func PostingsForMatchers()

in tsdb/querier.go [238:358]
93 lines of code
31 McCabe index (conditional complexity)

func PostingsForMatchers(ix IndexReader, ms ...*labels.Matcher) (index.Postings, error) {
	var its, notIts []index.Postings
	// See which label must be non-empty.
	// Optimization for case like {l=~".", l!="1"}.
	labelMustBeSet := make(map[string]bool, len(ms))
	for _, m := range ms {
		if !m.Matches("") {
			labelMustBeSet[m.Name] = true
		}
	}
	isSubtractingMatcher := func(m *labels.Matcher) bool {
		if !labelMustBeSet[m.Name] {
			return true
		}
		return (m.Type == labels.MatchNotEqual || m.Type == labels.MatchNotRegexp) && m.Matches("")
	}
	someSubtractingMatchers, someIntersectingMatchers := false, false
	for _, m := range ms {
		if isSubtractingMatcher(m) {
			someSubtractingMatchers = true
		} else {
			someIntersectingMatchers = true
		}
	}

	if someSubtractingMatchers && !someIntersectingMatchers {
		// If there's nothing to subtract from, add in everything and remove the notIts later.
		// We prefer to get AllPostings so that the base of subtraction (i.e. allPostings)
		// doesn't include series that may be added to the index reader during this function call.
		k, v := index.AllPostingsKey()
		allPostings, err := ix.Postings(k, v)
		if err != nil {
			return nil, err
		}
		its = append(its, allPostings)
	}

	// Sort matchers to have the intersecting matchers first.
	// This way the base for subtraction is smaller and
	// there is no chance that the set we subtract from
	// contains postings of series that didn't exist when
	// we constructed the set we subtract by.
	slices.SortStableFunc(ms, func(i, j *labels.Matcher) bool {
		return !isSubtractingMatcher(i) && isSubtractingMatcher(j)
	})

	for _, m := range ms {
		switch {
		case m.Name == "" && m.Value == "": // Special-case for AllPostings, used in tests at least.
			k, v := index.AllPostingsKey()
			allPostings, err := ix.Postings(k, v)
			if err != nil {
				return nil, err
			}
			its = append(its, allPostings)
		case labelMustBeSet[m.Name]:
			// If this matcher must be non-empty, we can be smarter.
			matchesEmpty := m.Matches("")
			isNot := m.Type == labels.MatchNotEqual || m.Type == labels.MatchNotRegexp
			switch {
			case isNot && matchesEmpty: // l!="foo"
				// If the label can't be empty and is a Not and the inner matcher
				// doesn't match empty, then subtract it out at the end.
				inverse, err := m.Inverse()
				if err != nil {
					return nil, err
				}

				it, err := postingsForMatcher(ix, inverse)
				if err != nil {
					return nil, err
				}
				notIts = append(notIts, it)
			case isNot && !matchesEmpty: // l!=""
				// If the label can't be empty and is a Not, but the inner matcher can
				// be empty we need to use inversePostingsForMatcher.
				inverse, err := m.Inverse()
				if err != nil {
					return nil, err
				}

				it, err := inversePostingsForMatcher(ix, inverse)
				if err != nil {
					return nil, err
				}
				if index.IsEmptyPostingsType(it) {
					return index.EmptyPostings(), nil
				}
				its = append(its, it)
			default: // l="a"
				// Non-Not matcher, use normal postingsForMatcher.
				it, err := postingsForMatcher(ix, m)
				if err != nil {
					return nil, err
				}
				if index.IsEmptyPostingsType(it) {
					return index.EmptyPostings(), nil
				}
				its = append(its, it)
			}
		default: // l=""
			// If the matchers for a labelname selects an empty value, it selects all
			// the series which don't have the label name set too. See:
			// https://github.com/prometheus/prometheus/issues/3575 and
			// https://github.com/prometheus/prometheus/pull/3578#issuecomment-351653555
			it, err := inversePostingsForMatcher(ix, m)
			if err != nil {
				return nil, err
			}
			notIts = append(notIts, it)
		}
	}

	it := index.Intersect(its...)

	for _, n := range notIts {
		it = index.Without(it, n)
	}

	return it, nil
}