-- Make a feed page cost a page, not the whole event.
--
-- The previous definition (016) computed like_count/comment_count with LEFT JOINs and a
-- GROUP BY. Postgres CAN push the `event_id = $1` qual and the keyset predicate through the
-- view — verified with EXPLAIN, it uses idx_upload_event_created_id — but it CANNOT push
-- ORDER BY ... LIMIT across a GroupAggregate. So every feed request aggregated every upload in
-- the event (times its likes and comments) and only then sorted and took 21 rows. The cost
-- grew with the event, not with the page, and page 1 — the most expensive one — is exactly
-- what refreshFeedInPlace refetches on every completed upload, from every open feed in the
-- venue.
--
-- Correlated scalar subqueries move the counts ABOVE the Limit in the plan: they are evaluated
-- once per returned row, so 21 index lookups instead of a full aggregation.
--
-- The rewrite is EXACTLY equivalent, not merely close:
--   * "like" is keyed (upload_id, user_id), so COUNT(DISTINCT l.user_id) == count(*).
--   * comment.id is the primary key, so COUNT(DISTINCT c.id) == count(*).
--   * one row per upload either way — the GROUP BY was on u.id.
-- Column names, order and types are unchanged (count(*) and COUNT(DISTINCT ...) are both
-- bigint), so no Rust code changes.
--
-- No new index needed: idx_like_upload plus the (upload_id, user_id) PK serve the like
-- subquery, and idx_comment_upload ... WHERE deleted_at IS NULL matches the comment
-- subquery's predicate exactly.
--
-- One thing a future editor needs to know: the hashtag-filtered feed joins upload_hashtag
-- against this view. That was safe before only because the GROUP BY collapsed the join
-- fan-out; it is safe now because the view is one row per upload and upload_hashtag is keyed
-- (upload_id, hashtag_id) with a single tag filtered. Adding a second tag filter would need
-- fresh thought.

-- Not CASCADE: if something ever comes to depend on this view, the migration should fail
-- loudly rather than silently drop it.
DROP VIEW IF EXISTS v_feed;
CREATE VIEW v_feed AS
SELECT
    u.id,
    u.event_id,
    u.user_id,
    usr.display_name AS uploader_name,
    usr.is_banned,
    usr.uploads_hidden,
    u.preview_path,
    u.thumbnail_path,
    u.display_path,
    u.mime_type,
    u.caption,
    u.created_at,
    (SELECT count(*) FROM "like" l WHERE l.upload_id = u.id)                           AS like_count,
    (SELECT count(*) FROM comment c WHERE c.upload_id = u.id AND c.deleted_at IS NULL) AS comment_count
FROM upload u
JOIN "user" usr ON u.user_id = usr.id
WHERE u.deleted_at IS NULL
  AND usr.uploads_hidden = FALSE
  AND usr.is_banned = FALSE;