Optimize usage dashboard: partial indexes, request cache, parallel histograms

- Add migration 14: partial covering indexes WHERE job_state='running'
  for user/project/subcluster groupings (tiny B-tree vs full table)
- Inline literal state value in BuildWhereClause so SQLite matches
  partial indexes instead of parameterized placeholders
- Add per-request statsGroupCache (sync.Once per filter+groupBy key)
  so identical grouped stats queries execute only once per GQL operation
- Parallelize 4 histogram queries in AddHistograms using errgroup
- Consolidate frontend from 6 GQL aliases to 2, sort+slice top-10
  client-side via $derived

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Entire-Checkpoint: 5b26a6e5ff10

internal/graph/stats_cache.go

@@ -0,0 +1,135 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package graph
+
+import (
+	"context"
+	"fmt"
+	"slices"
+	"sync"
+
+	"github.com/ClusterCockpit/cc-backend/internal/graph/model"
+)
+
+// statsGroupCache is a per-request cache for grouped JobsStatistics results.
+// It deduplicates identical (filter+groupBy) SQL queries that arise when the
+// frontend requests multiple sort/page slices of the same underlying data
+// (e.g. topUserJobs, topUserNodes, topUserAccs all group by USER).
+type statsGroupCache struct {
+	mu      sync.Mutex
+	entries map[string]*cacheEntry
+}
+
+type cacheEntry struct {
+	once   sync.Once
+	result []*model.JobsStatistics
+	err    error
+}
+
+type ctxKey int
+
+const statsGroupCacheKey ctxKey = iota
+
+// newStatsGroupCache creates a new empty cache.
+func newStatsGroupCache() *statsGroupCache {
+	return &statsGroupCache{
+		entries: make(map[string]*cacheEntry),
+	}
+}
+
+// WithStatsGroupCache injects a new cache into the context.
+func WithStatsGroupCache(ctx context.Context) context.Context {
+	return context.WithValue(ctx, statsGroupCacheKey, newStatsGroupCache())
+}
+
+// getStatsGroupCache retrieves the cache from context, or nil if absent.
+func getStatsGroupCache(ctx context.Context) *statsGroupCache {
+	if c, ok := ctx.Value(statsGroupCacheKey).(*statsGroupCache); ok {
+		return c
+	}
+	return nil
+}
+
+// cacheKey builds a deterministic string key from filter + groupBy.
+func statsCacheKey(filter []*model.JobFilter, groupBy *model.Aggregate) string {
+	return fmt.Sprintf("%v|%v", filter, *groupBy)
+}
+
+// getOrCompute returns cached results for the given key, computing them on
+// first access via the provided function.
+func (c *statsGroupCache) getOrCompute(
+	key string,
+	compute func() ([]*model.JobsStatistics, error),
+) ([]*model.JobsStatistics, error) {
+	c.mu.Lock()
+	entry, ok := c.entries[key]
+	if !ok {
+		entry = &cacheEntry{}
+		c.entries[key] = entry
+	}
+	c.mu.Unlock()
+
+	entry.once.Do(func() {
+		entry.result, entry.err = compute()
+	})
+	return entry.result, entry.err
+}
+
+// sortAndPageStats sorts a copy of allStats by the given sortBy field (descending)
+// and returns the requested page slice.
+func sortAndPageStats(allStats []*model.JobsStatistics, sortBy *model.SortByAggregate, page *model.PageRequest) []*model.JobsStatistics {
+	// Work on a shallow copy so the cached slice order is not mutated.
+	sorted := make([]*model.JobsStatistics, len(allStats))
+	copy(sorted, allStats)
+
+	if sortBy != nil {
+		getter := statsFieldGetter(*sortBy)
+		slices.SortFunc(sorted, func(a, b *model.JobsStatistics) int {
+			return getter(b) - getter(a) // descending
+		})
+	}
+
+	if page != nil && page.ItemsPerPage != -1 {
+		start := (page.Page - 1) * page.ItemsPerPage
+		if start >= len(sorted) {
+			return nil
+		}
+		end := start + page.ItemsPerPage
+		if end > len(sorted) {
+			end = len(sorted)
+		}
+		sorted = sorted[start:end]
+	}
+
+	return sorted
+}
+
+// statsFieldGetter returns a function that extracts the sortable int field
+// from a JobsStatistics struct for the given sort key.
+func statsFieldGetter(sortBy model.SortByAggregate) func(*model.JobsStatistics) int {
+	switch sortBy {
+	case model.SortByAggregateTotaljobs:
+		return func(s *model.JobsStatistics) int { return s.TotalJobs }
+	case model.SortByAggregateTotalusers:
+		return func(s *model.JobsStatistics) int { return s.TotalUsers }
+	case model.SortByAggregateTotalwalltime:
+		return func(s *model.JobsStatistics) int { return s.TotalWalltime }
+	case model.SortByAggregateTotalnodes:
+		return func(s *model.JobsStatistics) int { return s.TotalNodes }
+	case model.SortByAggregateTotalnodehours:
+		return func(s *model.JobsStatistics) int { return s.TotalNodeHours }
+	case model.SortByAggregateTotalcores:
+		return func(s *model.JobsStatistics) int { return s.TotalCores }
+	case model.SortByAggregateTotalcorehours:
+		return func(s *model.JobsStatistics) int { return s.TotalCoreHours }
+	case model.SortByAggregateTotalaccs:
+		return func(s *model.JobsStatistics) int { return s.TotalAccs }
+	case model.SortByAggregateTotalacchours:
+		return func(s *model.JobsStatistics) int { return s.TotalAccHours }
+	default:
+		return func(s *model.JobsStatistics) int { return s.TotalJobs }
+	}
+}