Vertical query merging and compaction (#370)

* Vertical series iterator Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Select overlapped blocks first in compactor Plan() Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Added vertical compaction Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Code cleanup and comments Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Fix review comments Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Fix tests Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Add benchmark for compaction Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Perform vertical compaction only when blocks are overlapping. Actions for vertical compaction: * Sorting chunk metas * Calling chunks.MergeOverlappingChunks on the chunks Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Benchmark for vertical compaction * BenchmarkNormalCompaction => BenchmarkCompaction * Moved the benchmark from db_test.go to compact_test.go Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Benchmark for query iterator and seek for non overlapping blocks Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Vertical query merge only for overlapping blocks Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Simplify logging in Compact(...) Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Updated CHANGELOG.md Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Calculate overlapping inside populateBlock Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * MinTime and MaxTime for BlockReader. Using this to find overlapping blocks in populateBlock() Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Sort blocks w.r.t. MinTime in reload() Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Log about overlapping in LeveledCompactor.write() instead of returning bool Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Log about overlapping inside LeveledCompactor.populateBlock() Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Fix review comments Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Refactor createBlock to take optional []Series Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * review1 Signed-off-by: Krasi Georgiev <kgeorgie@redhat.com> * Updated CHANGELOG and minor nits Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * nits Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Updated CHANGELOG Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Refactor iterator and seek benchmarks for Querier. Also has as overlapping blocks. Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Additional test case Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * genSeries takes optional labels. Updated BenchmarkQueryIterator and BenchmarkQuerySeek. Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Split genSeries into genSeries and populateSeries Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Check error in benchmark Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Fix review comments Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in> * Warn about overlapping blocks in reload() Signed-off-by: Ganesh Vernekar <cs15btech11018@iith.ac.in>
2024-12-23 12:44:05 -08:00 · 2019-02-14 18:59:41 +05:30 · 2019-02-14 18:59:41 +05:30 · c59ed492b2
parent 89ee5aaed4
commit c59ed492b2
13 changed files with 1164 additions and 127 deletions
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@ -1,4 +1,8 @@
 ## master / unreleased
+ - [ENHANCEMENT] Time-ovelapping blocks are now allowed. [#370](https://github.com/prometheus/tsdb/pull/370)
+   - Added `MergeChunks` function in `chunkenc/xor.go` to merge 2 time-overlapping chunks.
+   - Added `MergeOverlappingChunks` function in `chunks/chunks.go` to merge multiple time-overlapping Chunk Metas.
+   - Added `MinTime` and `MaxTime` method for `BlockReader`.
 - [CHANGE]  `NewLeveledCompactor` takes a context so that a compaction is canceled when closing the db.
 - [ENHANCEMENT] When closing the db any running compaction will be cancelled so it doesn't block.
 - [CHANGE] `prometheus_tsdb_storage_blocks_bytes_total` is now `prometheus_tsdb_storage_blocks_bytes`
@ -7,9 +11,9 @@
 - [CHANGE] New `WALSegmentSize` option to override the `DefaultOptions.WALSegmentSize`. Added to allow using smaller wal files. For example using tmpfs on a RPI to minimise the SD card wear out from the constant WAL writes. As part of this change the `DefaultOptions.WALSegmentSize` constant was also exposed.
 - [CHANGE] Empty blocks are not written during compaction [#374](https://github.com/prometheus/tsdb/pull/374)
 - [FEATURE]  Size base retention through `Options.MaxBytes`.  As part of this change:
-    - added new metrics - `prometheus_tsdb_storage_blocks_bytes_total`, `prometheus_tsdb_size_retentions_total`, `prometheus_tsdb_time_retentions_total`
-    - new public interface `SizeReader: Size() int64`
-    - `OpenBlock` signature changed to take a logger.
+   - Added new metrics - `prometheus_tsdb_storage_blocks_bytes_total`, `prometheus_tsdb_size_retentions_total`, `prometheus_tsdb_time_retentions_total`
+   - New public interface `SizeReader: Size() int64`
+   - `OpenBlock` signature changed to take a logger.
 - [REMOVED] `PrefixMatcher` is considered unused so was removed.
 - [CLEANUP] `Options.WALFlushInterval` is removed as it wasn't used anywhere.
 - [FEATURE] Add new `LiveReader` to WAL pacakge. Added to allow live tailing of a WAL segment, used by Prometheus Remote Write after refactor. The main difference between the new reader and the existing `Reader` is that for `LiveReader` a call to `Next()` that returns false does not mean that there will never be more data to read.
@ -24,4 +28,4 @@
 - [CHANGE] `Head.Init()` is changed to `Head.Init(minValidTime int64)`  
 - [CHANGE] `SymbolTable()` renamed to `SymbolTableSize()` to make the name consistent with the  `Block{ symbolTableSize uint64 }` field.
 - [CHANGE] `wal.Reader{}` now exposes `Segment()` for the current segment being read  and `Offset()` for the current offset.
-  -[FEATURE] tsdbutil analyze subcomand to find churn, high cardinality, etc.
+ - [FEATURE] tsdbutil analyze subcomand to find churn, high cardinality, etc.
--- a/block.go
+++ b/block.go
@ -135,6 +135,12 @@ type BlockReader interface {

 	// Tombstones returns a TombstoneReader over the block's deleted data.
 	Tombstones() (TombstoneReader, error)
+
+	// MinTime returns the min time of the block.
+	MinTime() int64
+
+	// MaxTime returns the max time of the block.
+	MaxTime() int64
 }

 // Appendable defines an entity to which data can be appended.
@ -363,6 +369,12 @@ func (pb *Block) Dir() string { return pb.dir }
 // Meta returns meta information about the block.
 func (pb *Block) Meta() BlockMeta { return pb.meta }

+// MinTime returns the min time of the meta.
+func (pb *Block) MinTime() int64 { return pb.meta.MinTime }
+
+// MaxTime returns the max time of the meta.
+func (pb *Block) MaxTime() int64 { return pb.meta.MaxTime }
+
 // Size returns the number of bytes that the block takes up.
 func (pb *Block) Size() int64 { return pb.meta.Stats.NumBytes }

--- a/block_test.go
+++ b/block_test.go
@ -101,8 +101,8 @@ func genSeries(totalSeries, labelCount int, mint, maxt int64) []Series {
 	if totalSeries == 0 || labelCount == 0 {
 		return nil
 	}
-	series := make([]Series, totalSeries)

+	series := make([]Series, totalSeries)
 	for i := 0; i < totalSeries; i++ {
 		lbls := make(map[string]string, labelCount)
 		for len(lbls) < labelCount {
@ -114,7 +114,26 @@ func genSeries(totalSeries, labelCount int, mint, maxt int64) []Series {
 		}
 		series[i] = newSeries(lbls, samples)
 	}
+	return series
+}

+// populateSeries generates series from given labels, mint and maxt.
+func populateSeries(lbls []map[string]string, mint, maxt int64) []Series {
+	if len(lbls) == 0 {
+		return nil
+	}
+
+	series := make([]Series, 0, len(lbls))
+	for _, lbl := range lbls {
+		if len(lbl) == 0 {
+			continue
+		}
+		samples := make([]tsdbutil.Sample, 0, maxt-mint+1)
+		for t := mint; t <= maxt; t++ {
+			samples = append(samples, sample{t: t, v: rand.Float64()})
+		}
+		series = append(series, newSeries(lbl, samples))
+	}
 	return series
 }

--- a/chunks/chunks.go
+++ b/chunks/chunks.go
@ -196,6 +196,84 @@ func (w *Writer) write(b []byte) error {
 	return err
 }

+// MergeOverlappingChunks removes the samples whose timestamp is overlapping.
+// The last appearing sample is retained in case there is overlapping.
+// This assumes that `chks []Meta` is sorted w.r.t. MinTime.
+func MergeOverlappingChunks(chks []Meta) ([]Meta, error) {
+	if len(chks) < 2 {
+		return chks, nil
+	}
+	newChks := make([]Meta, 0, len(chks)) // Will contain the merged chunks.
+	newChks = append(newChks, chks[0])
+	last := 0
+	for _, c := range chks[1:] {
+		// We need to check only the last chunk in newChks.
+		// Reason: (1) newChks[last-1].MaxTime < newChks[last].MinTime (non overlapping)
+		//         (2) As chks are sorted w.r.t. MinTime, newChks[last].MinTime < c.MinTime.
+		// So never overlaps with newChks[last-1] or anything before that.
+		if c.MinTime > newChks[last].MaxTime {
+			newChks = append(newChks, c)
+			continue
+		}
+		nc := &newChks[last]
+		if c.MaxTime > nc.MaxTime {
+			nc.MaxTime = c.MaxTime
+		}
+		chk, err := MergeChunks(nc.Chunk, c.Chunk)
+		if err != nil {
+			return nil, err
+		}
+		nc.Chunk = chk
+	}
+
+	return newChks, nil
+}
+
+// MergeChunks vertically merges a and b, i.e., if there is any sample
+// with same timestamp in both a and b, the sample in a is discarded.
+func MergeChunks(a, b chunkenc.Chunk) (*chunkenc.XORChunk, error) {
+	newChunk := chunkenc.NewXORChunk()
+	app, err := newChunk.Appender()
+	if err != nil {
+		return nil, err
+	}
+	ait := a.Iterator()
+	bit := b.Iterator()
+	aok, bok := ait.Next(), bit.Next()
+	for aok && bok {
+		at, av := ait.At()
+		bt, bv := bit.At()
+		if at < bt {
+			app.Append(at, av)
+			aok = ait.Next()
+		} else if bt < at {
+			app.Append(bt, bv)
+			bok = bit.Next()
+		} else {
+			app.Append(bt, bv)
+			aok = ait.Next()
+			bok = bit.Next()
+		}
+	}
+	for aok {
+		at, av := ait.At()
+		app.Append(at, av)
+		aok = ait.Next()
+	}
+	for bok {
+		bt, bv := bit.At()
+		app.Append(bt, bv)
+		bok = bit.Next()
+	}
+	if ait.Err() != nil {
+		return nil, ait.Err()
+	}
+	if bit.Err() != nil {
+		return nil, bit.Err()
+	}
+	return newChunk, nil
+}
+
 func (w *Writer) WriteChunks(chks ...Meta) error {
 	// Calculate maximum space we need and cut a new segment in case
 	// we don't fit into the current one.
--- a/compact.go
+++ b/compact.go
@ -17,6 +17,7 @@ import (
 	"context"
 	"fmt"
 	"io"
+	"math"
 	"math/rand"
 	"os"
 	"path/filepath"
@ -80,13 +81,14 @@ type LeveledCompactor struct {
 }

 type compactorMetrics struct {
-	ran              prometheus.Counter
-	populatingBlocks prometheus.Gauge
-	failed           prometheus.Counter
-	duration         prometheus.Histogram
-	chunkSize        prometheus.Histogram
-	chunkSamples     prometheus.Histogram
-	chunkRange       prometheus.Histogram
+	ran               prometheus.Counter
+	populatingBlocks  prometheus.Gauge
+	failed            prometheus.Counter
+	overlappingBlocks prometheus.Counter
+	duration          prometheus.Histogram
+	chunkSize         prometheus.Histogram
+	chunkSamples      prometheus.Histogram
+	chunkRange        prometheus.Histogram
 }

 func newCompactorMetrics(r prometheus.Registerer) *compactorMetrics {
@ -104,6 +106,10 @@ func newCompactorMetrics(r prometheus.Registerer) *compactorMetrics {
 		Name: "prometheus_tsdb_compactions_failed_total",
 		Help: "Total number of compactions that failed for the partition.",
 	})
+	m.overlappingBlocks = prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "prometheus_tsdb_vertical_compactions_total",
+		Help: "Total number of compactions done on overlapping blocks.",
+	})
 	m.duration = prometheus.NewHistogram(prometheus.HistogramOpts{
 		Name:    "prometheus_tsdb_compaction_duration_seconds",
 		Help:    "Duration of compaction runs",
@ -130,6 +136,7 @@ func newCompactorMetrics(r prometheus.Registerer) *compactorMetrics {
 			m.ran,
 			m.populatingBlocks,
 			m.failed,
+			m.overlappingBlocks,
 			m.duration,
 			m.chunkRange,
 			m.chunkSamples,
@ -147,6 +154,9 @@ func NewLeveledCompactor(ctx context.Context, r prometheus.Registerer, l log.Log
 	if pool == nil {
 		pool = chunkenc.NewPool()
 	}
+	if l == nil {
+		l = log.NewNopLogger()
+	}
 	return &LeveledCompactor{
 		ranges:    ranges,
 		chunkPool: pool,
@ -187,11 +197,15 @@ func (c *LeveledCompactor) plan(dms []dirMeta) ([]string, error) {
 		return dms[i].meta.MinTime < dms[j].meta.MinTime
 	})

+	res := c.selectOverlappingDirs(dms)
+	if len(res) > 0 {
+		return res, nil
+	}
+	// No overlapping blocks, do compaction the usual way.
 	// We do not include a recently created block with max(minTime), so the block which was just created from WAL.
 	// This gives users a window of a full block size to piece-wise backup new data without having to care about data overlap.
 	dms = dms[:len(dms)-1]

-	var res []string
 	for _, dm := range c.selectDirs(dms) {
 		res = append(res, dm.dir)
 	}
@ -252,6 +266,28 @@ func (c *LeveledCompactor) selectDirs(ds []dirMeta) []dirMeta {
 	return nil
 }

+// selectOverlappingDirs returns all dirs with overlaping time ranges.
+// It expects sorted input by mint.
+func (c *LeveledCompactor) selectOverlappingDirs(ds []dirMeta) []string {
+	if len(ds) < 2 {
+		return nil
+	}
+	var overlappingDirs []string
+	globalMaxt := ds[0].meta.MaxTime
+	for i, d := range ds[1:] {
+		if d.meta.MinTime < globalMaxt {
+			if len(overlappingDirs) == 0 { // When it is the first overlap, need to add the last one as well.
+				overlappingDirs = append(overlappingDirs, ds[i].dir)
+			}
+			overlappingDirs = append(overlappingDirs, d.dir)
+		}
+		if d.meta.MaxTime > globalMaxt {
+			globalMaxt = d.meta.MaxTime
+		}
+	}
+	return overlappingDirs
+}
+
 // splitByRange splits the directories by the time range. The range sequence starts at 0.
 //
 // For example, if we have blocks [0-10, 10-20, 50-60, 90-100] and the split range tr is 30
@ -299,12 +335,17 @@ func compactBlockMetas(uid ulid.ULID, blocks ...*BlockMeta) *BlockMeta {
 	res := &BlockMeta{
 		ULID:    uid,
 		MinTime: blocks[0].MinTime,
-		MaxTime: blocks[len(blocks)-1].MaxTime,
 	}

 	sources := map[ulid.ULID]struct{}{}
+	// For overlapping blocks, the Maxt can be
+	// in any block so we track it globally.
+	maxt := int64(math.MinInt64)

 	for _, b := range blocks {
+		if b.MaxTime > maxt {
+			maxt = b.MaxTime
+		}
 		if b.Compaction.Level > res.Compaction.Level {
 			res.Compaction.Level = b.Compaction.Level
 		}
@ -326,6 +367,7 @@ func compactBlockMetas(uid ulid.ULID, blocks ...*BlockMeta) *BlockMeta {
 		return res.Compaction.Sources[i].Compare(res.Compaction.Sources[j]) < 0
 	})

+	res.MaxTime = maxt
 	return res
 }

@ -603,15 +645,17 @@ func (c *LeveledCompactor) write(dest string, meta *BlockMeta, blocks ...BlockRe

 // populateBlock fills the index and chunk writers with new data gathered as the union
 // of the provided blocks. It returns meta information for the new block.
+// It expects sorted blocks input by mint.
 func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta, indexw IndexWriter, chunkw ChunkWriter) (err error) {
 	if len(blocks) == 0 {
 		return errors.New("cannot populate block from no readers")
 	}

 	var (
-		set        ChunkSeriesSet
-		allSymbols = make(map[string]struct{}, 1<<16)
-		closers    = []io.Closer{}
+		set         ChunkSeriesSet
+		allSymbols  = make(map[string]struct{}, 1<<16)
+		closers     = []io.Closer{}
+		overlapping bool
 	)
 	defer func() {
 		var merr MultiError
@ -622,6 +666,7 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 	}()
 	c.metrics.populatingBlocks.Set(1)

+	globalMaxt := blocks[0].MaxTime()
 	for i, b := range blocks {
 		select {
 		case <-c.ctx.Done():
@ -629,6 +674,17 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		default:
 		}

+		if !overlapping {
+			if i > 0 && b.MinTime() < globalMaxt {
+				c.metrics.overlappingBlocks.Inc()
+				overlapping = true
+				level.Warn(c.logger).Log("msg", "found overlapping blocks during compaction", "ulid", meta.ULID)
+			}
+			if b.MaxTime() > globalMaxt {
+				globalMaxt = b.MaxTime()
+			}
+		}
+
 		indexr, err := b.Index()
 		if err != nil {
 			return errors.Wrapf(err, "open index reader for block %s", b)
@ -692,6 +748,12 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 		}

 		lset, chks, dranges := set.At() // The chunks here are not fully deleted.
+		if overlapping {
+			// If blocks are overlapping, it is possible to have unsorted chunks.
+			sort.Slice(chks, func(i, j int) bool {
+				return chks[i].MinTime < chks[j].MinTime
+			})
+		}

 		// Skip the series with all deleted chunks.
 		if len(chks) == 0 {
@ -725,21 +787,28 @@ func (c *LeveledCompactor) populateBlock(blocks []BlockReader, meta *BlockMeta,
 			}
 		}

-		if err := chunkw.WriteChunks(chks...); err != nil {
+		mergedChks := chks
+		if overlapping {
+			mergedChks, err = chunks.MergeOverlappingChunks(chks)
+			if err != nil {
+				return errors.Wrap(err, "merge overlapping chunks")
+			}
+		}
+		if err := chunkw.WriteChunks(mergedChks...); err != nil {
 			return errors.Wrap(err, "write chunks")
 		}

-		if err := indexw.AddSeries(i, lset, chks...); err != nil {
+		if err := indexw.AddSeries(i, lset, mergedChks...); err != nil {
 			return errors.Wrap(err, "add series")
 		}

-		meta.Stats.NumChunks += uint64(len(chks))
+		meta.Stats.NumChunks += uint64(len(mergedChks))
 		meta.Stats.NumSeries++
-		for _, chk := range chks {
+		for _, chk := range mergedChks {
 			meta.Stats.NumSamples += uint64(chk.Chunk.NumSamples())
 		}

-		for _, chk := range chks {
+		for _, chk := range mergedChks {
 			if err := c.chunkPool.Put(chk.Chunk); err != nil {
 				return errors.Wrap(err, "put chunk")
 			}
--- a/compact_test.go
+++ b/compact_test.go
@ -15,6 +15,7 @@ package tsdb

 import (
 	"context"
+	"fmt"
 	"io/ioutil"
 	"math"
 	"os"
@ -311,13 +312,46 @@ func TestLeveledCompactor_plan(t *testing.T) {
 			},
 			expected: []string{"7", "8"},
 		},
+		// For overlapping blocks.
+		{
+			metas: []dirMeta{
+				metaRange("1", 0, 20, nil),
+				metaRange("2", 19, 40, nil),
+				metaRange("3", 40, 60, nil),
+			},
+			expected: []string{"1", "2"},
+		},
+		{
+			metas: []dirMeta{
+				metaRange("1", 0, 20, nil),
+				metaRange("2", 20, 40, nil),
+				metaRange("3", 30, 50, nil),
+			},
+			expected: []string{"2", "3"},
+		},
+		{
+			metas: []dirMeta{
+				metaRange("1", 0, 20, nil),
+				metaRange("2", 10, 40, nil),
+				metaRange("3", 30, 50, nil),
+			},
+			expected: []string{"1", "2", "3"},
+		},
+		{
+			metas: []dirMeta{
+				metaRange("5", 0, 360, nil),
+				metaRange("6", 340, 560, nil),
+				metaRange("7", 360, 420, nil),
+				metaRange("8", 420, 540, nil),
+			},
+			expected: []string{"5", "6", "7", "8"},
+		},
 	}

 	for _, c := range cases {
 		if !t.Run("", func(t *testing.T) {
 			res, err := compactor.plan(c.metas)
 			testutil.Ok(t, err)
-
 			testutil.Equals(t, c.expected, res)
 		}) {
 			return
@ -410,6 +444,8 @@ type erringBReader struct{}
 func (erringBReader) Index() (IndexReader, error)          { return nil, errors.New("index") }
 func (erringBReader) Chunks() (ChunkReader, error)         { return nil, errors.New("chunks") }
 func (erringBReader) Tombstones() (TombstoneReader, error) { return nil, errors.New("tombstones") }
+func (erringBReader) MinTime() int64                       { return 0 }
+func (erringBReader) MaxTime() int64                       { return 0 }

 type nopChunkWriter struct{}

@ -422,9 +458,8 @@ func TestCompaction_populateBlock(t *testing.T) {
 		inputSeriesSamples [][]seriesSamples
 		compactMinTime     int64
 		compactMaxTime     int64 // When not defined the test runner sets a default of math.MaxInt64.
-
-		expSeriesSamples []seriesSamples
-		expErr           error
+		expSeriesSamples   []seriesSamples
+		expErr             error
 	}{
 		{
 			title:              "Populate block from empty input should return error.",
@ -500,16 +535,6 @@ func TestCompaction_populateBlock(t *testing.T) {
 		{
 			title: "Populate from two blocks showing that order is maintained.",
 			inputSeriesSamples: [][]seriesSamples{
-				{
-					{
-						lset:   map[string]string{"a": "b"},
-						chunks: [][]sample{{{t: 21}, {t: 30}}},
-					},
-					{
-						lset:   map[string]string{"a": "c"},
-						chunks: [][]sample{{{t: 40}, {t: 45}}},
-					},
-				},
 				{
 					{
 						lset:   map[string]string{"a": "b"},
@ -520,20 +545,30 @@ func TestCompaction_populateBlock(t *testing.T) {
 						chunks: [][]sample{{{t: 1}, {t: 9}}, {{t: 10}, {t: 19}}},
 					},
 				},
+				{
+					{
+						lset:   map[string]string{"a": "b"},
+						chunks: [][]sample{{{t: 21}, {t: 30}}},
+					},
+					{
+						lset:   map[string]string{"a": "c"},
+						chunks: [][]sample{{{t: 40}, {t: 45}}},
+					},
+				},
 			},
 			expSeriesSamples: []seriesSamples{
 				{
 					lset:   map[string]string{"a": "b"},
-					chunks: [][]sample{{{t: 21}, {t: 30}}, {{t: 0}, {t: 10}}, {{t: 11}, {t: 20}}},
+					chunks: [][]sample{{{t: 0}, {t: 10}}, {{t: 11}, {t: 20}}, {{t: 21}, {t: 30}}},
 				},
 				{
 					lset:   map[string]string{"a": "c"},
-					chunks: [][]sample{{{t: 40}, {t: 45}}, {{t: 1}, {t: 9}}, {{t: 10}, {t: 19}}},
+					chunks: [][]sample{{{t: 1}, {t: 9}}, {{t: 10}, {t: 19}}, {{t: 40}, {t: 45}}},
 				},
 			},
 		},
 		{
-			title: "Populate from two blocks showing that order or series is sorted.",
+			title: "Populate from two blocks showing that order of series is sorted.",
 			inputSeriesSamples: [][]seriesSamples{
 				{
 					{
@ -647,8 +682,8 @@ func TestCompaction_populateBlock(t *testing.T) {
 		if ok := t.Run(tc.title, func(t *testing.T) {
 			blocks := make([]BlockReader, 0, len(tc.inputSeriesSamples))
 			for _, b := range tc.inputSeriesSamples {
-				ir, cr := createIdxChkReaders(b)
-				blocks = append(blocks, &mockBReader{ir: ir, cr: cr})
+				ir, cr, mint, maxt := createIdxChkReaders(b)
+				blocks = append(blocks, &mockBReader{ir: ir, cr: cr, mint: mint, maxt: maxt})
 			}

 			c, err := NewLeveledCompactor(context.Background(), nil, nil, []int64{0}, nil)
@ -690,6 +725,78 @@ func TestCompaction_populateBlock(t *testing.T) {
 	}
 }

+func BenchmarkCompaction(b *testing.B) {
+	cases := []struct {
+		ranges         [][2]int64
+		compactionType string
+	}{
+		{
+			ranges:         [][2]int64{{0, 100}, {200, 300}, {400, 500}, {600, 700}},
+			compactionType: "normal",
+		},
+		{
+			ranges:         [][2]int64{{0, 1000}, {2000, 3000}, {4000, 5000}, {6000, 7000}},
+			compactionType: "normal",
+		},
+		{
+			ranges:         [][2]int64{{0, 10000}, {20000, 30000}, {40000, 50000}, {60000, 70000}},
+			compactionType: "normal",
+		},
+		{
+			ranges:         [][2]int64{{0, 100000}, {200000, 300000}, {400000, 500000}, {600000, 700000}},
+			compactionType: "normal",
+		},
+		// 40% overlaps.
+		{
+			ranges:         [][2]int64{{0, 100}, {60, 160}, {120, 220}, {180, 280}},
+			compactionType: "vertical",
+		},
+		{
+			ranges:         [][2]int64{{0, 1000}, {600, 1600}, {1200, 2200}, {1800, 2800}},
+			compactionType: "vertical",
+		},
+		{
+			ranges:         [][2]int64{{0, 10000}, {6000, 16000}, {12000, 22000}, {18000, 28000}},
+			compactionType: "vertical",
+		},
+		{
+			ranges:         [][2]int64{{0, 100000}, {60000, 160000}, {120000, 220000}, {180000, 280000}},
+			compactionType: "vertical",
+		},
+	}
+
+	nSeries := 10000
+	for _, c := range cases {
+		nBlocks := len(c.ranges)
+		b.Run(fmt.Sprintf("type=%s,blocks=%d,series=%d,samplesPerSeriesPerBlock=%d", c.compactionType, nBlocks, nSeries, c.ranges[0][1]-c.ranges[0][0]+1), func(b *testing.B) {
+			dir, err := ioutil.TempDir("", "bench_compaction")
+			testutil.Ok(b, err)
+			defer func() {
+				testutil.Ok(b, os.RemoveAll(dir))
+			}()
+			blockDirs := make([]string, 0, len(c.ranges))
+			var blocks []*Block
+			for _, r := range c.ranges {
+				block, err := OpenBlock(nil, createBlock(b, dir, genSeries(nSeries, 10, r[0], r[1])), nil)
+				testutil.Ok(b, err)
+				blocks = append(blocks, block)
+				defer func() {
+					testutil.Ok(b, block.Close())
+				}()
+				blockDirs = append(blockDirs, block.Dir())
+			}
+
+			c, err := NewLeveledCompactor(context.Background(), nil, log.NewNopLogger(), []int64{0}, nil)
+			testutil.Ok(b, err)
+
+			b.ResetTimer()
+			b.ReportAllocs()
+			_, err = c.Compact(dir, blockDirs, blocks)
+			testutil.Ok(b, err)
+		})
+	}
+}
+
 // TestDisableAutoCompactions checks that we can
 // disable and enable the auto compaction.
 // This is needed for unit tests that rely on
--- a/db.go
+++ b/db.go
@ -546,11 +546,8 @@ func (db *DB) reload() (err error) {
 	db.metrics.blocksBytes.Set(float64(blocksSize))

 	sort.Slice(loadable, func(i, j int) bool {
-		return loadable[i].Meta().MaxTime < loadable[j].Meta().MaxTime
+		return loadable[i].Meta().MinTime < loadable[j].Meta().MinTime
 	})
-	if err := validateBlockSequence(loadable); err != nil {
-		return errors.Wrap(err, "invalid block sequence")
-	}

 	// Swap new blocks first for subsequently created readers to be seen.
 	db.mtx.Lock()
@ -558,6 +555,14 @@ func (db *DB) reload() (err error) {
 	db.blocks = loadable
 	db.mtx.Unlock()

+	blockMetas := make([]BlockMeta, 0, len(loadable))
+	for _, b := range loadable {
+		blockMetas = append(blockMetas, b.Meta())
+	}
+	if overlaps := OverlappingBlocks(blockMetas); len(overlaps) > 0 {
+		level.Warn(db.logger).Log("msg", "overlapping blocks found during reload", "detail", overlaps.String())
+	}
+
 	for _, b := range oldBlocks {
 		if _, ok := deletable[b.Meta().ULID]; ok {
 			deletable[b.Meta().ULID] = b
@ -694,25 +699,6 @@ func (db *DB) deleteBlocks(blocks map[ulid.ULID]*Block) error {
 	return nil
 }

-// validateBlockSequence returns error if given block meta files indicate that some blocks overlaps within sequence.
-func validateBlockSequence(bs []*Block) error {
-	if len(bs) <= 1 {
-		return nil
-	}
-
-	var metas []BlockMeta
-	for _, b := range bs {
-		metas = append(metas, b.meta)
-	}
-
-	overlaps := OverlappingBlocks(metas)
-	if len(overlaps) > 0 {
-		return errors.Errorf("block time ranges overlap: %s", overlaps)
-	}
-
-	return nil
-}
-
 // TimeRange specifies minTime and maxTime range.
 type TimeRange struct {
 	Min, Max int64
@ -909,6 +895,7 @@ func (db *DB) Snapshot(dir string, withHead bool) error {
 // A goroutine must not handle more than one open Querier.
 func (db *DB) Querier(mint, maxt int64) (Querier, error) {
 	var blocks []BlockReader
+	var blockMetas []BlockMeta

 	db.mtx.RLock()
 	defer db.mtx.RUnlock()
@ -916,6 +903,7 @@ func (db *DB) Querier(mint, maxt int64) (Querier, error) {
 	for _, b := range db.blocks {
 		if b.OverlapsClosedInterval(mint, maxt) {
 			blocks = append(blocks, b)
+			blockMetas = append(blockMetas, b.Meta())
 		}
 	}
 	if maxt >= db.head.MinTime() {
@ -926,22 +914,31 @@ func (db *DB) Querier(mint, maxt int64) (Querier, error) {
 		})
 	}

-	sq := &querier{
-		blocks: make([]Querier, 0, len(blocks)),
-	}
+	blockQueriers := make([]Querier, 0, len(blocks))
 	for _, b := range blocks {
 		q, err := NewBlockQuerier(b, mint, maxt)
 		if err == nil {
-			sq.blocks = append(sq.blocks, q)
+			blockQueriers = append(blockQueriers, q)
 			continue
 		}
 		// If we fail, all previously opened queriers must be closed.
-		for _, q := range sq.blocks {
+		for _, q := range blockQueriers {
 			q.Close()
 		}
 		return nil, errors.Wrapf(err, "open querier for block %s", b)
 	}
-	return sq, nil
+
+	if len(OverlappingBlocks(blockMetas)) > 0 {
+		return &verticalQuerier{
+			querier: querier{
+				blocks: blockQueriers,
+			},
+		}, nil
+	}
+
+	return &querier{
+		blocks: blockQueriers,
+	}, nil
 }

 func rangeForTimestamp(t int64, width int64) (maxt int64) {
--- a/db_test.go
+++ b/db_test.go
@ -52,16 +52,19 @@ func openTestDB(t testing.TB, opts *Options) (db *DB, close func()) {
 }

 // query runs a matcher query against the querier and fully expands its data.
-func query(t testing.TB, q Querier, matchers ...labels.Matcher) map[string][]sample {
+func query(t testing.TB, q Querier, matchers ...labels.Matcher) map[string][]tsdbutil.Sample {
 	ss, err := q.Select(matchers...)
+	defer func() {
+		testutil.Ok(t, q.Close())
+	}()
 	testutil.Ok(t, err)

-	result := map[string][]sample{}
+	result := map[string][]tsdbutil.Sample{}

 	for ss.Next() {
 		series := ss.At()

-		samples := []sample{}
+		samples := []tsdbutil.Sample{}
 		it := series.Iterator()
 		for it.Next() {
 			t, v := it.At()
@ -124,9 +127,7 @@ func TestDataAvailableOnlyAfterCommit(t *testing.T) {
 	querier, err := db.Querier(0, 1)
 	testutil.Ok(t, err)
 	seriesSet := query(t, querier, labels.NewEqualMatcher("foo", "bar"))
-
-	testutil.Equals(t, map[string][]sample{}, seriesSet)
-	testutil.Ok(t, querier.Close())
+	testutil.Equals(t, map[string][]tsdbutil.Sample{}, seriesSet)

 	err = app.Commit()
 	testutil.Ok(t, err)
@ -137,7 +138,7 @@ func TestDataAvailableOnlyAfterCommit(t *testing.T) {

 	seriesSet = query(t, querier, labels.NewEqualMatcher("foo", "bar"))

-	testutil.Equals(t, map[string][]sample{`{foo="bar"}`: {{t: 0, v: 0}}}, seriesSet)
+	testutil.Equals(t, map[string][]tsdbutil.Sample{`{foo="bar"}`: {sample{t: 0, v: 0}}}, seriesSet)
 }

 func TestDataNotAvailableAfterRollback(t *testing.T) {
@ -160,7 +161,7 @@ func TestDataNotAvailableAfterRollback(t *testing.T) {

 	seriesSet := query(t, querier, labels.NewEqualMatcher("foo", "bar"))

-	testutil.Equals(t, map[string][]sample{}, seriesSet)
+	testutil.Equals(t, map[string][]tsdbutil.Sample{}, seriesSet)
 }

 func TestDBAppenderAddRef(t *testing.T) {
@ -207,17 +208,15 @@ func TestDBAppenderAddRef(t *testing.T) {

 	res := query(t, q, labels.NewEqualMatcher("a", "b"))

-	testutil.Equals(t, map[string][]sample{
+	testutil.Equals(t, map[string][]tsdbutil.Sample{
 		labels.FromStrings("a", "b").String(): {
-			{t: 123, v: 0},
-			{t: 124, v: 1},
-			{t: 125, v: 0},
-			{t: 133, v: 1},
-			{t: 143, v: 2},
+			sample{t: 123, v: 0},
+			sample{t: 124, v: 1},
+			sample{t: 125, v: 0},
+			sample{t: 133, v: 1},
+			sample{t: 143, v: 2},
 		},
 	}, res)
-
-	testutil.Ok(t, q.Close())
 }

 func TestDeleteSimple(t *testing.T) {
@ -398,12 +397,10 @@ func TestSkippingInvalidValuesInSameTxn(t *testing.T) {

 	ssMap := query(t, q, labels.NewEqualMatcher("a", "b"))

-	testutil.Equals(t, map[string][]sample{
-		labels.New(labels.Label{"a", "b"}).String(): {{0, 1}},
+	testutil.Equals(t, map[string][]tsdbutil.Sample{
+		labels.New(labels.Label{"a", "b"}).String(): {sample{0, 1}},
 	}, ssMap)

-	testutil.Ok(t, q.Close())
-
 	// Append Out of Order Value.
 	app = db.Appender()
 	_, err = app.Add(labels.Labels{{"a", "b"}}, 10, 3)
@ -417,10 +414,9 @@ func TestSkippingInvalidValuesInSameTxn(t *testing.T) {

 	ssMap = query(t, q, labels.NewEqualMatcher("a", "b"))

-	testutil.Equals(t, map[string][]sample{
-		labels.New(labels.Label{"a", "b"}).String(): {{0, 1}, {10, 3}},
+	testutil.Equals(t, map[string][]tsdbutil.Sample{
+		labels.New(labels.Label{"a", "b"}).String(): {sample{0, 1}, sample{10, 3}},
 	}, ssMap)
-	testutil.Ok(t, q.Close())
 }

 func TestDB_Snapshot(t *testing.T) {
@ -610,9 +606,9 @@ func TestDB_e2e(t *testing.T) {
 		},
 	}

-	seriesMap := map[string][]sample{}
+	seriesMap := map[string][]tsdbutil.Sample{}
 	for _, l := range lbls {
-		seriesMap[labels.New(l...).String()] = []sample{}
+		seriesMap[labels.New(l...).String()] = []tsdbutil.Sample{}
 	}

 	db, delete := openTestDB(t, nil)
@ -625,7 +621,7 @@ func TestDB_e2e(t *testing.T) {

 	for _, l := range lbls {
 		lset := labels.New(l...)
-		series := []sample{}
+		series := []tsdbutil.Sample{}

 		ts := rand.Int63n(300)
 		for i := 0; i < numDatapoints; i++ {
@ -682,7 +678,7 @@ func TestDB_e2e(t *testing.T) {
 			mint := rand.Int63n(300)
 			maxt := mint + rand.Int63n(timeInterval*int64(numDatapoints))

-			expected := map[string][]sample{}
+			expected := map[string][]tsdbutil.Sample{}

 			// Build the mockSeriesSet.
 			for _, m := range matched {
@ -698,7 +694,7 @@ func TestDB_e2e(t *testing.T) {
 			ss, err := q.Select(qry.ms...)
 			testutil.Ok(t, err)

-			result := map[string][]sample{}
+			result := map[string][]tsdbutil.Sample{}

 			for ss.Next() {
 				x := ss.At()
@ -1666,6 +1662,310 @@ func TestCorrectNumTombstones(t *testing.T) {
 	testutil.Equals(t, uint64(3), db.blocks[0].meta.Stats.NumTombstones)
 }

+func TestVerticalCompaction(t *testing.T) {
+	cases := []struct {
+		blockSeries [][]Series
+		expSeries   map[string][]tsdbutil.Sample
+	}{
+		// Case 0
+		// |--------------|
+		//        |----------------|
+		{
+			blockSeries: [][]Series{
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{7, 0}, sample{8, 0}, sample{9, 0},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{7, 99},
+						sample{8, 99}, sample{9, 99}, sample{10, 99}, sample{11, 99},
+						sample{12, 99}, sample{13, 99}, sample{14, 99},
+					}),
+				},
+			},
+			expSeries: map[string][]tsdbutil.Sample{`{a="b"}`: {
+				sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{3, 99},
+				sample{4, 0}, sample{5, 99}, sample{6, 99}, sample{7, 99},
+				sample{8, 99}, sample{9, 99}, sample{10, 99}, sample{11, 99},
+				sample{12, 99}, sample{13, 99}, sample{14, 99},
+			}},
+		},
+		// Case 1
+		// |-------------------------------|
+		//        |----------------|
+		{
+			blockSeries: [][]Series{
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{7, 0}, sample{8, 0}, sample{9, 0},
+						sample{11, 0}, sample{13, 0}, sample{17, 0},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{7, 99},
+						sample{8, 99}, sample{9, 99}, sample{10, 99},
+					}),
+				},
+			},
+			expSeries: map[string][]tsdbutil.Sample{`{a="b"}`: {
+				sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{3, 99},
+				sample{4, 0}, sample{5, 99}, sample{6, 99}, sample{7, 99},
+				sample{8, 99}, sample{9, 99}, sample{10, 99}, sample{11, 0},
+				sample{13, 0}, sample{17, 0},
+			}},
+		},
+		// Case 2
+		// |-------------------------------|
+		//        |------------|
+		//                           |--------------------|
+		{
+			blockSeries: [][]Series{
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{7, 0}, sample{8, 0}, sample{9, 0},
+						sample{11, 0}, sample{13, 0}, sample{17, 0},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{7, 99},
+						sample{8, 99}, sample{9, 99},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{14, 59}, sample{15, 59}, sample{17, 59}, sample{20, 59},
+						sample{21, 59}, sample{22, 59},
+					}),
+				},
+			},
+			expSeries: map[string][]tsdbutil.Sample{`{a="b"}`: {
+				sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{3, 99},
+				sample{4, 0}, sample{5, 99}, sample{6, 99}, sample{7, 99},
+				sample{8, 99}, sample{9, 99}, sample{11, 0}, sample{13, 0},
+				sample{14, 59}, sample{15, 59}, sample{17, 59}, sample{20, 59},
+				sample{21, 59}, sample{22, 59},
+			}},
+		},
+		// Case 3
+		// |-------------------|
+		//                           |--------------------|
+		//               |----------------|
+		{
+			blockSeries: [][]Series{
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{8, 0}, sample{9, 0},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{14, 59}, sample{15, 59}, sample{17, 59}, sample{20, 59},
+						sample{21, 59}, sample{22, 59},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{5, 99}, sample{6, 99}, sample{7, 99}, sample{8, 99},
+						sample{9, 99}, sample{10, 99}, sample{13, 99}, sample{15, 99},
+						sample{16, 99}, sample{17, 99},
+					}),
+				},
+			},
+			expSeries: map[string][]tsdbutil.Sample{`{a="b"}`: {
+				sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+				sample{5, 99}, sample{6, 99}, sample{7, 99}, sample{8, 99},
+				sample{9, 99}, sample{10, 99}, sample{13, 99}, sample{14, 59},
+				sample{15, 59}, sample{16, 99}, sample{17, 59}, sample{20, 59},
+				sample{21, 59}, sample{22, 59},
+			}},
+		},
+		// Case 4
+		// |-------------------------------------|
+		//            |------------|
+		//      |-------------------------|
+		{
+			blockSeries: [][]Series{
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{8, 0}, sample{9, 0}, sample{10, 0},
+						sample{13, 0}, sample{15, 0}, sample{16, 0}, sample{17, 0},
+						sample{20, 0}, sample{22, 0},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{7, 59}, sample{8, 59}, sample{9, 59}, sample{10, 59},
+						sample{11, 59},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{8, 99},
+						sample{9, 99}, sample{10, 99}, sample{13, 99}, sample{15, 99},
+						sample{16, 99}, sample{17, 99},
+					}),
+				},
+			},
+			expSeries: map[string][]tsdbutil.Sample{`{a="b"}`: {
+				sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{3, 99},
+				sample{4, 0}, sample{5, 99}, sample{6, 99}, sample{7, 59},
+				sample{8, 59}, sample{9, 59}, sample{10, 59}, sample{11, 59},
+				sample{13, 99}, sample{15, 99}, sample{16, 99}, sample{17, 99},
+				sample{20, 0}, sample{22, 0},
+			}},
+		},
+		// Case 5: series are merged properly when there are multiple series.
+		// |-------------------------------------|
+		//            |------------|
+		//      |-------------------------|
+		{
+			blockSeries: [][]Series{
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{8, 0}, sample{9, 0}, sample{10, 0},
+						sample{13, 0}, sample{15, 0}, sample{16, 0}, sample{17, 0},
+						sample{20, 0}, sample{22, 0},
+					}),
+					newSeries(map[string]string{"b": "c"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{8, 0}, sample{9, 0}, sample{10, 0},
+						sample{13, 0}, sample{15, 0}, sample{16, 0}, sample{17, 0},
+						sample{20, 0}, sample{22, 0},
+					}),
+					newSeries(map[string]string{"c": "d"}, []tsdbutil.Sample{
+						sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+						sample{5, 0}, sample{8, 0}, sample{9, 0}, sample{10, 0},
+						sample{13, 0}, sample{15, 0}, sample{16, 0}, sample{17, 0},
+						sample{20, 0}, sample{22, 0},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"__name__": "a"}, []tsdbutil.Sample{
+						sample{7, 59}, sample{8, 59}, sample{9, 59}, sample{10, 59},
+						sample{11, 59},
+					}),
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{7, 59}, sample{8, 59}, sample{9, 59}, sample{10, 59},
+						sample{11, 59},
+					}),
+					newSeries(map[string]string{"aa": "bb"}, []tsdbutil.Sample{
+						sample{7, 59}, sample{8, 59}, sample{9, 59}, sample{10, 59},
+						sample{11, 59},
+					}),
+					newSeries(map[string]string{"c": "d"}, []tsdbutil.Sample{
+						sample{7, 59}, sample{8, 59}, sample{9, 59}, sample{10, 59},
+						sample{11, 59},
+					}),
+				},
+				[]Series{
+					newSeries(map[string]string{"a": "b"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{8, 99},
+						sample{9, 99}, sample{10, 99}, sample{13, 99}, sample{15, 99},
+						sample{16, 99}, sample{17, 99},
+					}),
+					newSeries(map[string]string{"aa": "bb"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{8, 99},
+						sample{9, 99}, sample{10, 99}, sample{13, 99}, sample{15, 99},
+						sample{16, 99}, sample{17, 99},
+					}),
+					newSeries(map[string]string{"c": "d"}, []tsdbutil.Sample{
+						sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{8, 99},
+						sample{9, 99}, sample{10, 99}, sample{13, 99}, sample{15, 99},
+						sample{16, 99}, sample{17, 99},
+					}),
+				},
+			},
+			expSeries: map[string][]tsdbutil.Sample{
+				`{__name__="a"}`: {
+					sample{7, 59}, sample{8, 59}, sample{9, 59}, sample{10, 59},
+					sample{11, 59},
+				},
+				`{a="b"}`: {
+					sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{3, 99},
+					sample{4, 0}, sample{5, 99}, sample{6, 99}, sample{7, 59},
+					sample{8, 59}, sample{9, 59}, sample{10, 59}, sample{11, 59},
+					sample{13, 99}, sample{15, 99}, sample{16, 99}, sample{17, 99},
+					sample{20, 0}, sample{22, 0},
+				},
+				`{aa="bb"}`: {
+					sample{3, 99}, sample{5, 99}, sample{6, 99}, sample{7, 59},
+					sample{8, 59}, sample{9, 59}, sample{10, 59}, sample{11, 59},
+					sample{13, 99}, sample{15, 99}, sample{16, 99}, sample{17, 99},
+				},
+				`{b="c"}`: {
+					sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{4, 0},
+					sample{5, 0}, sample{8, 0}, sample{9, 0}, sample{10, 0},
+					sample{13, 0}, sample{15, 0}, sample{16, 0}, sample{17, 0},
+					sample{20, 0}, sample{22, 0},
+				},
+				`{c="d"}`: {
+					sample{0, 0}, sample{1, 0}, sample{2, 0}, sample{3, 99},
+					sample{4, 0}, sample{5, 99}, sample{6, 99}, sample{7, 59},
+					sample{8, 59}, sample{9, 59}, sample{10, 59}, sample{11, 59},
+					sample{13, 99}, sample{15, 99}, sample{16, 99}, sample{17, 99},
+					sample{20, 0}, sample{22, 0},
+				},
+			},
+		},
+	}
+
+	defaultMatcher := labels.NewMustRegexpMatcher("__name__", ".*")
+	for _, c := range cases {
+		if ok := t.Run("", func(t *testing.T) {
+
+			tmpdir, err := ioutil.TempDir("", "data")
+			testutil.Ok(t, err)
+			defer func() {
+				testutil.Ok(t, os.RemoveAll(tmpdir))
+			}()
+
+			for _, series := range c.blockSeries {
+				createBlock(t, tmpdir, series)
+			}
+			db, err := Open(tmpdir, nil, nil, nil)
+			testutil.Ok(t, err)
+			defer func() {
+				testutil.Ok(t, db.Close())
+			}()
+			db.DisableCompactions()
+			testutil.Assert(t, len(db.blocks) == len(c.blockSeries), "Wrong number of blocks [before compact].")
+
+			// Vertical Query Merging test.
+			querier, err := db.Querier(0, 100)
+			testutil.Ok(t, err)
+			actSeries := query(t, querier, defaultMatcher)
+			testutil.Equals(t, c.expSeries, actSeries)
+
+			// Vertical compaction.
+			lc := db.compactor.(*LeveledCompactor)
+			testutil.Equals(t, 0, int(prom_testutil.ToFloat64(lc.metrics.overlappingBlocks)), "overlapping blocks count should be still 0 here")
+			err = db.compact()
+			testutil.Ok(t, err)
+			testutil.Equals(t, 1, len(db.Blocks()), "Wrong number of blocks [after compact]")
+
+			testutil.Equals(t, 1, int(prom_testutil.ToFloat64(lc.metrics.overlappingBlocks)), "overlapping blocks count mismatch")
+
+			// Query test after merging the overlapping blocks.
+			querier, err = db.Querier(0, 100)
+			testutil.Ok(t, err)
+			actSeries = query(t, querier, defaultMatcher)
+			testutil.Equals(t, c.expSeries, actSeries)
+		}); !ok {
+			return
+		}
+	}
+}
+
 // TestBlockRanges checks the following use cases:
 //  - No samples can be added with timestamps lower than the last block maxt.
 //  - The compactor doesn't create overlapping blocks
--- a/head.go
+++ b/head.go
@ -616,6 +616,14 @@ func (h *rangeHead) Tombstones() (TombstoneReader, error) {
 	return emptyTombstoneReader, nil
 }

+func (h *rangeHead) MinTime() int64 {
+	return h.mint
+}
+
+func (h *rangeHead) MaxTime() int64 {
+	return h.maxt
+}
+
 // initAppender is a helper to initialize the time bounds of the head
 // upon the first sample it receives.
 type initAppender struct {
--- a/head_test.go
+++ b/head_test.go
@ -489,7 +489,7 @@ func TestDeleteUntilCurMax(t *testing.T) {
 	it := exps.Iterator()
 	ressmpls, err := expandSeriesIterator(it)
 	testutil.Ok(t, err)
-	testutil.Equals(t, []sample{{11, 1}}, ressmpls)
+	testutil.Equals(t, []tsdbutil.Sample{sample{11, 1}}, ressmpls)
 }
 func TestDelete_e2e(t *testing.T) {
 	numDatapoints := 1000
@ -650,16 +650,16 @@ func TestDelete_e2e(t *testing.T) {
 	}
 }

-func boundedSamples(full []sample, mint, maxt int64) []sample {
+func boundedSamples(full []tsdbutil.Sample, mint, maxt int64) []tsdbutil.Sample {
 	for len(full) > 0 {
-		if full[0].t >= mint {
+		if full[0].T() >= mint {
 			break
 		}
 		full = full[1:]
 	}
 	for i, s := range full {
 		// labels.Labelinate on the first sample larger than maxt.
-		if s.t > maxt {
+		if s.T() > maxt {
 			return full[:i]
 		}
 	}
--- a/mocks_test.go
+++ b/mocks_test.go
@ -64,10 +64,14 @@ func (mockIndexWriter) WritePostings(name, value string, it index.Postings) erro
 func (mockIndexWriter) Close() error                                              { return nil }

 type mockBReader struct {
-	ir IndexReader
-	cr ChunkReader
+	ir   IndexReader
+	cr   ChunkReader
+	mint int64
+	maxt int64
 }

 func (r *mockBReader) Index() (IndexReader, error)          { return r.ir, nil }
 func (r *mockBReader) Chunks() (ChunkReader, error)         { return r.cr, nil }
 func (r *mockBReader) Tombstones() (TombstoneReader, error) { return newMemTombstones(), nil }
+func (r *mockBReader) MinTime() int64                       { return r.mint }
+func (r *mockBReader) MaxTime() int64                       { return r.maxt }
--- a/querier.go
+++ b/querier.go
@ -111,7 +111,6 @@ func (q *querier) LabelValuesFor(string, labels.Label) ([]string, error) {

 func (q *querier) Select(ms ...labels.Matcher) (SeriesSet, error) {
 	return q.sel(q.blocks, ms)
-
 }

 func (q *querier) sel(qs []Querier, ms []labels.Matcher) (SeriesSet, error) {
@ -143,6 +142,36 @@ func (q *querier) Close() error {
 	return merr.Err()
 }

+// verticalQuerier aggregates querying results from time blocks within
+// a single partition. The block time ranges can be overlapping.
+type verticalQuerier struct {
+	querier
+}
+
+func (q *verticalQuerier) Select(ms ...labels.Matcher) (SeriesSet, error) {
+	return q.sel(q.blocks, ms)
+}
+
+func (q *verticalQuerier) sel(qs []Querier, ms []labels.Matcher) (SeriesSet, error) {
+	if len(qs) == 0 {
+		return EmptySeriesSet(), nil
+	}
+	if len(qs) == 1 {
+		return qs[0].Select(ms...)
+	}
+	l := len(qs) / 2
+
+	a, err := q.sel(qs[:l], ms)
+	if err != nil {
+		return nil, err
+	}
+	b, err := q.sel(qs[l:], ms)
+	if err != nil {
+		return nil, err
+	}
+	return newMergedVerticalSeriesSet(a, b), nil
+}
+
 // NewBlockQuerier returns a querier against the reader.
 func NewBlockQuerier(b BlockReader, mint, maxt int64) (Querier, error) {
 	indexr, err := b.Index()
@ -444,6 +473,72 @@ func (s *mergedSeriesSet) Next() bool {
 	return true
 }

+type mergedVerticalSeriesSet struct {
+	a, b         SeriesSet
+	cur          Series
+	adone, bdone bool
+}
+
+// NewMergedVerticalSeriesSet takes two series sets as a single series set.
+// The input series sets must be sorted and
+// the time ranges of the series can be overlapping.
+func NewMergedVerticalSeriesSet(a, b SeriesSet) SeriesSet {
+	return newMergedVerticalSeriesSet(a, b)
+}
+
+func newMergedVerticalSeriesSet(a, b SeriesSet) *mergedVerticalSeriesSet {
+	s := &mergedVerticalSeriesSet{a: a, b: b}
+	// Initialize first elements of both sets as Next() needs
+	// one element look-ahead.
+	s.adone = !s.a.Next()
+	s.bdone = !s.b.Next()
+
+	return s
+}
+
+func (s *mergedVerticalSeriesSet) At() Series {
+	return s.cur
+}
+
+func (s *mergedVerticalSeriesSet) Err() error {
+	if s.a.Err() != nil {
+		return s.a.Err()
+	}
+	return s.b.Err()
+}
+
+func (s *mergedVerticalSeriesSet) compare() int {
+	if s.adone {
+		return 1
+	}
+	if s.bdone {
+		return -1
+	}
+	return labels.Compare(s.a.At().Labels(), s.b.At().Labels())
+}
+
+func (s *mergedVerticalSeriesSet) Next() bool {
+	if s.adone && s.bdone || s.Err() != nil {
+		return false
+	}
+
+	d := s.compare()
+
+	// Both sets contain the current series. Chain them into a single one.
+	if d > 0 {
+		s.cur = s.b.At()
+		s.bdone = !s.b.Next()
+	} else if d < 0 {
+		s.cur = s.a.At()
+		s.adone = !s.a.Next()
+	} else {
+		s.cur = &verticalChainedSeries{series: []Series{s.a.At(), s.b.At()}}
+		s.adone = !s.a.Next()
+		s.bdone = !s.b.Next()
+	}
+	return true
+}
+
 // ChunkSeriesSet exposes the chunks and intervals of a series instead of the
 // actual series itself.
 type ChunkSeriesSet interface {
@ -739,6 +834,100 @@ func (it *chainedSeriesIterator) Err() error {
 	return it.cur.Err()
 }

+// verticalChainedSeries implements a series for a list of time-sorted, time-overlapping series.
+// They all must have the same labels.
+type verticalChainedSeries struct {
+	series []Series
+}
+
+func (s *verticalChainedSeries) Labels() labels.Labels {
+	return s.series[0].Labels()
+}
+
+func (s *verticalChainedSeries) Iterator() SeriesIterator {
+	return newVerticalMergeSeriesIterator(s.series...)
+}
+
+// verticalMergeSeriesIterator implements a series iterater over a list
+// of time-sorted, time-overlapping iterators.
+type verticalMergeSeriesIterator struct {
+	a, b                  SeriesIterator
+	aok, bok, initialized bool
+
+	curT int64
+	curV float64
+}
+
+func newVerticalMergeSeriesIterator(s ...Series) SeriesIterator {
+	if len(s) == 1 {
+		return s[0].Iterator()
+	} else if len(s) == 2 {
+		return &verticalMergeSeriesIterator{
+			a: s[0].Iterator(),
+			b: s[1].Iterator(),
+		}
+	}
+	return &verticalMergeSeriesIterator{
+		a: s[0].Iterator(),
+		b: newVerticalMergeSeriesIterator(s[1:]...),
+	}
+}
+
+func (it *verticalMergeSeriesIterator) Seek(t int64) bool {
+	it.aok, it.bok = it.a.Seek(t), it.b.Seek(t)
+	it.initialized = true
+	return it.Next()
+}
+
+func (it *verticalMergeSeriesIterator) Next() bool {
+	if !it.initialized {
+		it.aok = it.a.Next()
+		it.bok = it.b.Next()
+		it.initialized = true
+	}
+
+	if !it.aok && !it.bok {
+		return false
+	}
+
+	if !it.aok {
+		it.curT, it.curV = it.b.At()
+		it.bok = it.b.Next()
+		return true
+	}
+	if !it.bok {
+		it.curT, it.curV = it.a.At()
+		it.aok = it.a.Next()
+		return true
+	}
+
+	acurT, acurV := it.a.At()
+	bcurT, bcurV := it.b.At()
+	if acurT < bcurT {
+		it.curT, it.curV = acurT, acurV
+		it.aok = it.a.Next()
+	} else if acurT > bcurT {
+		it.curT, it.curV = bcurT, bcurV
+		it.bok = it.b.Next()
+	} else {
+		it.curT, it.curV = bcurT, bcurV
+		it.aok = it.a.Next()
+		it.bok = it.b.Next()
+	}
+	return true
+}
+
+func (it *verticalMergeSeriesIterator) At() (t int64, v float64) {
+	return it.curT, it.curV
+}
+
+func (it *verticalMergeSeriesIterator) Err() error {
+	if it.a.Err() != nil {
+		return it.a.Err()
+	}
+	return it.b.Err()
+}
+
 // chunkSeriesIterator implements a series iterator on top
 // of a list of time-sorted, non-overlapping chunks.
 type chunkSeriesIterator struct {
--- a/querier_test.go
+++ b/querier_test.go
@ -178,7 +178,7 @@ Outer:
 	}
 }

-func expandSeriesIterator(it SeriesIterator) (r []sample, err error) {
+func expandSeriesIterator(it SeriesIterator) (r []tsdbutil.Sample, err error) {
 	for it.Next() {
 		t, v := it.At()
 		r = append(r, sample{t: t, v: v})
@ -194,7 +194,7 @@ type seriesSamples struct {

 // Index: labels -> postings -> chunkMetas -> chunkRef
 // ChunkReader: ref -> vals
-func createIdxChkReaders(tc []seriesSamples) (IndexReader, ChunkReader) {
+func createIdxChkReaders(tc []seriesSamples) (IndexReader, ChunkReader, int64, int64) {
 	sort.Slice(tc, func(i, j int) bool {
 		return labels.Compare(labels.FromMap(tc[i].lset), labels.FromMap(tc[i].lset)) < 0
 	})
@ -203,6 +203,8 @@ func createIdxChkReaders(tc []seriesSamples) (IndexReader, ChunkReader) {
 	chkReader := mockChunkReader(make(map[uint64]chunkenc.Chunk))
 	lblIdx := make(map[string]stringset)
 	mi := newMockIndex()
+	blockMint := int64(math.MaxInt64)
+	blockMaxt := int64(math.MinInt64)

 	for i, s := range tc {
 		i = i + 1 // 0 is not a valid posting.
@ -211,6 +213,13 @@ func createIdxChkReaders(tc []seriesSamples) (IndexReader, ChunkReader) {
 			// Collisions can be there, but for tests, its fine.
 			ref := rand.Uint64()

+			if chk[0].t < blockMint {
+				blockMint = chk[0].t
+			}
+			if chk[len(chk)-1].t > blockMaxt {
+				blockMaxt = chk[len(chk)-1].t
+			}
+
 			metas = append(metas, chunks.Meta{
 				MinTime: chk[0].t,
 				MaxTime: chk[len(chk)-1].t,
@ -248,7 +257,7 @@ func createIdxChkReaders(tc []seriesSamples) (IndexReader, ChunkReader) {
 		return mi.WritePostings(l.Name, l.Value, p)
 	})

-	return mi, chkReader
+	return mi, chkReader, blockMint, blockMaxt
 }

 func TestBlockQuerier(t *testing.T) {
@ -355,7 +364,7 @@ func TestBlockQuerier(t *testing.T) {

 Outer:
 	for _, c := range cases.queries {
-		ir, cr := createIdxChkReaders(cases.data)
+		ir, cr, _, _ := createIdxChkReaders(cases.data)
 		querier := &blockQuerier{
 			index:      ir,
 			chunks:     cr,
@ -517,7 +526,7 @@ func TestBlockQuerierDelete(t *testing.T) {

 Outer:
 	for _, c := range cases.queries {
-		ir, cr := createIdxChkReaders(cases.data)
+		ir, cr, _, _ := createIdxChkReaders(cases.data)
 		querier := &blockQuerier{
 			index:      ir,
 			chunks:     cr,
@ -924,6 +933,46 @@ func TestSeriesIterator(t *testing.T) {
 	})

 	t.Run("Chain", func(t *testing.T) {
+		// Extra cases for overlapping series.
+		itcasesExtra := []struct {
+			a, b, c    []tsdbutil.Sample
+			exp        []tsdbutil.Sample
+			mint, maxt int64
+		}{
+			{
+				a: []tsdbutil.Sample{
+					sample{1, 2}, sample{2, 3}, sample{3, 5}, sample{6, 1},
+				},
+				b: []tsdbutil.Sample{
+					sample{5, 49}, sample{7, 89}, sample{9, 8},
+				},
+				c: []tsdbutil.Sample{
+					sample{2, 33}, sample{4, 44}, sample{10, 3},
+				},
+
+				exp: []tsdbutil.Sample{
+					sample{1, 2}, sample{2, 33}, sample{3, 5}, sample{4, 44}, sample{5, 49}, sample{6, 1}, sample{7, 89}, sample{9, 8}, sample{10, 3},
+				},
+				mint: math.MinInt64,
+				maxt: math.MaxInt64,
+			},
+			{
+				a: []tsdbutil.Sample{
+					sample{1, 2}, sample{2, 3}, sample{9, 5}, sample{13, 1},
+				},
+				b: []tsdbutil.Sample{},
+				c: []tsdbutil.Sample{
+					sample{1, 23}, sample{2, 342}, sample{3, 25}, sample{6, 11},
+				},
+
+				exp: []tsdbutil.Sample{
+					sample{1, 23}, sample{2, 342}, sample{3, 25}, sample{6, 11}, sample{9, 5}, sample{13, 1},
+				},
+				mint: math.MinInt64,
+				maxt: math.MaxInt64,
+			},
+		}
+
 		for _, tc := range itcases {
 			a, b, c := itSeries{newListSeriesIterator(tc.a)},
 				itSeries{newListSeriesIterator(tc.b)},
@ -939,30 +988,55 @@ func TestSeriesIterator(t *testing.T) {
 			testutil.Equals(t, smplExp, smplRes)
 		}

+		for _, tc := range append(itcases, itcasesExtra...) {
+			a, b, c := itSeries{newListSeriesIterator(tc.a)},
+				itSeries{newListSeriesIterator(tc.b)},
+				itSeries{newListSeriesIterator(tc.c)}
+
+			res := newVerticalMergeSeriesIterator(a, b, c)
+			exp := newListSeriesIterator([]tsdbutil.Sample(tc.exp))
+
+			smplExp, errExp := expandSeriesIterator(exp)
+			smplRes, errRes := expandSeriesIterator(res)
+
+			testutil.Equals(t, errExp, errRes)
+			testutil.Equals(t, smplExp, smplRes)
+		}
+
 		t.Run("Seek", func(t *testing.T) {
 			for _, tc := range seekcases {
-				a, b, c := itSeries{newListSeriesIterator(tc.a)},
-					itSeries{newListSeriesIterator(tc.b)},
-					itSeries{newListSeriesIterator(tc.c)}
+				ress := []SeriesIterator{
+					newChainedSeriesIterator(
+						itSeries{newListSeriesIterator(tc.a)},
+						itSeries{newListSeriesIterator(tc.b)},
+						itSeries{newListSeriesIterator(tc.c)},
+					),
+					newVerticalMergeSeriesIterator(
+						itSeries{newListSeriesIterator(tc.a)},
+						itSeries{newListSeriesIterator(tc.b)},
+						itSeries{newListSeriesIterator(tc.c)},
+					),
+				}

-				res := newChainedSeriesIterator(a, b, c)
-				exp := newListSeriesIterator(tc.exp)
+				for _, res := range ress {
+					exp := newListSeriesIterator(tc.exp)

-				testutil.Equals(t, tc.success, res.Seek(tc.seek))
+					testutil.Equals(t, tc.success, res.Seek(tc.seek))

-				if tc.success {
-					// Init the list and then proceed to check.
-					remaining := exp.Next()
-					testutil.Assert(t, remaining == true, "")
+					if tc.success {
+						// Init the list and then proceed to check.
+						remaining := exp.Next()
+						testutil.Assert(t, remaining == true, "")

-					for remaining {
-						sExp, eExp := exp.At()
-						sRes, eRes := res.At()
-						testutil.Equals(t, eExp, eRes)
-						testutil.Equals(t, sExp, sRes)
+						for remaining {
+							sExp, eExp := exp.At()
+							sRes, eRes := res.At()
+							testutil.Equals(t, eExp, eRes)
+							testutil.Equals(t, sExp, sRes)

-						remaining = exp.Next()
-						testutil.Equals(t, remaining, res.Next())
+							remaining = exp.Next()
+							testutil.Equals(t, remaining, res.Next())
+						}
 					}
 				}
 			}
@ -1159,6 +1233,7 @@ func BenchmarkPersistedQueries(b *testing.B) {
 				dir, err := ioutil.TempDir("", "bench_persisted")
 				testutil.Ok(b, err)
 				defer os.RemoveAll(dir)
+
 				block, err := OpenBlock(nil, createBlock(b, dir, genSeries(nSeries, 10, 1, int64(nSamples))), nil)
 				testutil.Ok(b, err)
 				defer block.Close()
@ -1439,3 +1514,178 @@ func (it *listSeriesIterator) Seek(t int64) bool {
 func (it *listSeriesIterator) Err() error {
 	return nil
 }
+
+func BenchmarkQueryIterator(b *testing.B) {
+	cases := []struct {
+		numBlocks                   int
+		numSeries                   int
+		numSamplesPerSeriesPerBlock int
+		overlapPercentages          []int // >=0, <=100, this is w.r.t. the previous block.
+	}{
+		{
+			numBlocks:                   20,
+			numSeries:                   1000,
+			numSamplesPerSeriesPerBlock: 20000,
+			overlapPercentages:          []int{0, 10, 30},
+		},
+	}
+
+	for _, c := range cases {
+		for _, overlapPercentage := range c.overlapPercentages {
+			benchMsg := fmt.Sprintf("nBlocks=%d,nSeries=%d,numSamplesPerSeriesPerBlock=%d,overlap=%d%%",
+				c.numBlocks, c.numSeries, c.numSamplesPerSeriesPerBlock, overlapPercentage)
+
+			b.Run(benchMsg, func(b *testing.B) {
+				dir, err := ioutil.TempDir("", "bench_query_iterator")
+				testutil.Ok(b, err)
+				defer func() {
+					testutil.Ok(b, os.RemoveAll(dir))
+				}()
+
+				var (
+					blocks          []*Block
+					overlapDelta    = int64(overlapPercentage * c.numSamplesPerSeriesPerBlock / 100)
+					prefilledLabels []map[string]string
+					generatedSeries []Series
+				)
+				for i := int64(0); i < int64(c.numBlocks); i++ {
+					offset := i * overlapDelta
+					mint := i*int64(c.numSamplesPerSeriesPerBlock) - offset
+					maxt := mint + int64(c.numSamplesPerSeriesPerBlock) - 1
+					if len(prefilledLabels) == 0 {
+						generatedSeries = genSeries(c.numSeries, 10, mint, maxt)
+						for _, s := range generatedSeries {
+							prefilledLabels = append(prefilledLabels, s.Labels().Map())
+						}
+					} else {
+						generatedSeries = populateSeries(prefilledLabels, mint, maxt)
+					}
+					block, err := OpenBlock(nil, createBlock(b, dir, generatedSeries), nil)
+					testutil.Ok(b, err)
+					blocks = append(blocks, block)
+					defer block.Close()
+				}
+
+				que := &querier{
+					blocks: make([]Querier, 0, len(blocks)),
+				}
+				for _, blk := range blocks {
+					q, err := NewBlockQuerier(blk, math.MinInt64, math.MaxInt64)
+					testutil.Ok(b, err)
+					que.blocks = append(que.blocks, q)
+				}
+
+				var sq Querier = que
+				if overlapPercentage > 0 {
+					sq = &verticalQuerier{
+						querier: *que,
+					}
+				}
+				defer sq.Close()
+
+				b.ResetTimer()
+				b.ReportAllocs()
+
+				ss, err := sq.Select(labels.NewMustRegexpMatcher("__name__", ".*"))
+				testutil.Ok(b, err)
+				for ss.Next() {
+					it := ss.At().Iterator()
+					for it.Next() {
+					}
+					testutil.Ok(b, it.Err())
+				}
+				testutil.Ok(b, ss.Err())
+				testutil.Ok(b, err)
+			})
+		}
+	}
+}
+
+func BenchmarkQuerySeek(b *testing.B) {
+	cases := []struct {
+		numBlocks                   int
+		numSeries                   int
+		numSamplesPerSeriesPerBlock int
+		overlapPercentages          []int // >=0, <=100, this is w.r.t. the previous block.
+	}{
+		{
+			numBlocks:                   20,
+			numSeries:                   100,
+			numSamplesPerSeriesPerBlock: 2000,
+			overlapPercentages:          []int{0, 10, 30, 50},
+		},
+	}
+
+	for _, c := range cases {
+		for _, overlapPercentage := range c.overlapPercentages {
+			benchMsg := fmt.Sprintf("nBlocks=%d,nSeries=%d,numSamplesPerSeriesPerBlock=%d,overlap=%d%%",
+				c.numBlocks, c.numSeries, c.numSamplesPerSeriesPerBlock, overlapPercentage)
+
+			b.Run(benchMsg, func(b *testing.B) {
+				dir, err := ioutil.TempDir("", "bench_query_iterator")
+				testutil.Ok(b, err)
+				defer func() {
+					testutil.Ok(b, os.RemoveAll(dir))
+				}()
+
+				var (
+					blocks          []*Block
+					overlapDelta    = int64(overlapPercentage * c.numSamplesPerSeriesPerBlock / 100)
+					prefilledLabels []map[string]string
+					generatedSeries []Series
+				)
+				for i := int64(0); i < int64(c.numBlocks); i++ {
+					offset := i * overlapDelta
+					mint := i*int64(c.numSamplesPerSeriesPerBlock) - offset
+					maxt := mint + int64(c.numSamplesPerSeriesPerBlock) - 1
+					if len(prefilledLabels) == 0 {
+						generatedSeries = genSeries(c.numSeries, 10, mint, maxt)
+						for _, s := range generatedSeries {
+							prefilledLabels = append(prefilledLabels, s.Labels().Map())
+						}
+					} else {
+						generatedSeries = populateSeries(prefilledLabels, mint, maxt)
+					}
+					block, err := OpenBlock(nil, createBlock(b, dir, generatedSeries), nil)
+					testutil.Ok(b, err)
+					blocks = append(blocks, block)
+					defer block.Close()
+				}
+
+				que := &querier{
+					blocks: make([]Querier, 0, len(blocks)),
+				}
+				for _, blk := range blocks {
+					q, err := NewBlockQuerier(blk, math.MinInt64, math.MaxInt64)
+					testutil.Ok(b, err)
+					que.blocks = append(que.blocks, q)
+				}
+
+				var sq Querier = que
+				if overlapPercentage > 0 {
+					sq = &verticalQuerier{
+						querier: *que,
+					}
+				}
+				defer sq.Close()
+
+				mint := blocks[0].meta.MinTime
+				maxt := blocks[len(blocks)-1].meta.MaxTime
+
+				b.ResetTimer()
+				b.ReportAllocs()
+
+				ss, err := sq.Select(labels.NewMustRegexpMatcher("__name__", ".*"))
+				for ss.Next() {
+					it := ss.At().Iterator()
+					for t := mint; t <= maxt; t++ {
+						it.Seek(t)
+					}
+					testutil.Ok(b, it.Err())
+				}
+				testutil.Ok(b, ss.Err())
+				testutil.Ok(b, err)
+			})
+		}
+	}
+}