Efficient deletion of a Firestore collection.

main.go

package main
import (
	"context"
	"fmt"
	"sync" 

	"cloud.google.com/go/firestore"
	firebase "firebase.google.com/go"
)
var (
	gcpProject = "foo"
	collection = "bar"
)
func deleteCollection(ctx context.Context, client *firestore.Client,
	ref *firestore.CollectionRef) {
	type Deletes struct {
		sync.Mutex
		count int
	}
	var (
		lastSeen *firestore.DocumentSnapshot
		wg       sync.WaitGroup
		del      Deletes
	)
	for {
		q := ref.Select().OrderBy(firestore.DocumentID, firestore.Asc).Limit(500)
		if lastSeen != nil {
			q = q.StartAfter(lastSeen)
		}
		docs, err := q.Documents(ctx).GetAll()
		if err != nil || len(docs) == 0 {
			break
		}
		lastSeen = docs[len(docs)-1]
		wg.Add(1)
		go func(docs []*firestore.DocumentSnapshot) {
			defer wg.Done()
			numDeleted := 0
			batch := client.Batch()
			for _, doc := range docs {
				batch.Delete(doc.Ref)
				numDeleted++
			}
			if numDeleted == 0 {
				return
			}
			_, err := batch.Commit(ctx)
			if err == nil {
				del.Lock()
				del.count += len(docs)
				del.Unlock()
				fmt.Printf("\rDocuments deleted: %d", del.count)
			}
		}(docs)
	}
	wg.Wait()
	fmt.Println("\nDone!")
}
func main() {
	ctx := context.Background()
	conf := &firebase.Config{ProjectID: gcpProject}
	app, err := firebase.NewApp(ctx, conf)
	if err != nil {
		fmt.Println(err)
		return
	}
	client, err := app.Firestore(ctx)
	if err != nil {
		fmt.Println(err)
		return
	}
	deleteCollection(ctx, client, client.Collection(collection))
}