Attempt to reduce memory footprint of compaction

block.go

@@ -75,7 +75,9 @@ type IndexReader interface {
 	// The Postings here contain the offsets to the series inside the index.
 	// Found IDs are not strictly required to point to a valid Series, e.g. during
 	// background garbage collections.
-	Postings(name, value string) (index.Postings, error)
+	// 'reusePosts' is the Postings object that can be re-used instead of allocating
+	// a new object.
+	Postings(name, value string, reusePosts index.Postings) (index.Postings, error)
 
 	// SortedPostings returns a postings list that is reordered to be sorted
 	// by the label set of the underlying series.
@@ -143,6 +145,9 @@ type BlockReader interface {
 
 	// MaxTime returns the max time of the block.
 	MaxTime() int64
+
+	// NumSeries returns the total number of series in the block.
+	NumSeries() uint64
 }
 
 // Appendable defines an entity to which data can be appended.
@@ -451,8 +456,8 @@ func (r blockIndexReader) LabelValues(names ...string) (index.StringTuples, erro
 	return st, errors.Wrapf(err, "block: %s", r.b.Meta().ULID)
 }
 
-func (r blockIndexReader) Postings(name, value string) (index.Postings, error) {
-	p, err := r.ir.Postings(name, value)
+func (r blockIndexReader) Postings(name, value string, reusePosts index.Postings) (index.Postings, error) {
+	p, err := r.ir.Postings(name, value, reusePosts)
 	if err != nil {
 		return p, errors.Wrapf(err, "block: %s", r.b.Meta().ULID)
 	}
@@ -648,6 +653,11 @@ func (pb *Block) LabelNames() ([]string, error) {
 	return pb.indexr.LabelNames()
 }
 
+// NumSeries returns number of series in the block.
+func (pb *Block) NumSeries() uint64 {
+	return pb.meta.Stats.NumSeries
+}
+
 func clampInterval(a, b, mint, maxt int64) (int64, int64) {
 	if a < mint {
 		a = mint

cmd/tsdb/main.go

@@ -34,6 +34,7 @@ import (
 	"github.com/pkg/errors"
 	"github.com/prometheus/tsdb"
 	"github.com/prometheus/tsdb/chunks"
+	"github.com/prometheus/tsdb/index"
 	"github.com/prometheus/tsdb/labels"
 	"gopkg.in/alecthomas/kingpin.v2"
 )
@@ -456,7 +457,8 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 	labelpairsUncovered := map[string]uint64{}
 	labelpairsCount := map[string]uint64{}
 	entries := 0
-	p, err := ir.Postings("", "") // The special all key.
+	apkName, apkValue := index.AllPostingsKey()
+	p, err := ir.Postings(apkName, apkValue, nil) // The special all key.
 	if err != nil {
 		exitWithError(err)
 	}
@@ -552,7 +554,7 @@ func analyzeBlock(b *tsdb.Block, limit int) {
 			exitWithError(err)
 		}
 		for _, n := range names {
-			postings, err := ir.Postings("__name__", n)
+			postings, err := ir.Postings("__name__", n, nil)
 			if err != nil {
 				exitWithError(err)
 			}