TABLESAMPLE SYSTEM_ROWS reads CONTIGUOUS rows from each sampled page, so
sequentially-imported near-duplicates (multi-image posts, variant sets) came
back adjacent and clustered in the showcase ("three near-identical in a row").
Sample limit*5 rows (spanning more pages) then ORDER BY random() before taking
limit — breaks the physical adjacency for much better spread, still cheap
(random() over a few hundred rows, not the whole table).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
The showcase/gallery/artist/series/post-feed APIs were constructing
thumbnail URLs from (sha256, mime). The MIME-based extension predicate
("png if image/png or image/gif else jpg") DISAGREED with the
thumbnailer's actual on-disk extension predicate ("png if alpha else
jpg"). Result: every PNG source without transparency 404'd (URL asked
.png, disk had .jpg); every WebP/AVIF source with transparency 404'd
(URL asked .jpg, disk had .png) — despite the thumbnail file existing
on disk.
The backfill task couldn't catch these because backfill checks the
ACTUAL thumbnail_path stored on the record (correct), not the URL the
browser fetches (broken derivation). So records with valid on-disk
thumbnails kept showing as broken in the UI no matter how many times
backfill ran.
Operator-flagged 2026-05-30: "the generate thumbnails function appears
to not catch all of the failed thumbnail cases" — turned out to not be
a backfill bug at all.
Fix: thumbnail_url now takes (thumbnail_path, sha256, mime) and returns
the stored path verbatim — Quart serves /images/* 1:1 from the volume
(frontend.py:20-36), so the URL IS the disk path. Falls back to the old
sha256+mime derivation only when thumbnail_path is NULL (thumbnailer
hasn't run yet); that URL will 404 in the browser until backfill catches
it, same as before the path was tracked.
All 8 callers updated: showcase_service, gallery_service (2 sites),
artist_service, series_service, post_feed_service, tag_directory_service,
artist_directory_service. The four sites whose query was raw-tuple now
also SELECT ImageRecord.thumbnail_path.
Net effect: every record that has a valid on-disk thumbnail will now
render correctly, regardless of which extension the thumbnailer chose,
without any DB migration or backfill rerun needed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen