diff --git a/cmd/mongo_struct.go b/cmd/mongo_struct.go
index afe4fee..7a3eeea 100644
--- a/cmd/mongo_struct.go
+++ b/cmd/mongo_struct.go
@@ -1,11 +1,24 @@
 package logmower
 
 import (
+	"context"
 	"time"
 
 	"go.mongodb.org/mongo-driver/bson"
+	"go.mongodb.org/mongo-driver/mongo"
 )
 
+func initializeIndexes(ctx context.Context, col *mongo.Collection) error {
+	ind := col.Indexes()
+
+	// (does not create duplicates)
+	_, err := ind.CreateOne(mongoTimeoutCtx(ctx), mongo.IndexModel{
+		Keys: bson.D{{Key: mongoKeyFileBasename, Value: 1}, {Key: mongoKeyOffset, Value: -1}},
+	})
+
+	return err
+}
+
 // when editing, also edit everything in this file!
 type mLog struct {
 	HostInfo    HostInfo
diff --git a/cmd/mower.go b/cmd/mower.go
index d5d2b87..da05476 100644
--- a/cmd/mower.go
+++ b/cmd/mower.go
@@ -124,6 +124,11 @@ var App = &cli.App{
 
 		state.db = dbClient.Database(uriParsed.Path).Collection("logs")
 
+		err = initializeIndexes(ctx.Context, state.db)
+		if err != nil {
+			l.Fatal("initializing indexes", zap.Error(err))
+		}
+
 		state.hostInfo, err = getHostInfo(ctx.String("node-name"))
 		if err != nil {
 			l.Fatal("gathering host info", zap.Error(err))