Achievement Rarity

The gallery at /achievements shows each achievement's category, points, description, progress and unlock date (achievements). What it cannot say is whether an unlock is special: "First post" and a hidden clicker milestone look the same once earned. Steam's answer is the global achievement percentage — the share of a game's players who have each achievement, listed from most common to rarest — and it is the number people quote when they show an achievement off.

What it adds

• One grouped read. achievement.readAchievementRarities returns, per achievement, the count of users with a non-null unlockedAt over the count of users, in one GROUP BY achievementId over userAchievements plus one count() over users — the same shape as readPointsLeaderboard's aggregate, and cheap enough to run on the page's load with no stored counter to keep in step.
• On the card, under the points: "n% of users", with a Rare mark in the warning colour under a threshold named as a constant beside the other achievement constants. A hidden achievement a viewer has not unlocked shows its rarity too, as Steam does — it says how hard it is without saying what it is.
• Sort. The gallery gains a sort — Category (today's grouping, the default) and Rarest first.
• The leaderboard is unchanged: points already weight hard achievements, and rarity is a per-achievement fact.

The percentage is over every registered user, not over users of the product the achievement belongs to; a per-product denominator needs a notion of "has used the clicker" the schema does not keep, and a whole-platform share is still an honest reading of rarity here.

What is deliberately not in it

• No stored rarity column. A count over an indexed table on a page view is cheaper than a counter every unlock has to maintain transactionally.
• No rarity in the unlock toast. The toast fires inside the mutation that unlocked it; adding a count to that path puts an aggregate on every trigger.

Key files

Sources

• Steamworks — Stats and achievements — global achievement statistics: the percentage of a game's players with each achievement, ordered common to rarest, and hidden achievements kept off a profile until earned.