perf(db): the channel manager and the feed sidebar read an index, not the video heap

GET /api/channels summed every subscribed channel's videos by reading each video's heap page
(681 MB on prod): 51-61 ms warm, but 2.25 s on a first open and 6.4 s under EXPLAIN once the
table had left memory. /feed/summary and /feed/years did the same (~140 ms warm, ~800 ms cold).
Migration 0095 replaces ix_videos_channel_pub_desc with the same key INCLUDING the columns
those queries read, so they are Index Only Scans over a 25 MB index. A test holds the queries
and the index's column list together.

The admin Configuration page did a db.get per setting (two for an unset one): it now reads
every override in one query (~40 ms of ~55 on prod).

backend/app/models.py

@@ -330,10 +330,12 @@ class Video(Base, TimestampMixin):
     __tablename__ = "videos"
 
     id: Mapped[str] = mapped_column(String(16), primary_key=True)  # video id
-    # No standalone channel_id index: the composite `ix_videos_channel_pub_desc`
-    # (channel_id, published_at DESC NULLS LAST, id) from migration 0067 leads with channel_id, so it
-    # serves every channel_id lookup (FK cascade, sync upsert, channel-detail) AND the per-channel feed
-    # sort. Like the search_vector GIN, this composite is migration-defined, not an inline `index=True`.
+    # No standalone channel_id index: the composite `ix_videos_channel_pub_cover`
+    # (channel_id, published_at DESC NULLS LAST, id) from migration 0095 leads with channel_id, so it
+    # serves every channel_id lookup (FK cascade, sync upsert, channel-detail) and the per-channel feed
+    # sort. Its INCLUDEd columns make the channel manager's sums and the feed sidebar's counts Index Only
+    # Scans (no heap): a column those queries start reading belongs in it (test_channel_list_index_only).
+    # Like the search_vector GIN, this composite is migration-defined, not an inline `index=True`.
     channel_id: Mapped[str] = mapped_column(ForeignKey("channels.id", ondelete="CASCADE"))
     title: Mapped[str | None] = mapped_column(Text)  # normalized for display (see app.titles)
     original_title: Mapped[str | None] = mapped_column(Text)  # raw YouTube title, preserved

backend/app/routes/channels.py

@@ -6,7 +6,7 @@ from datetime import datetime, timezone
 
 from fastapi import APIRouter, BackgroundTasks, Depends
 from pydantic import BaseModel
-from sqlalchemy import func, select, update
+from sqlalchemy import Select, func, select, update
 from sqlalchemy.orm import Session
 
 from app.collation import collated, sorted_words
@@ -40,6 +40,22 @@ router = APIRouter(prefix="/api/channels", tags=["channels"])
 log = logs.get_logger(__name__)
 
 
+def channel_aggregates(channel_ids: list[str]) -> Select:
+    """Per-channel stored count, last upload and total duration, in one grouped pass. Every column it
+    reads is in `ix_videos_channel_pub_cover` (migration 0095), so Postgres answers it from the index
+    alone; a test holds that (test_channel_list_index_only)."""
+    return (
+        select(
+            Video.channel_id,
+            func.count(Video.id),
+            func.max(Video.published_at),
+            func.coalesce(func.sum(Video.duration_seconds), 0),
+        )
+        .where(Video.channel_id.in_(channel_ids))
+        .group_by(Video.channel_id)
+    )
+
+
 @router.get("")
 def list_channels(user: User = Depends(current_user), db: Session = Depends(get_db)) -> list[dict]:
     rows = db.execute(
@@ -49,24 +65,15 @@ def list_channels(user: User = Depends(current_user), db: Session = Depends(get_
         .order_by(Subscription.priority.desc(), collated(Channel.title, lang_for_user(user)))
     ).all()
 
-    # Per-channel aggregates over the shared catalog in ONE grouped pass: stored count, last
-    # upload, total duration. Cheap — a single grouped scan, so computed on read rather than
-    # denormalised (measure-first: reads aren't frequent/hot enough to warrant cached columns).
+    # Per-channel aggregates over the shared catalog (channel_aggregates), computed on read rather than
+    # denormalised: an Index Only Scan over a ~20 MB index, where it used to read every video's heap page
+    # (681 MB on prod, a 2–6 s first open once something else had pushed it out of memory, E31 S4d).
     # (The Normal/Shorts/Live breakdown moved to the channel-detail page in R14, so it is no longer
     # computed here — the manager table dropped its N/S/L column.)
     channel_ids = [c.id for _, c in rows]
     agg: dict[str, dict] = {}
     if channel_ids:
-        for cid, n, last_up, total_dur in db.execute(
-            select(
-                Video.channel_id,
-                func.count(Video.id),
-                func.max(Video.published_at),
-                func.coalesce(func.sum(Video.duration_seconds), 0),
-            )
-            .where(Video.channel_id.in_(channel_ids))
-            .group_by(Video.channel_id)
-        ).all():
+        for cid, n, last_up, total_dur in db.execute(channel_aggregates(channel_ids)).all():
             agg[cid] = {
                 "stored": n,
                 "last_video_at": iso(last_up),

backend/app/sysconfig.py

@@ -10,6 +10,7 @@ stored in the DB and stay env-only).
 
 from dataclasses import dataclass
 
+from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app import logs, security
@@ -307,7 +308,11 @@ def _raw_override(db: Session, key: str) -> str | None:
     per key so the operator gets a real signal without a per-request traceback. Recovery is documented in
     docs/self-hosting.md: the key must never change once secrets are stored. (B-core-2)
     """
-    row = db.get(SystemConfig, key)
+    return _raw_of(db.get(SystemConfig, key), key)
+
+
+def _raw_of(row: SystemConfig | None, key: str) -> str | None:
+    """`_raw_override` over a row already read (describe() reads them all in one query)."""
     if row is None or row.value is None:
         return None
     if not row.encrypted:
@@ -327,8 +332,11 @@ def _raw_override(db: Session, key: str) -> str | None:
 
 def get(db: Session, key: str):
     """Effective value: the DB override (typed) if set, else the env/config default."""
-    s = _BY_KEY[key]
-    raw = _raw_override(db, key)
+    return _effective(_BY_KEY[key], _raw_override(db, key))
+
+
+def _effective(s: ConfigSpec, raw: str | None):
+    """`get` over an override already read."""
     if raw is None:
         return _default(s)
     if raw == "" and not s.allow_empty:
@@ -420,8 +428,12 @@ def describe(db: Session) -> dict:
     """Admin-UI metadata + current values, grouped. Secret values are never echoed —
     only whether an override is set and whether an env default exists."""
     groups: dict[str, list[dict]] = {}
+    # Every override in one query: a db.get per key was one round trip each (two for a key with no
+    # row, which the session cannot cache), ~40 ms of the page's ~55 on prod (E31 S4d).
+    rows = {r.key: r for r in db.scalars(select(SystemConfig))}
     for s in SPECS:
-        is_set = db.get(SystemConfig, s.key) is not None
+        row = rows.get(s.key)
+        is_set = row is not None
         item: dict = {
             "key": s.key,
             "type": s.type,
@@ -436,7 +448,7 @@ def describe(db: Session) -> dict:
         if s.secret:
             item["default_is_set"] = bool(_default(s))
         else:
-            item["value"] = get(db, s.key)
+            item["value"] = _effective(s, _raw_of(row, s.key))
             item["default"] = _default(s)
         groups.setdefault(s.group, []).append(item)
     sections = [