Order index segments in multi segments builder in descending order.

src/m3ninx/index/segment/builder/multi_segments_builder.go

@@ -138,6 +138,12 @@ func (b *builderFromSegments) AddSegments(segments []segment.Segment) error {
 		b.segmentsOffset += postings.ID(added)
 	}
 
+	// Sort segments in descending order in terms of size so the multi segments
+	// terms iter more efficiently builds its postings list.
+	sort.Slice(b.segments, func(i, j int) bool {
+		return b.segments[i].segment.Size() > b.segments[j].segment.Size()
+	})
+
 	// Make sure the terms iter has all the segments to combine data from
 	b.termsIter.reset(b.segments)
 

src/m3ninx/index/segment/builder/multi_segments_builder_test.go

@@ -0,0 +1,79 @@
+// Copyright (c) 2020 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package builder
+
+import (
+	"sort"
+	"testing"
+
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/require"
+
+	"github.com/m3db/m3/src/m3ninx/index"
+	"github.com/m3db/m3/src/m3ninx/index/segment"
+	"github.com/m3db/m3/src/m3ninx/postings"
+)
+
+// TestMultiSegmentsBuilderSortedBySizeDescending ensures that segments in the
+// multi segments builder are in descending order by size for optimal compaction.
+func TestMultiSegmentsBuilderSortedBySizeDescending(t *testing.T) {
+	ctrl := gomock.NewController(t)
+	defer ctrl.Finish()
+
+	b := NewBuilderFromSegments(NewOptions()).(*builderFromSegments)
+
+	numSegments := 5
+	segs := make([]segment.Segment, 0, numSegments)
+	for i := 1; i <= numSegments; i++ {
+		pi := postings.NewMockIterator(ctrl)
+		gomock.InOrder(
+			pi.EXPECT().Next().Return(false),
+			pi.EXPECT().Close().Return(nil),
+		)
+		r := segment.NewMockReader(ctrl)
+		it := index.NewIDDocIterator(nil, pi)
+		gomock.InOrder(
+			r.EXPECT().AllDocs().Return(it, nil),
+			r.EXPECT().Close().Return(nil),
+		)
+
+		seg := segment.NewMockSegment(ctrl)
+		seg.EXPECT().Reader().Return(r, nil)
+		seg.EXPECT().Size().Return(int64(i * 10)).AnyTimes()
+		seg.EXPECT().TermsIterable().Return(nil)
+		segs = append(segs, seg)
+	}
+	require.NoError(t, b.AddSegments(segs))
+
+	// Sort segments in descending order by size.
+	sort.Slice(segs, func(i, j int) bool {
+		return segs[i].Size() > segs[j].Size()
+	})
+
+	actualSegs := make([]segment.Segment, 0, numSegments)
+	for _, segMD := range b.segments {
+		actualSegs = append(actualSegs, segMD.segment)
+	}
+
+	for i := 0; i < numSegments; i++ {
+		require.Equal(t, segs[i].Size(), actualSegs[i].Size())
+	}
+}