Fix hot_rank algorithm pushing downvoted content off the feed. (#4085)

* Fix hot_rank algorithm pushing downvoted content off the feed.

- Max hot_rank algorithm now uses max(2, score) rather than greatest(1, score + 3)
- Fixes #4084

* Fixing SQL format.

* Adding 2 to bias the positive scores.

migrations/2023-10-23-184941_hot_rank_greatest_fix/down.sql

@@ -0,0 +1,19 @@
+CREATE OR REPLACE FUNCTION hot_rank (score numeric, published timestamp with time zone)
+    RETURNS float
+    AS $$
+DECLARE
+    hours_diff numeric := EXTRACT(EPOCH FROM (now() - published)) / 3600;
+BEGIN
+    -- 24 * 7 = 168, so after a week, it will default to 0.
+    IF (hours_diff > 0 AND hours_diff < 168) THEN
+        RETURN log(greatest (1, score + 3)) / power((hours_diff + 2), 1.8);
+    ELSE
+        -- if the post is from the future, set hot score to 0. otherwise you can game the post to
+        -- always be on top even with only 1 vote by setting it to the future
+        RETURN 0.0;
+    END IF;
+END;
+$$
+LANGUAGE plpgsql
+IMMUTABLE PARALLEL SAFE;
+

migrations/2023-10-23-184941_hot_rank_greatest_fix/up.sql

@@ -0,0 +1,25 @@
+-- The hot_rank algorithm currently uses greatest(1, score + 3)
+-- This greatest of 1 incorrect because log10(1) is zero,
+-- so it will push negative-voted comments / posts to the bottom, IE hot_rank = 0
+-- The update_scheduled_ranks will never recalculate them, because it ignores content
+-- with hot_rank = 0
+CREATE OR REPLACE FUNCTION hot_rank (score numeric, published timestamp with time zone)
+    RETURNS float
+    AS $$
+DECLARE
+    hours_diff numeric := EXTRACT(EPOCH FROM (now() - published)) / 3600;
+BEGIN
+    -- 24 * 7 = 168, so after a week, it will default to 0.
+    IF (hours_diff > 0 AND hours_diff < 168) THEN
+        -- Use greatest(2,score), so that the hot_rank will be positive and not ignored.
+        RETURN log(greatest (2, score + 2)) / power((hours_diff + 2), 1.8);
+    ELSE
+        -- if the post is from the future, set hot score to 0. otherwise you can game the post to
+        -- always be on top even with only 1 vote by setting it to the future
+        RETURN 0.0;
+    END IF;
+END;
+$$
+LANGUAGE plpgsql
+IMMUTABLE PARALLEL SAFE;
+