only rebuild chart leaderboard when ranking changes (#1671)

* only rebuild chart leaderboard when ranking changes

* document partition invariant for leaderboard refresh

Adds an explanatory comment noting that the ranking-change check assumes
an UPDATE never moves a pb between (chart_id, lens) partitions (true
today), plus a trailing newline. Keeps DISTINCT for clarity.

---------

Co-authored-by: zk <zkldi@proton.me>

db/migrations/20260615024243_only_rebuild_leaderboard_score_change.sql

@@ -0,0 +1,35 @@
+CREATE OR REPLACE FUNCTION refresh_leaderboard_stmt_after_pb_update()
+RETURNS TRIGGER
+LANGUAGE plpgsql
+SET search_path = public
+AS $$
+BEGIN
+    -- Only rebuild partitions whose leaderboard ordering actually changed. The
+    -- ordering columns mirror refresh_chart_leaderboard_partition's ORDER BY, so a
+    -- non-ranking update (e.g. rivalRank in calculated_data, highlight) leaves the
+    -- leaderboard intact and is skipped entirely. row_id is pb's PK and is never
+    -- mutated on UPDATE, so OLD and NEW transition rows pair up 1:1.
+    --
+    -- NOTE: this assumes an UPDATE never changes a row's (chart_id, lens), i.e. a
+    -- pb never moves between partitions. That holds today (updates only touch score
+    -- data). If a partition-moving UPDATE is ever introduced, this must also refresh
+    -- the partition the row left, or the old partition's cached ranks go stale.
+    PERFORM refresh_chart_leaderboard_partition (d.chart_id, d.lens)
+    FROM (
+        SELECT DISTINCT
+            n.chart_id,
+            n.lens
+        FROM pb_updated_new n
+        JOIN pb_updated_old o ON o.row_id = n.row_id
+        WHERE n.ranking_value IS DISTINCT FROM o.ranking_value
+           OR n.ranking_value_tb1 IS DISTINCT FROM o.ranking_value_tb1
+           OR n.ranking_value_tb2 IS DISTINCT FROM o.ranking_value_tb2
+           OR n.ranking_value_tb3 IS DISTINCT FROM o.ranking_value_tb3
+           OR n.ranking_value_tb4 IS DISTINCT FROM o.ranking_value_tb4
+           OR n.ranking_value_tb5 IS DISTINCT FROM o.ranking_value_tb5
+           OR n.time_achieved IS DISTINCT FROM o.time_achieved
+    ) AS d;
+
+    RETURN NULL;
+END;
+$$;