func analyzeBlock()

in tsdb/cmd/tsdb/main.go [467:605]
122 lines of code
23 McCabe index (conditional complexity)

func analyzeBlock(b tsdb.BlockReader, limit int) error {
	meta := b.Meta()
	fmt.Printf("Block ID: %s\n", meta.ULID)
	// Presume 1ms resolution that Prometheus uses.
	fmt.Printf("Duration: %s\n", (time.Duration(meta.MaxTime-meta.MinTime) * 1e6).String())
	fmt.Printf("Series: %d\n", meta.Stats.NumSeries)
	ir, err := b.Index()
	if err != nil {
		return err
	}
	defer ir.Close()

	allLabelNames, err := ir.LabelNames()
	if err != nil {
		return err
	}
	fmt.Printf("Label names: %d\n", len(allLabelNames))

	type postingInfo struct {
		key    string
		metric uint64
	}
	postingInfos := []postingInfo{}

	printInfo := func(postingInfos []postingInfo) {
		sort.Slice(postingInfos, func(i, j int) bool { return postingInfos[i].metric > postingInfos[j].metric })

		for i, pc := range postingInfos {
			fmt.Printf("%d %s\n", pc.metric, pc.key)
			if i >= limit {
				break
			}
		}
	}

	labelsUncovered := map[string]uint64{}
	labelpairsUncovered := map[string]uint64{}
	labelpairsCount := map[string]uint64{}
	entries := 0
	p, err := ir.Postings("", "") // The special all key.
	if err != nil {
		return err
	}
	lbls := labels.Labels{}
	chks := []chunks.Meta{}
	for p.Next() {
		if err = ir.Series(p.At(), &lbls, &chks); err != nil {
			return err
		}
		// Amount of the block time range not covered by this series.
		uncovered := uint64(meta.MaxTime-meta.MinTime) - uint64(chks[len(chks)-1].MaxTime-chks[0].MinTime)
		for _, lbl := range lbls {
			key := lbl.Name + "=" + lbl.Value
			labelsUncovered[lbl.Name] += uncovered
			labelpairsUncovered[key] += uncovered
			labelpairsCount[key]++
			entries++
		}
	}
	if p.Err() != nil {
		return p.Err()
	}
	fmt.Printf("Postings (unique label pairs): %d\n", len(labelpairsUncovered))
	fmt.Printf("Postings entries (total label pairs): %d\n", entries)

	postingInfos = postingInfos[:0]
	for k, m := range labelpairsUncovered {
		postingInfos = append(postingInfos, postingInfo{k, uint64(float64(m) / float64(meta.MaxTime-meta.MinTime))})
	}

	fmt.Printf("\nLabel pairs most involved in churning:\n")
	printInfo(postingInfos)

	postingInfos = postingInfos[:0]
	for k, m := range labelsUncovered {
		postingInfos = append(postingInfos, postingInfo{k, uint64(float64(m) / float64(meta.MaxTime-meta.MinTime))})
	}

	fmt.Printf("\nLabel names most involved in churning:\n")
	printInfo(postingInfos)

	postingInfos = postingInfos[:0]
	for k, m := range labelpairsCount {
		postingInfos = append(postingInfos, postingInfo{k, m})
	}

	fmt.Printf("\nMost common label pairs:\n")
	printInfo(postingInfos)

	postingInfos = postingInfos[:0]
	for _, n := range allLabelNames {
		values, err := ir.LabelValues(n)
		if err != nil {
			return err
		}
		var cumulativeLength uint64
		for _, str := range values {
			cumulativeLength += uint64(len(str))
		}
		postingInfos = append(postingInfos, postingInfo{n, cumulativeLength})
	}

	fmt.Printf("\nLabel names with highest cumulative label value length:\n")
	printInfo(postingInfos)

	postingInfos = postingInfos[:0]
	for _, n := range allLabelNames {
		lv, err := ir.LabelValues(n)
		if err != nil {
			return err
		}
		postingInfos = append(postingInfos, postingInfo{n, uint64(len(lv))})
	}
	fmt.Printf("\nHighest cardinality labels:\n")
	printInfo(postingInfos)

	postingInfos = postingInfos[:0]
	lv, err := ir.LabelValues("__name__")
	if err != nil {
		return err
	}
	for _, n := range lv {
		postings, err := ir.Postings("__name__", n)
		if err != nil {
			return err
		}
		count := 0
		for postings.Next() {
			count++
		}
		if postings.Err() != nil {
			return postings.Err()
		}
		postingInfos = append(postingInfos, postingInfo{n, uint64(count)})
	}
	fmt.Printf("\nHighest cardinality metric names:\n")
	printInfo(postingInfos)
	return nil
}