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.
@@ -448,8 +450,8 @@ 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)
 	}

cmd/tsdb/main.go

@@ -35,6 +35,7 @@ import (
 	"github.com/prometheus/tsdb"
 	"github.com/prometheus/tsdb/chunks"
 	tsdb_errors "github.com/prometheus/tsdb/errors"
+	"github.com/prometheus/tsdb/index"
 	"github.com/prometheus/tsdb/labels"
 	"gopkg.in/alecthomas/kingpin.v2"
 )
@@ -500,7 +501,8 @@ func analyzeBlock(b tsdb.BlockReader, limit int) error {
 	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 {
 		return err
 	}
@@ -596,7 +598,7 @@ func analyzeBlock(b tsdb.BlockReader, limit int) error {
 			return err
 		}
 		for _, n := range names {
-			postings, err := ir.Postings("__name__", n)
+			postings, err := ir.Postings("__name__", n, nil)
 			if err != nil {
 				return err
 			}