"::text IN (${cumulativeList}) THEN SUM(s."value") ELSE AVG(s."value") END)::double precision`,
+    `(CASE WHEN m."type"::text IN (${cumulativeList}) THEN SUM(m."value") ELSE AVG(m."value") END)::double precision`,
   );
   const rankRaw = Prisma.raw(
-    buildSourceRankCase(priorityJson, 'm."type"', 'm."source"'),
+    buildSourceRankCase(priorityJson, 'p."type"', 'p."source"'),
   );
   const typeFilter = type
     ? Prisma.sql`AND m."type" = ${type}::measurement_type`
     : Prisma.empty;
   // measured_at is a UTC wall-clock timestamp: pin it to UTC, re-read it in
-  // the user's zone and truncate there. The bucket is computed once in `src`
-  // and referenced by name afterwards, so the zone parameter never has to
-  // match itself across SELECT and GROUP BY.
+  // the user's zone and truncate there.
+  //
+  // Fold first, pick second. Every source is folded per bucket in one hashed
+  // pass (a few rows per day, however dense the stream), and only then does
+  // each bucket keep its ladder-canonical source. The fold of the canonical
+  // source's rows is exactly the fold of that source's group, so the result
+  // is the same as collapsing the rows first; what changes is the cost. A
+  // year of per-minute heart rate is half a million rows, and ranking them
+  // before the fold sorted them twice (about 0.55 s); folding first never
+  // sorts more than one row per source per day.
   const buckets = await prisma.$queryRaw<
     Array<{
       type: string;
@@ -213,41 +220,38 @@ export async function readLiveBuckets(opts: {
       max_value: number | null;
     }>
   >`
-    WITH src AS (
+    WITH per_source AS (
       SELECT
         m."type",
         m."source",
-        m."value",
-        date_trunc(${unit}, (m."measured_at" AT TIME ZONE 'UTC') AT TIME ZONE ${timeZone}) AS d
+        date_trunc(${unit}, (m."measured_at" AT TIME ZONE 'UTC') AT TIME ZONE ${timeZone}) AS d,
+        ${aggregator} AS avg,
+        COUNT(*)::int AS cnt,
+        MIN(m."value") AS min_value,
+        MAX(m."value") AS max_value
       FROM measurements m
       WHERE m."user_id" = ${userId}
         AND m."measured_at" >= ${from}
         AND m."measured_at" <= ${to}
         AND m."deleted_at" IS NULL
         ${typeFilter}
+      GROUP BY m."type", m."source", 3
     ),
     canon AS (
-      SELECT DISTINCT ON (m."type", m.d)
-        m."type"   AS t,
-        m.d        AS d,
-        m."source" AS canon
-      FROM src m
-      ORDER BY m."type", m.d, (${rankRaw}), m."source"
+      SELECT DISTINCT ON (p."type", p.d)
+        p."type", p.d, p.avg, p.cnt, p.min_value, p.max_value
+      FROM per_source p
+      ORDER BY p."type", p.d, (${rankRaw}), p."source"
     )
     SELECT
-      s."type"::text AS type,
-      s.d AT TIME ZONE ${timeZone} AS bucket_start,
-      ${aggregator} AS avg,
-      COUNT(*)::int AS cnt,
-      MIN(s."value") AS min_value,
-      MAX(s."value") AS max_value
-    FROM src s
-    JOIN canon c
-      ON c.t = s."type"
-      AND c.d = s.d
-      AND c.canon = s."source"
-    GROUP BY s."type", s.d
-    ORDER BY s.d ASC
+      c."type"::text AS type,
+      c.d AT TIME ZONE ${timeZone} AS bucket_start,
+      c.avg,
+      c.cnt,
+      c.min_value,
+      c.max_value
+    FROM canon c
+    ORDER BY c.d ASC
     LIMIT ${cap}
   `;
   return buckets.map((b) => {
diff --git a/tests/integration/daily-series-user-tz.test.ts b/tests/integration/daily-series-user-tz.test.ts
index e96270b81..4ae147839 100644
--- a/tests/integration/daily-series-user-tz.test.ts
+++ b/tests/integration/daily-series-user-tz.test.ts
@@ -86,6 +86,12 @@ function byLocalDay(
 
 const EXPECTED = { "2026-09-10": 1100, "2026-09-11": 10 };
 
+/** The ladder puts Withings first for pulse, so its two readings are the day. */
+const CANON_EXPECTED = [
+  { day: "2026-09-10", value: 95, count: 2, min: 90, max: 100 },
+  { day: "2026-09-11", value: 64, count: 1, min: 64, max: 64 },
+];
+
 describe("daily series buckets by the user's calendar day (#1026)", () => {
   for (const tz of ["Europe/Berlin", "Asia/Kolkata", "America/New_York"]) {
     it(`chart daily read agrees with the local day (${tz})`, async () => {
@@ -124,4 +130,58 @@ describe("daily series buckets by the user's calendar day (#1026)", () => {
       expect(byLocalDay(rows, tz)).toEqual(EXPECTED);
     });
   }
+
+  it("collapses a day to its canonical source before folding it", async () => {
+    // Two sources on one day: only the ladder's pick is folded, and the
+    // mean, count and spread all come from that one source.
+    const prisma = getPrismaClient();
+    const user = await prisma.user.create({
+      data: {
+        username: "tz-canon",
+        email: "tz-canon@example.test",
+        role: "USER",
+        timezone: "Europe/Berlin",
+      },
+    });
+    const at = (h: number) => localHmAsUtc(DAY, "Europe/Berlin", h, 0);
+    await prisma.measurement.createMany({
+      data: [
+        { value: 60, source: "APPLE_HEALTH" as const, measuredAt: at(8) },
+        { value: 70, source: "APPLE_HEALTH" as const, measuredAt: at(9) },
+        { value: 90, source: "WITHINGS" as const, measuredAt: at(10) },
+        { value: 100, source: "WITHINGS" as const, measuredAt: at(12) },
+        { value: 50, source: "MANUAL" as const, measuredAt: at(11) },
+        // The next day has one source only.
+        {
+          value: 64,
+          source: "WITHINGS" as const,
+          measuredAt: localHmAsUtc(NEXT_DAY, "Europe/Berlin", 7, 0),
+        },
+      ].map((r) => ({
+        ...r,
+        userId: user.id,
+        type: "PULSE" as const,
+        unit: "bpm",
+      })),
+    });
+    const rows = await readLiveBuckets({
+      userId: user.id,
+      type: "PULSE",
+      from: new Date("2026-09-05T00:00:00Z"),
+      to: new Date("2026-09-15T00:00:00Z"),
+      cap: 366,
+      priorityJson: null,
+      grain: "daily",
+      timeZone: "Europe/Berlin",
+    });
+    expect(
+      rows.map((r) => ({
+        day: userDayKey(new Date(r.measuredAt), "Europe/Berlin"),
+        value: r.value,
+        count: r.count,
+        min: r.minValue,
+        max: r.maxValue,
+      })),
+    ).toEqual(CANON_EXPECTED);
+  });
 });