-- Post likes.
--
-- `post_likes` is the membership table (one row per user-per-post like),
-- written synchronously so a viewer's like state and the "liked by" list are
-- immediately consistent. The UNIQUE(post_id, user_id) makes liking idempotent.
--
-- `posts.likes_count` is a denormalized counter that is NOT updated on every
-- like (that hot-row UPDATE doesn't scale for popular posts). Instead each
-- like/unlike nudges a per-post delta in Redis, and a background worker folds
-- the deltas into this column every 15 minutes. Reads return
-- likes_count + the still-pending Redis delta, so the displayed total is
-- always current.
ALTER TABLE posts ADD COLUMN IF NOT EXISTS likes_count INTEGER NOT NULL DEFAULT 0;

CREATE TABLE IF NOT EXISTS post_likes (
    id         UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
    post_id    UUID         NOT NULL REFERENCES posts(id) ON DELETE CASCADE,
    user_id    UUID         NOT NULL REFERENCES users(id) ON DELETE CASCADE,
    created_at TIMESTAMPTZ  NOT NULL DEFAULT now(),
    UNIQUE (post_id, user_id)
);

-- Listing "who liked this post", newest first.
CREATE INDEX IF NOT EXISTS post_likes_post_idx ON post_likes (post_id, created_at DESC);
-- "Have I liked these posts?" — viewer-state hydration across a feed.
CREATE INDEX IF NOT EXISTS post_likes_user_idx ON post_likes (user_id);