Stream symbols during compaction. (#6468)

Rather than buffer up symbols in RAM, do it one by one
during compaction. Then use the reader's symbol handling
for symbol lookups during the rest of the index write.

There is some slowdown in compaction, due to having to look through a file
rather than a hash lookup. This is noise to the overall cost of compacting
series with thousands of samples though.

benchmark                                                                                   old ns/op       new ns/op       delta
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=101-4        539917175       675341565       +25.08%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=1001-4       2441815993      2477453524      +1.46%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=2001-4       3978543559      3922909687      -1.40%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=5001-4       8430219716      8586610007      +1.86%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=101-4      1786424591      1909552782      +6.89%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=1001-4     5328998202      6020839950      +12.98%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=2001-4     10085059958     11085278690     +9.92%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=5001-4     25497010155     27018079806     +5.97%
BenchmarkCompactionFromHead/labelnames=1,labelvalues=100000-4                               2427391406      2817217987      +16.06%
BenchmarkCompactionFromHead/labelnames=10,labelvalues=10000-4                               2592965497      2538805050      -2.09%
BenchmarkCompactionFromHead/labelnames=100,labelvalues=1000-4                               2437388343      2668012858      +9.46%
BenchmarkCompactionFromHead/labelnames=1000,labelvalues=100-4                               2317095324      2787423966      +20.30%
BenchmarkCompactionFromHead/labelnames=10000,labelvalues=10-4                               2600239857      2096973860      -19.35%

benchmark                                                                                   old allocs     new allocs     delta
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=101-4        500851         470794         -6.00%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=1001-4       821527         791451         -3.66%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=2001-4       1141562        1111508        -2.63%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=5001-4       2141576        2111504        -1.40%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=101-4      871466         841424         -3.45%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=1001-4     1941428        1911415        -1.55%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=2001-4     3071573        3041510        -0.98%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=5001-4     6771648        6741509        -0.45%
BenchmarkCompactionFromHead/labelnames=1,labelvalues=100000-4                               731493         824888         +12.77%
BenchmarkCompactionFromHead/labelnames=10,labelvalues=10000-4                               793918         887311         +11.76%
BenchmarkCompactionFromHead/labelnames=100,labelvalues=1000-4                               811842         905204         +11.50%
BenchmarkCompactionFromHead/labelnames=1000,labelvalues=100-4                               832244         925081         +11.16%
BenchmarkCompactionFromHead/labelnames=10000,labelvalues=10-4                               921553         1019162        +10.59%

benchmark                                                                                   old bytes      new bytes      delta
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=101-4        40532648       35698276       -11.93%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=1001-4       60340216       53409568       -11.49%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=2001-4       81087336       72065552       -11.13%
BenchmarkCompaction/type=normal,blocks=4,series=10000,samplesPerSeriesPerBlock=5001-4       142485576      120878544      -15.16%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=101-4      208661368      203831136      -2.31%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=1001-4     347345904      340484696      -1.98%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=2001-4     585185856      576244648      -1.53%
BenchmarkCompaction/type=vertical,blocks=4,series=10000,samplesPerSeriesPerBlock=5001-4     1357641792     1358966528     +0.10%
BenchmarkCompactionFromHead/labelnames=1,labelvalues=100000-4                               126486664      119666744      -5.39%
BenchmarkCompactionFromHead/labelnames=10,labelvalues=10000-4                               122323192      115117224      -5.89%
BenchmarkCompactionFromHead/labelnames=100,labelvalues=1000-4                               126404504      119469864      -5.49%
BenchmarkCompactionFromHead/labelnames=1000,labelvalues=100-4                               119047832      112230408      -5.73%
BenchmarkCompactionFromHead/labelnames=10000,labelvalues=10-4                               136576016      116634800      -14.60%

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

tsdb/block.go

@@ -39,8 +39,8 @@ import (
 // The methods must be called in the order they are specified in.
 type IndexWriter interface {
 	// AddSymbols registers all string symbols that are encountered in series
-	// and other indices.
+	// and other indices. Symbols must be added in sorted order.
-	AddSymbols(sym map[string]struct{}) error
+	AddSymbol(sym string) error
 
 	// AddSeries populates the index writer with a series and its offsets
 	// of chunks that the index can reference.
@@ -61,9 +61,10 @@ type IndexWriter interface {
 
 // IndexReader provides reading access of serialized index data.
 type IndexReader interface {
-	// Symbols returns a set of string symbols that may occur in series' labels
+	// Symbols return an iterator over sorted string symbols that may occur in
-	// and indices.
+	// series' labels and indices. It is not safe to use the returned strings
-	Symbols() (map[string]struct{}, error)
+	// beyond the lifetime of the index reader.
+	Symbols() index.StringIter
 
 	// LabelValues returns sorted possible label values.
 	LabelValues(names ...string) (index.StringTuples, error)
@@ -432,9 +433,8 @@ type blockIndexReader struct {
 	b  *Block
 }
 
-func (r blockIndexReader) Symbols() (map[string]struct{}, error) {
+func (r blockIndexReader) Symbols() index.StringIter {
-	s, err := r.ir.Symbols()
+	return r.ir.Symbols()
-	return s, errors.Wrapf(err, "block: %s", r.b.Meta().ULID)
 }
 
 func (r blockIndexReader) LabelValues(names ...string) (index.StringTuples, error) {

tsdb/compact.go

@@ -650,7 +650,7 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 
 	var (
 		set         ChunkSeriesSet
-		allSymbols  = make(map[string]struct{}, 1<<16)
+		symbols     index.StringIter
 		closers     = []io.Closer{}
 		overlapping bool
 	)
@@ -700,14 +700,6 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		}
 		closers = append(closers, tombsr)
 
-		symbols, err := indexr.Symbols()
-		if err != nil {
-			return errors.Wrap(err, "read symbols")
-		}
-		for s := range symbols {
-			allSymbols[s] = struct{}{}
-		}
-
 		k, v := index.AllPostingsKey()
 		all, err := indexr.Postings(k, v)
 		if err != nil {
@@ -716,15 +708,18 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		all = indexr.SortedPostings(all)
 
 		s := newCompactionSeriesSet(indexr, chunkr, tombsr, all)
+		syms := indexr.Symbols()
 
 		if i == 0 {
 			set = s
+			symbols = syms
 			continue
 		}
 		set, err = newCompactionMerger(set, s)
 		if err != nil {
 			return err
 		}
+		symbols = newMergedStringIter(symbols, syms)
 	}
 
 	var (
@@ -732,8 +727,13 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		ref    = uint64(0)
 	)
 
-	if err := indexw.AddSymbols(allSymbols); err != nil {
+	for symbols.Next() {
-		return errors.Wrap(err, "add symbols")
+		if err := indexw.AddSymbol(symbols.At()); err != nil {
+			return errors.Wrap(err, "add symbol")
+		}
+	}
+	if symbols.Err() != nil {
+		return errors.Wrap(symbols.Err(), "next symbol")
 	}
 
 	delIter := &deletedIterator{}
@@ -1026,3 +1026,47 @@ func (c *compactionMerger) Err() error {
 func (c *compactionMerger) At() (labels.Labels, []chunks.Meta, tombstones.Intervals) {
 	return c.l, c.c, c.intervals
 }
+
+func newMergedStringIter(a index.StringIter, b index.StringIter) index.StringIter {
+	return &mergedStringIter{a: a, b: b, aok: a.Next(), bok: b.Next()}
+}
+
+type mergedStringIter struct {
+	a        index.StringIter
+	b        index.StringIter
+	aok, bok bool
+	cur      string
+}
+
+func (m *mergedStringIter) Next() bool {
+	if (!m.aok && !m.bok) || (m.Err() != nil) {
+		return false
+	}
+
+	if !m.aok {
+		m.cur = m.b.At()
+		m.bok = m.b.Next()
+	} else if !m.bok {
+		m.cur = m.a.At()
+		m.aok = m.a.Next()
+	} else if m.b.At() > m.a.At() {
+		m.cur = m.a.At()
+		m.aok = m.a.Next()
+	} else if m.a.At() > m.b.At() {
+		m.cur = m.b.At()
+		m.bok = m.b.Next()
+	} else { // Equal.
+		m.cur = m.b.At()
+		m.aok = m.a.Next()
+		m.bok = m.b.Next()
+	}
+
+	return true
+}
+func (m mergedStringIter) At() string { return m.cur }
+func (m mergedStringIter) Err() error {
+	if m.a.Err() != nil {
+		return m.a.Err()
+	}
+	return m.b.Err()
+}

tsdb/docs/format/index.md

@@ -126,7 +126,7 @@ After the labels, the number of indexed chunks is encoded, followed by a sequenc
 ### Label Index
 
 A label index section indexes the existing (combined) values for one or more label names.
-The `#names` field determines the number of indexed label names, followed by the total number of entries in the `#entries` field. The body holds #entries / #names tuples of symbol table references, each tuple being of #names length. The value tuples are sorted in lexicographically increasing order.
+The `#names` field determines the number of indexed label names, followed by the total number of entries in the `#entries` field. The body holds #entries / #names tuples of symbol table references, each tuple being of #names length. The value tuples are sorted in lexicographically increasing order. This is no longer used.
 
 ```
 ┌───────────────┬────────────────┬────────────────┐
@@ -181,7 +181,7 @@ The sequence of postings sections is finalized by a [postings offset table](#pos
 
 A label offset table stores a sequence of label offset entries.
 Every label offset entry holds the label name and the offset to its values in the label index section.
-They are used to track label index sections. They are read into memory when an index file is loaded.
+They are used to track label index sections. This is no longer used.
 
 ```
 ┌─────────────────────┬──────────────────────┐

tsdb/head.go

@@ -1339,16 +1339,17 @@ func (h *headIndexReader) Close() error {
 	return nil
 }
 
-func (h *headIndexReader) Symbols() (map[string]struct{}, error) {
+func (h *headIndexReader) Symbols() index.StringIter {
 	h.head.symMtx.RLock()
-	defer h.head.symMtx.RUnlock()
+	res := make([]string, 0, len(h.head.symbols))
-
-	res := make(map[string]struct{}, len(h.head.symbols))
 
 	for s := range h.head.symbols {
-		res[s] = struct{}{}
+		res = append(res, s)
 	}
-	return res, nil
+	h.head.symMtx.RUnlock()
+
+	sort.Strings(res)
+	return index.NewStringListIter(res)
 }
 
 // LabelValues returns the possible label values

tsdb/index/index.go

@@ -123,11 +123,14 @@ type Writer struct {
 	buf1 encoding.Encbuf
 	buf2 encoding.Encbuf
 
-	symbols        map[string]uint32 // symbol offsets
+	numSymbols int
-	reverseSymbols map[uint32]string
+	symbols    *Symbols
-	labelIndexes   []labelIndexHashEntry // label index offsets
+	symbolFile *fileutil.MmapFile
-	postings       []postingsHashEntry   // postings lists offsets
+	lastSymbol string
-	labelNames     map[string]uint64     // label names, and their usage
+
+	labelIndexes []labelIndexHashEntry // label index offsets
+	postings     []postingsHashEntry   // postings lists offsets
+	labelNames   map[string]uint64     // label names, and their usage
 
 	// Hold last series to validate that clients insert new series in order.
 	lastSeries labels.Labels
@@ -275,7 +278,13 @@ func (w *Writer) ensureStage(s indexWriterStage) error {
 	switch s {
 	case idxStageSymbols:
 		w.toc.Symbols = w.pos
+		if err := w.startSymbols(); err != nil {
+			return err
+		}
 	case idxStageSeries:
+		if err := w.finishSymbols(); err != nil {
+			return err
+		}
 		w.toc.Series = w.pos
 
 	case idxStageLabelIndex:
@@ -339,16 +348,16 @@ func (w *Writer) AddSeries(ref uint64, lset labels.Labels, chunks ...chunks.Meta
 
 	for _, l := range lset {
 		// here we have an index for the symbol file if v2, otherwise it's an offset
-		index, ok := w.symbols[l.Name]
+		index, err := w.symbols.ReverseLookup(l.Name)
-		if !ok {
+		if err != nil {
-			return errors.Errorf("symbol entry for %q does not exist", l.Name)
+			return errors.Errorf("symbol entry for %q does not exist, %v", l.Name, err)
 		}
 		w.labelNames[l.Name]++
 		w.buf2.PutUvarint32(index)
 
-		index, ok = w.symbols[l.Value]
+		index, err = w.symbols.ReverseLookup(l.Value)
-		if !ok {
+		if err != nil {
-			return errors.Errorf("symbol entry for %q does not exist", l.Value)
+			return errors.Errorf("symbol entry for %q does not exist, %v", l.Value, err)
 		}
 		w.buf2.PutUvarint32(index)
 	}
@@ -388,58 +397,66 @@ func (w *Writer) AddSeries(ref uint64, lset labels.Labels, chunks ...chunks.Meta
 	return nil
 }
 
-func (w *Writer) AddSymbols(sym map[string]struct{}) error {
+func (w *Writer) startSymbols() error {
+	// We are at w.toc.Symbols.
+	// Leave 4 bytes of space for the length, and another 4 for the number of symbols
+	// which will both be calculated later.
+	return w.write([]byte("alenblen"))
+}
+
+func (w *Writer) AddSymbol(sym string) error {
 	if err := w.ensureStage(idxStageSymbols); err != nil {
 		return err
 	}
-	// Generate sorted list of strings we will store as reference table.
+	if w.numSymbols != 0 && sym <= w.lastSymbol {
-	symbols := make([]string, 0, len(sym))
+		return errors.Errorf("symbol %q out-of-order", sym)
-
-	for s := range sym {
-		symbols = append(symbols, s)
 	}
-	sort.Strings(symbols)
+	w.lastSymbol = sym
-
+	w.numSymbols++
-	startPos := w.pos
-	// Leave 4 bytes of space for the length, which will be calculated later.
-	if err := w.write([]byte("alen")); err != nil {
-		return err
-	}
-	w.crc32.Reset()
-
 	w.buf1.Reset()
-	w.buf1.PutBE32int(len(symbols))
+	w.buf1.PutUvarintStr(sym)
-	w.buf1.WriteToHash(w.crc32)
-	if err := w.write(w.buf1.Get()); err != nil {
-		return err
-	}
-
-	w.symbols = make(map[string]uint32, len(symbols))
-	w.reverseSymbols = make(map[uint32]string, len(symbols))
-
-	for index, s := range symbols {
-		w.symbols[s] = uint32(index)
-		w.reverseSymbols[uint32(index)] = s
-		w.buf1.Reset()
-		w.buf1.PutUvarintStr(s)
-		w.buf1.WriteToHash(w.crc32)
-		if err := w.write(w.buf1.Get()); err != nil {
-			return err
-		}
-	}
-
-	// Write out the length.
-	w.buf1.Reset()
-	w.buf1.PutBE32int(int(w.pos - startPos - 4))
-	if err := w.writeAt(w.buf1.Get(), startPos); err != nil {
-		return err
-	}
-
-	w.buf1.Reset()
-	w.buf1.PutHashSum(w.crc32)
 	return w.write(w.buf1.Get())
 }
 
+func (w *Writer) finishSymbols() error {
+	// Write out the length and symbol count.
+	w.buf1.Reset()
+	w.buf1.PutBE32int(int(w.pos - w.toc.Symbols - 4))
+	w.buf1.PutBE32int(int(w.numSymbols))
+	if err := w.writeAt(w.buf1.Get(), w.toc.Symbols); err != nil {
+		return err
+	}
+
+	hashPos := w.pos
+	// Leave space for the hash. We can only calculate it
+	// now that the number of symbols is known, so mmap and do it from there.
+	if err := w.write([]byte("hash")); err != nil {
+		return err
+	}
+	if err := w.fbuf.Flush(); err != nil {
+		return err
+	}
+
+	var err error
+	w.symbolFile, err = fileutil.OpenMmapFile(w.f.Name())
+	if err != nil {
+		return err
+	}
+	hash := crc32.Checksum(w.symbolFile.Bytes()[w.toc.Symbols+4:hashPos], castagnoliTable)
+	w.buf1.Reset()
+	w.buf1.PutBE32(hash)
+	if err := w.writeAt(w.buf1.Get(), hashPos); err != nil {
+		return err
+	}
+
+	// Load in the symbol table efficiently for the rest of the index writing.
+	w.symbols, err = NewSymbols(realByteSlice(w.symbolFile.Bytes()), FormatV2, int(w.toc.Symbols))
+	if err != nil {
+		return errors.Wrap(err, "read symbols")
+	}
+	return nil
+}
+
 func (w *Writer) WriteLabelIndex(names []string, values []string) error {
 	if len(values)%len(names) != 0 {
 		return errors.Errorf("invalid value list length %d for %d names", len(values), len(names))
@@ -481,12 +498,12 @@ func (w *Writer) WriteLabelIndex(names []string, values []string) error {
 
 	// here we have an index for the symbol file if v2, otherwise it's an offset
 	for _, v := range valt.entries {
-		index, ok := w.symbols[v]
+		sid, err := w.symbols.ReverseLookup(v)
-		if !ok {
+		if err != nil {
-			return errors.Errorf("symbol entry for %q does not exist", v)
+			return errors.Errorf("symbol entry for %q does not exist: %v", v, err)
 		}
 		w.buf1.Reset()
-		w.buf1.PutBE32(index)
+		w.buf1.PutBE32(sid)
 		w.buf1.WriteToHash(w.crc32)
 		if err := w.write(w.buf1.Get()); err != nil {
 			return err
@@ -585,7 +602,7 @@ func (w *Writer) writePostingsOffsetTable() error {
 	return w.write(w.buf1.Get())
 }
 
-const indexTOCLen = 6*8 + 4
+const indexTOCLen = 6*8 + crc32.Size
 
 func (w *Writer) writeTOC() error {
 	w.buf1.Reset()
@@ -629,8 +646,8 @@ func (w *Writer) writePostings() error {
 			return errors.Errorf("series not 16-byte aligned at %d", startPos)
 		}
 		offsets = append(offsets, uint32(startPos/16))
-		// Skip to next series. The 4 is for the CRC32.
+		// Skip to next series.
-		d.Skip(d.Uvarint() + 4)
+		d.Skip(d.Uvarint() + crc32.Size)
 		if err := d.Err(); err != nil {
 			return nil
 		}
@@ -654,9 +671,13 @@ func (w *Writer) writePostings() error {
 			names = names[1:]
 		}
 
-		nameSymbols := map[uint32]struct{}{}
+		nameSymbols := map[uint32]string{}
 		for _, name := range batchNames {
-			nameSymbols[w.symbols[name]] = struct{}{}
+			sid, err := w.symbols.ReverseLookup(name)
+			if err != nil {
+				return err
+			}
+			nameSymbols[sid] = name
 		}
 		// Label name -> label value -> positions.
 		postings := map[uint32]map[uint32][]uint32{}
@@ -679,14 +700,11 @@ func (w *Writer) writePostings() error {
 					if _, ok := postings[lno]; !ok {
 						postings[lno] = map[uint32][]uint32{}
 					}
-					if _, ok := postings[lno][lvo]; !ok {
-						postings[lno][lvo] = []uint32{}
-					}
 					postings[lno][lvo] = append(postings[lno][lvo], uint32(startPos/16))
 				}
 			}
-			// Skip to next series. The 4 is for the CRC32.
+			// Skip to next series.
-			d.Skip(l - (startLen - d.Len()) + 4)
+			d.Skip(l - (startLen - d.Len()) + crc32.Size)
 			if err := d.Err(); err != nil {
 				return nil
 			}
@@ -694,15 +712,23 @@ func (w *Writer) writePostings() error {
 
 		for _, name := range batchNames {
 			// Write out postings for this label name.
-			values := make([]uint32, 0, len(postings[w.symbols[name]]))
+			sid, err := w.symbols.ReverseLookup(name)
-			for v := range postings[w.symbols[name]] {
+			if err != nil {
+				return err
+			}
+			values := make([]uint32, 0, len(postings[sid]))
+			for v := range postings[sid] {
 				values = append(values, v)
 
 			}
 			// Symbol numbers are in order, so the strings will also be in order.
 			sort.Sort(uint32slice(values))
 			for _, v := range values {
-				if err := w.writePosting(name, w.reverseSymbols[v], postings[w.symbols[name]][v]); err != nil {
+				value, err := w.symbols.Lookup(v)
+				if err != nil {
+					return err
+				}
+				if err := w.writePosting(name, value, postings[sid][v]); err != nil {
 					return err
 				}
 			}
@@ -765,6 +791,11 @@ func (w *Writer) Close() error {
 	if err := w.ensureStage(idxStageDone); err != nil {
 		return err
 	}
+	if w.symbolFile != nil {
+		if err := w.symbolFile.Close(); err != nil {
+			return err
+		}
+	}
 	if err := w.fbuf.Flush(); err != nil {
 		return err
 	}
@@ -782,6 +813,18 @@ type StringTuples interface {
 	At(i int) ([]string, error)
 }
 
+// StringIter iterates over a sorted list of strings.
+type StringIter interface {
+	// Next advances the iterator and returns true if another value was found.
+	Next() bool
+
+	// At returns the value at the current iterator position.
+	At() string
+
+	// Err returns the last error of the iterator.
+	Err() error
+}
+
 type Reader struct {
 	b   ByteSlice
 	toc *TOC
@@ -1038,6 +1081,9 @@ func (s Symbols) Lookup(o uint32) (string, error) {
 }
 
 func (s Symbols) ReverseLookup(sym string) (uint32, error) {
+	if len(s.offsets) == 0 {
+		return 0, errors.Errorf("unknown symbol %q - no symbols", sym)
+	}
 	i := sort.Search(len(s.offsets), func(i int) bool {
 		// Any decoding errors here will be lost, however
 		// we already read through all of this at startup.
@@ -1077,24 +1123,43 @@ func (s Symbols) ReverseLookup(sym string) (uint32, error) {
 	return uint32(s.bs.Len() - lastLen), nil
 }
 
-func (s Symbols) All() (map[string]struct{}, error) {
-	d := encoding.NewDecbufAt(s.bs, s.off, castagnoliTable)
-	cnt := d.Be32int()
-	res := make(map[string]struct{}, cnt)
-	for d.Err() == nil && cnt > 0 {
-		res[d.UvarintStr()] = struct{}{}
-		cnt--
-	}
-	if d.Err() != nil {
-		return nil, d.Err()
-	}
-	return res, nil
-}
-
 func (s Symbols) Size() int {
 	return len(s.offsets) * 8
 }
 
+func (s Symbols) Iter() StringIter {
+	d := encoding.NewDecbufAt(s.bs, s.off, castagnoliTable)
+	cnt := d.Be32int()
+	return &symbolsIter{
+		d:   d,
+		cnt: cnt,
+	}
+}
+
+// symbolsIter implements StringIter.
+type symbolsIter struct {
+	d   encoding.Decbuf
+	cnt int
+	cur string
+	err error
+}
+
+func (s *symbolsIter) Next() bool {
+	if s.cnt == 0 || s.err != nil {
+		return false
+	}
+	s.cur = yoloString(s.d.UvarintBytes())
+	s.cnt--
+	if s.d.Err() != nil {
+		s.err = s.d.Err()
+		return false
+	}
+	return true
+}
+
+func (s symbolsIter) At() string { return s.cur }
+func (s symbolsIter) Err() error { return s.err }
+
 // ReadOffsetTable reads an offset table and at the given position calls f for each
 // found entry. If f returns an error it stops decoding and returns the received error.
 func ReadOffsetTable(bs ByteSlice, off uint64, f func([]string, uint64, int) error) error {
@@ -1137,9 +1202,9 @@ func (r *Reader) lookupSymbol(o uint32) (string, error) {
 	return r.symbols.Lookup(o)
 }
 
-// Symbols returns a set of symbols that exist within the index.
+// Symbols returns an iterator over the symbols that exist within the index.
-func (r *Reader) Symbols() (map[string]struct{}, error) {
+func (r *Reader) Symbols() StringIter {
-	return r.symbols.All()
+	return r.symbols.Iter()
 }
 
 // SymbolTableSize returns the symbol table size in bytes.
@@ -1359,6 +1424,28 @@ func (t *stringTuples) Less(i, j int) bool {
 	return false
 }
 
+// NewStringListIterator returns a StringIter for the given sorted list of strings.
+func NewStringListIter(s []string) StringIter {
+	return &stringListIter{l: s}
+}
+
+// symbolsIter implements StringIter.
+type stringListIter struct {
+	l   []string
+	cur string
+}
+
+func (s *stringListIter) Next() bool {
+	if len(s.l) == 0 {
+		return false
+	}
+	s.cur = s.l[0]
+	s.l = s.l[1:]
+	return true
+}
+func (s stringListIter) At() string { return s.cur }
+func (s stringListIter) Err() error { return nil }
+
 // Decoder provides decoding methods for the v1 and v2 index file format.
 //
 // It currently does not contain decoding methods for all entry types but can be extended

tsdb/index/index_test.go

@@ -186,15 +186,12 @@ func TestIndexRW_Postings(t *testing.T) {
 		labels.FromStrings("a", "1", "b", "4"),
 	}
 
-	err = iw.AddSymbols(map[string]struct{}{
+	testutil.Ok(t, iw.AddSymbol("1"))
-		"a": {},
+	testutil.Ok(t, iw.AddSymbol("2"))
-		"b": {},
+	testutil.Ok(t, iw.AddSymbol("3"))
-		"1": {},
+	testutil.Ok(t, iw.AddSymbol("4"))
-		"2": {},
+	testutil.Ok(t, iw.AddSymbol("a"))
-		"3": {},
+	testutil.Ok(t, iw.AddSymbol("b"))
-		"4": {},
-	})
-	testutil.Ok(t, err)
 
 	// Postings lists are only written if a series with the respective
 	// reference was added before.
@@ -280,7 +277,14 @@ func TestPostingsMany(t *testing.T) {
 	symbols["i"] = struct{}{}
 	symbols["foo"] = struct{}{}
 	symbols["bar"] = struct{}{}
-	testutil.Ok(t, iw.AddSymbols(symbols))
+	syms := []string{}
+	for s := range symbols {
+		syms = append(syms, s)
+	}
+	sort.Strings(syms)
+	for _, s := range syms {
+		testutil.Ok(t, iw.AddSymbol(s))
+	}
 
 	for i, s := range series {
 		testutil.Ok(t, iw.AddSeries(uint64(i), s))
@@ -390,7 +394,14 @@ func TestPersistence_index_e2e(t *testing.T) {
 	iw, err := NewWriter(context.Background(), filepath.Join(dir, indexFilename))
 	testutil.Ok(t, err)
 
-	testutil.Ok(t, iw.AddSymbols(symbols))
+	syms := []string{}
+	for s := range symbols {
+		syms = append(syms, s)
+	}
+	sort.Strings(syms)
+	for _, s := range syms {
+		testutil.Ok(t, iw.AddSymbol(s))
+	}
 
 	// Population procedure as done by compaction.
 	var (
@@ -479,14 +490,18 @@ func TestPersistence_index_e2e(t *testing.T) {
 		}
 	}
 
-	gotSymbols, err := ir.Symbols()
+	gotSymbols := []string{}
-	testutil.Ok(t, err)
+	it := ir.Symbols()
-
+	for it.Next() {
-	testutil.Equals(t, len(mi.symbols), len(gotSymbols))
+		gotSymbols = append(gotSymbols, it.At())
-	for s := range mi.symbols {
-		_, ok := gotSymbols[s]
-		testutil.Assert(t, ok, "")
 	}
+	testutil.Ok(t, it.Err())
+	expSymbols := []string{}
+	for s := range mi.symbols {
+		expSymbols = append(expSymbols, s)
+	}
+	sort.Strings(expSymbols)
+	testutil.Equals(t, expSymbols, gotSymbols)
 
 	testutil.Ok(t, ir.Close())
 }

tsdb/mocks_test.go

@@ -24,7 +24,7 @@ type mockIndexWriter struct {
 	series []seriesSamples
 }
 
-func (mockIndexWriter) AddSymbols(sym map[string]struct{}) error { return nil }
+func (mockIndexWriter) AddSymbol(sym string) error { return nil }
 func (m *mockIndexWriter) AddSeries(ref uint64, l labels.Labels, chunks ...chunks.Meta) error {
 	i := -1
 	for j, s := range m.series {

tsdb/querier_test.go

@@ -1316,8 +1316,13 @@ func newMockIndex() mockIndex {
 	return ix
 }
 
-func (m mockIndex) Symbols() (map[string]struct{}, error) {
+func (m mockIndex) Symbols() index.StringIter {
-	return m.symbols, nil
+	l := []string{}
+	for s := range m.symbols {
+		l = append(l, s)
+	}
+	sort.Strings(l)
+	return index.NewStringListIter(l)
 }
 
 func (m *mockIndex) AddSeries(ref uint64, l labels.Labels, chunks ...chunks.Meta) error {