perf(compare): fetch only on school-set changes; use SSR payload; parallel page fetches

- Metric changes no longer refire /api/compare (the data is already
  client-side; the picker is presentational) — the fetch effect depends
  only on the URN set, with URL sync split into its own effect.
- The initial client fetch is skipped when the SSR payload already covers
  the selected schools; national averages + benchmarks now arrive via SSR
  props so nothing is lost by skipping.
- page.tsx fetches comparison and metrics in parallel.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/app/compare/page.tsx

@@ -32,26 +32,24 @@ export default async function ComparePage({ searchParams }: ComparePageProps) {
   const selectedMetric = metricParam || 'rwm_expected_pct';
 
   try {
-    // Fetch comparison data if URNs provided
-    let comparisonData = null;
-    if (urns.length > 0) {
-      try {
-        const response = await fetchComparison(urnsParam!);
-        comparisonData = response.comparison;
-      } catch (error) {
+    // Fetch comparison + metrics in parallel — they are independent.
+    const [comparisonResponse, metricsResponse] = await Promise.all([
+      urns.length > 0
+        ? fetchComparison(urnsParam!).catch((error) => {
             console.error('Failed to fetch comparison:', error);
-      }
-    }
+            return null;
+          })
+        : Promise.resolve(null),
+      fetchMetrics(),
+    ]);
 
-    // Fetch available metrics
-    const metricsResponse = await fetchMetrics();
-
-    // Metrics is already an array
     const metricsArray = metricsResponse?.metrics || [];
 
     return (
       <ComparisonView
-        initialData={comparisonData}
+        initialData={comparisonResponse?.comparison ?? null}
+        initialNationalAverages={comparisonResponse?.national_averages}
+        initialBenchmarks={comparisonResponse?.benchmarks}
         initialUrns={urns}
         metrics={metricsArray}
         selectedMetric={selectedMetric}

nextjs-app/components/ComparisonView.tsx

@@ -35,6 +35,8 @@ import styles from './ComparisonView.module.css';
 
 interface ComparisonViewProps {
   initialData: Record<string, ComparisonData> | null;
+  initialNationalAverages?: NationalAverages;
+  initialBenchmarks?: Benchmarks;
   initialUrns: number[];
   metrics: MetricDefinition[];
   selectedMetric: string;
@@ -42,6 +44,8 @@ interface ComparisonViewProps {
 
 export function ComparisonView({
   initialData,
+  initialNationalAverages,
+  initialBenchmarks,
   initialUrns,
   metrics,
   selectedMetric: initialMetric,
@@ -54,8 +58,10 @@ export function ComparisonView({
   const [selectedMetric, setSelectedMetric] = useState(initialMetric);
   const [isModalOpen, setIsModalOpen] = useState(false);
   const [comparisonData, setComparisonData] = useState(initialData);
-  const [nationalAverages, setNationalAverages] = useState<NationalAverages | undefined>();
-  const [benchmarks, setBenchmarks] = useState<Benchmarks | undefined>();
+  const [nationalAverages, setNationalAverages] = useState<NationalAverages | undefined>(
+    initialNationalAverages,
+  );
+  const [benchmarks, setBenchmarks] = useState<Benchmarks | undefined>(initialBenchmarks);
   const [shareConfirm, setShareConfirm] = useState(false);
   const [comparePhase, setComparePhase] = useState<'primary' | 'secondary'>('primary');
   // Tracks whether the user has explicitly clicked a phase tab.
@@ -81,13 +87,16 @@ export function ComparisonView({
     }
   }, [isInitialized]); // eslint-disable-line react-hooks/exhaustive-deps
 
-  // Sync URL with selected schools + metric, and (re)fetch the comparison.
+  const urnKey = selectedSchools.map((s) => s.urn).join(',');
+
+  // Sync the URL with the selection + metric. Pure navigation state — no
+  // fetching here: metric changes are presentational (the data is already
+  // client-side) and must not refire the comparison request.
   useEffect(() => {
-    const urns = selectedSchools.map((s) => s.urn).join(',');
     const params = new URLSearchParams(searchParams);
 
-    if (urns) {
-      params.set('urns', urns);
+    if (urnKey) {
+      params.set('urns', urnKey);
     } else {
       params.delete('urns');
     }
@@ -96,9 +105,28 @@ export function ComparisonView({
 
     const newUrl = `${pathname}?${params.toString()}`;
     router.replace(newUrl, { scroll: false });
+  }, [urnKey, selectedMetric, pathname, searchParams, router]);
 
-    if (selectedSchools.length > 0) {
-      fetchComparison(urns, { cache: 'no-store' })
+  // Fetch only when the school set changes. The very first run is skipped
+  // when the SSR payload already covers the current set — no double-fetch
+  // of data the server just rendered.
+  const firstFetchRef = useRef(true);
+  useEffect(() => {
+    if (!urnKey) {
+      setComparisonData(null);
+      setNationalAverages(undefined);
+      setBenchmarks(undefined);
+      return;
+    }
+
+    if (firstFetchRef.current) {
+      firstFetchRef.current = false;
+      const ssrUrns = new Set(Object.keys(initialData ?? {}));
+      const covered = urnKey.split(',').every((urn) => ssrUrns.has(urn));
+      if (covered && ssrUrns.size > 0) return;
+    }
+
+    fetchComparison(urnKey, { cache: 'no-store' })
       .then((data) => {
         setComparisonData(data.comparison);
         setNationalAverages(data.national_averages);
@@ -110,12 +138,8 @@ export function ComparisonView({
         // destroy a working comparison the user is looking at.
         console.error('Failed to fetch comparison:', err);
       });
-    } else {
-      setComparisonData(null);
-      setNationalAverages(undefined);
-      setBenchmarks(undefined);
-    }
-  }, [selectedSchools, selectedMetric, pathname, searchParams, router]);
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [urnKey]);
 
   // Classify schools by phase using comparison data
   const classifySchool = (school: School): 'primary' | 'secondary' => {