From f0f7b798a4bbe31e58314d7c2e6fa7ad3fcbcdf7 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 10:23:14 +0100
Subject: [PATCH 01/38] WIP

---
 internal/backends/sqlite/collection.go | 28 ++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index 244de22084c3..1be3b404633a 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -164,9 +164,33 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 
 // Delete implements backends.Collection interface.
 func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams) (*backends.DeleteResult, error) {
-	// TODO https://github.com/FerretDB/FerretDB/issues/2751
+	db := c.r.DatabaseGetExisting(ctx, c.dbName)
+	if db == nil {
+		return nil, lazyerrors.Errorf("no database %q", c.dbName)
+	}
+
+	tableName := c.r.CollectionToTable(c.name)
+
+	query := fmt.Sprintf(`DELETE FROM %q WHERE json_extract(_ferretdb_sjson, '$._id') = ?`, tableName)
+
+	var deleted int64
+
+	for _, id := range params.IDs {
+		res, err := db.ExecContext(ctx, query, id)
+		if err != nil {
+			return nil, lazyerrors.Error(err)
+		}
+
+		rowsAffected, err := res.RowsAffected()
+		if err != nil {
+			return nil, lazyerrors.Error(err)
+		}
+
+		deleted += rowsAffected
+	}
+
 	return &backends.DeleteResult{
-		Deleted: 0,
+		Deleted: deleted,
 	}, nil
 }
 

From bf4fd41f966401629bc51182d866b70c62acf417 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 11:12:40 +0100
Subject: [PATCH 02/38] WIP

---
 internal/handlers/sqlite/msg_delete.go | 134 ++++++++++++++++++++++++-
 1 file changed, 133 insertions(+), 1 deletion(-)

diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 261b588d9f51..e3da3faa3268 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -16,12 +16,144 @@ package sqlite
 
 import (
 	"context"
+	"errors"
 
+	"github.com/FerretDB/FerretDB/internal/backends"
+	"github.com/FerretDB/FerretDB/internal/handlers/common"
+	"github.com/FerretDB/FerretDB/internal/handlers/commonerrors"
+	"github.com/FerretDB/FerretDB/internal/types"
+	"github.com/FerretDB/FerretDB/internal/util/iterator"
+	"github.com/FerretDB/FerretDB/internal/util/lazyerrors"
 	"github.com/FerretDB/FerretDB/internal/util/must"
 	"github.com/FerretDB/FerretDB/internal/wire"
 )
 
 // MsgDelete implements HandlerInterface.
 func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg, error) {
-	return nil, notImplemented(must.NotFail(msg.Document()).Command())
+	document, err := msg.Document()
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	params, err := common.GetDeleteParams(document, h.L)
+	if err != nil {
+		return nil, lazyerrors.Error(err)
+	}
+
+	var deleted int32
+	var delErrors commonerrors.WriteErrors
+
+	db := h.b.Database(params.DB)
+	defer db.Close()
+
+	coll := db.Collection(params.Collection)
+
+	// process every delete filter
+	for i, deleteParams := range params.Deletes {
+		del, err := execDelete(ctx, &execDeleteParams{
+			coll,
+			deleteParams.Filter,
+			params.Collection,
+			deleteParams.Limited,
+		})
+		if err == nil {
+			deleted += del
+			continue
+		}
+
+		delErrors.Append(err, int32(i))
+
+		if params.Ordered {
+			break
+		}
+	}
+
+	replyDoc := must.NotFail(types.NewDocument(
+		"ok", float64(1),
+	))
+
+	if delErrors.Len() > 0 {
+		replyDoc = delErrors.Document()
+	}
+
+	replyDoc.Set("n", deleted)
+
+	var reply wire.OpMsg
+	must.NoError(reply.SetSections(wire.OpMsgSection{
+		Documents: []*types.Document{replyDoc},
+	}))
+
+	return &reply, nil
+}
+
+// execDeleteParams contains parameters for execDelete function.
+type execDeleteParams struct {
+	coll       backends.Collection
+	filter     *types.Document
+	collection string
+	limited    bool
+}
+
+// execDelete fetches documents, filters them out, limits them (if needed) and deletes them.
+// If limit is true, only the first matched document is chosen for deletion, otherwise all matched documents are chosen.
+// It returns the number of deleted documents or an error.
+func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
+	var deleted int32
+
+	// filter is used to filter documents on the FerretDB side,
+	// qp.Filter is used to filter documents on the PostgreSQL side (query pushdown).
+	filter := dp.filter
+
+	// query documents here
+	res, err := dp.coll.Query(ctx, nil)
+	if err != nil {
+		return 0, err
+	}
+
+	iter := res.Iter
+
+	defer iter.Close()
+
+	ids := make([]any, 0, 16)
+
+	for {
+		var doc *types.Document
+		if _, doc, err = iter.Next(); err != nil {
+			if errors.Is(err, iterator.ErrIteratorDone) {
+				break
+			}
+
+			return 0, err
+		}
+
+		var matches bool
+		if matches, err = common.FilterDocument(doc, filter); err != nil {
+			return 0, err
+		}
+
+		if !matches {
+			continue
+		}
+
+		ids = append(ids, must.NotFail(doc.Get("_id")))
+
+		// if limit is set, no need to fetch all the documents
+		if dp.limited {
+			break
+		}
+	}
+
+	// if no documents matched, there is nothing to delete
+	if len(ids) == 0 {
+		return 0, nil
+	}
+
+	deleteRes, err := dp.coll.Delete(ctx, &backends.DeleteParams{IDs: ids})
+	if err != nil {
+		return 0, err
+	}
+
+	deleted = int32(deleteRes.Deleted)
+
+	return deleted, nil
 }

From 687e56603ec7849d07a301ca190ab4dc227f1d22 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 11:53:39 +0100
Subject: [PATCH 03/38] WIP

---
 internal/backends/sqlite/collection.go | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index 1be3b404633a..e660eb69a996 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -19,6 +19,8 @@ import (
 	"errors"
 	"fmt"
 
+	"modernc.org/sqlite"
+
 	"github.com/FerretDB/FerretDB/internal/backends"
 	"github.com/FerretDB/FerretDB/internal/backends/sqlite/metadata"
 	"github.com/FerretDB/FerretDB/internal/handlers/sjson"
@@ -59,6 +61,14 @@ func (c *collection) Query(ctx context.Context, params *backends.QueryParams) (*
 
 	rows, err := db.QueryContext(ctx, query)
 	if err != nil {
+		var serr *sqlite.Error
+		// No such table returned, so we can return empty result.
+		if errors.As(err, &serr) && serr.Code() == 1 { // TODO: use constant to check error code
+			return &backends.QueryResult{
+				Iter: newQueryIterator(ctx, nil),
+			}, nil
+		}
+
 		return nil, lazyerrors.Error(err)
 	}
 

From ec75f20334c8878d2b63fe819984f7f58e1569dc Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 12:36:57 +0100
Subject: [PATCH 04/38] WIP

---
 internal/backends/sqlite/collection.go | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index e660eb69a996..c48da98981da 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -20,6 +20,7 @@ import (
 	"fmt"
 
 	"modernc.org/sqlite"
+	sqlitelib "modernc.org/sqlite/lib"
 
 	"github.com/FerretDB/FerretDB/internal/backends"
 	"github.com/FerretDB/FerretDB/internal/backends/sqlite/metadata"
@@ -61,12 +62,10 @@ func (c *collection) Query(ctx context.Context, params *backends.QueryParams) (*
 
 	rows, err := db.QueryContext(ctx, query)
 	if err != nil {
-		var serr *sqlite.Error
-		// No such table returned, so we can return empty result.
-		if errors.As(err, &serr) && serr.Code() == 1 { // TODO: use constant to check error code
-			return &backends.QueryResult{
-				Iter: newQueryIterator(ctx, nil),
-			}, nil
+		// No such table, return empty result.
+		var e *sqlite.Error
+		if errors.As(err, &e) && e.Code() == sqlitelib.SQLITE_ERROR {
+			return &backends.QueryResult{Iter: newQueryIterator(ctx, nil)}, nil
 		}
 
 		return nil, lazyerrors.Error(err)

From 3466a13f37458dc79936d1a8b1273ea25bf6e3f9 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 12:46:54 +0100
Subject: [PATCH 05/38] WIP

---
 internal/handlers/sqlite/msg_delete.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index e3da3faa3268..e1acdc747f63 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -148,6 +148,9 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		return 0, nil
 	}
 
+	// close iterator to free db connection.
+	iter.Close()
+
 	deleteRes, err := dp.coll.Delete(ctx, &backends.DeleteParams{IDs: ids})
 	if err != nil {
 		return 0, err

From 6002aeb4562424c92fff35537ed77a35a3714676 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 12:46:54 +0100
Subject: [PATCH 06/38] WIP

---
 internal/backends/sqlite/collection.go | 5 ++++-
 internal/handlers/sqlite/msg_delete.go | 3 +++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index c48da98981da..fb9ea5c11916 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -18,6 +18,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strings"
 
 	"modernc.org/sqlite"
 	sqlitelib "modernc.org/sqlite/lib"
@@ -185,7 +186,9 @@ func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams)
 	var deleted int64
 
 	for _, id := range params.IDs {
-		res, err := db.ExecContext(ctx, query, id)
+		idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
+
+		res, err := db.ExecContext(ctx, query, idArg)
 		if err != nil {
 			return nil, lazyerrors.Error(err)
 		}
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index e3da3faa3268..e1acdc747f63 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -148,6 +148,9 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		return 0, nil
 	}
 
+	// close iterator to free db connection.
+	iter.Close()
+
 	deleteRes, err := dp.coll.Delete(ctx, &backends.DeleteParams{IDs: ids})
 	if err != nil {
 		return 0, err

From d99302abcd8c5f1c775975af643849a48599d797 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 17:14:19 +0100
Subject: [PATCH 07/38] WIP

---
 internal/handlers/sqlite/msg_delete.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index e1acdc747f63..d489b6e05a0c 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -118,6 +118,7 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 
 	for {
 		var doc *types.Document
+
 		if _, doc, err = iter.Next(); err != nil {
 			if errors.Is(err, iterator.ErrIteratorDone) {
 				break
@@ -127,6 +128,7 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		}
 
 		var matches bool
+
 		if matches, err = common.FilterDocument(doc, filter); err != nil {
 			return 0, err
 		}

From f7edb5ab4cc77a97ccef4b74d85cb337f984e1d9 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 17:42:00 +0100
Subject: [PATCH 08/38] WIP

---
 internal/handlers/sqlite/msg_delete.go | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index d489b6e05a0c..04ec865bccc9 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -141,6 +141,8 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 
 		// if limit is set, no need to fetch all the documents
 		if dp.limited {
+			iter.Close()
+
 			break
 		}
 	}
@@ -150,9 +152,6 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		return 0, nil
 	}
 
-	// close iterator to free db connection.
-	iter.Close()
-
 	deleteRes, err := dp.coll.Delete(ctx, &backends.DeleteParams{IDs: ids})
 	if err != nil {
 		return 0, err

From b957c63a33f22e9260c1a76678824a9474fc11cf Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 17:43:12 +0100
Subject: [PATCH 09/38] WIP

---
 internal/backends/sqlite/query_iterator.go | 44 ++++++++++++----------
 internal/handlers/sqlite/msg_delete.go     |  8 ++--
 2 files changed, 27 insertions(+), 25 deletions(-)

diff --git a/internal/backends/sqlite/query_iterator.go b/internal/backends/sqlite/query_iterator.go
index 022068ecb447..3a87e1ffd15b 100644
--- a/internal/backends/sqlite/query_iterator.go
+++ b/internal/backends/sqlite/query_iterator.go
@@ -23,26 +23,29 @@ import (
 	"github.com/FerretDB/FerretDB/internal/types"
 	"github.com/FerretDB/FerretDB/internal/util/iterator"
 	"github.com/FerretDB/FerretDB/internal/util/lazyerrors"
+	"github.com/FerretDB/FerretDB/internal/util/observability"
 	"github.com/FerretDB/FerretDB/internal/util/resource"
 )
 
 // queryIterator implements iterator.Interface to fetch documents from the database.
-//
-//nolint:vet // for readability
 type queryIterator struct {
-	ctx context.Context
-
-	m    sync.Mutex
-	rows *sql.Rows
+	// the order of fields is weird to make the struct smaller due to alignment
 
+	ctx   context.Context
+	rows  *sql.Rows // protected by m
 	token *resource.Token
+	m     sync.Mutex
 }
 
 // newQueryIterator returns a new queryIterator for the given *sql.Rows.
 //
 // Iterator's Close method closes rows.
+// They are also closed by the Next method on any error, including context cancellation,
+// to make sure that the database connection is released as early as possible.
+// In that case, the iterator's Close method should still be called.
 //
 // Nil rows are possible and return already done iterator.
+// It still should be Close'd.
 func newQueryIterator(ctx context.Context, rows *sql.Rows) types.DocumentsIterator {
 	iter := &queryIterator{
 		ctx:   ctx,
@@ -55,15 +58,9 @@ func newQueryIterator(ctx context.Context, rows *sql.Rows) types.DocumentsIterat
 }
 
 // Next implements iterator.Interface.
-//
-// Errors (possibly wrapped) are:
-//   - iterator.ErrIteratorDone;
-//   - context.Canceled;
-//   - context.DeadlineExceeded;
-//   - something else.
-//
-// Otherwise, the next document is returned.
 func (iter *queryIterator) Next() (struct{}, *types.Document, error) {
+	defer observability.FuncCall(iter.ctx)()
+
 	iter.m.Lock()
 	defer iter.m.Unlock()
 
@@ -75,28 +72,31 @@ func (iter *queryIterator) Next() (struct{}, *types.Document, error) {
 	}
 
 	if err := context.Cause(iter.ctx); err != nil {
+		iter.close()
 		return unused, nil, lazyerrors.Error(err)
 	}
 
 	if !iter.rows.Next() {
-		if err := iter.rows.Err(); err != nil {
-			return unused, nil, lazyerrors.Error(err)
-		}
+		err := iter.rows.Err()
 
-		// to avoid context cancellation changing the next `Next()` error
-		// from `iterator.ErrIteratorDone` to `context.Canceled`
 		iter.close()
 
-		return unused, nil, iterator.ErrIteratorDone
+		if err == nil {
+			err = iterator.ErrIteratorDone
+		}
+
+		return unused, nil, lazyerrors.Error(err)
 	}
 
 	var b []byte
 	if err := iter.rows.Scan(&b); err != nil {
+		iter.close()
 		return unused, nil, lazyerrors.Error(err)
 	}
 
 	doc, err := sjson.Unmarshal(b)
 	if err != nil {
+		iter.close()
 		return unused, nil, lazyerrors.Error(err)
 	}
 
@@ -105,6 +105,8 @@ func (iter *queryIterator) Next() (struct{}, *types.Document, error) {
 
 // Close implements iterator.Interface.
 func (iter *queryIterator) Close() {
+	defer observability.FuncCall(iter.ctx)()
+
 	iter.m.Lock()
 	defer iter.m.Unlock()
 
@@ -115,6 +117,8 @@ func (iter *queryIterator) Close() {
 //
 // This should be called only when the caller already holds the mutex.
 func (iter *queryIterator) close() {
+	defer observability.FuncCall(iter.ctx)()
+
 	if iter.rows != nil {
 		iter.rows.Close()
 		iter.rows = nil
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 04ec865bccc9..5afea5911426 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -110,16 +110,14 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		return 0, err
 	}
 
-	iter := res.Iter
-
-	defer iter.Close()
+	defer res.Iter.Close()
 
 	ids := make([]any, 0, 16)
 
 	for {
 		var doc *types.Document
 
-		if _, doc, err = iter.Next(); err != nil {
+		if _, doc, err = res.Iter.Next(); err != nil {
 			if errors.Is(err, iterator.ErrIteratorDone) {
 				break
 			}
@@ -141,7 +139,7 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 
 		// if limit is set, no need to fetch all the documents
 		if dp.limited {
-			iter.Close()
+			res.Iter.Close()
 
 			break
 		}

From feb245c6842ab160365cb5305fec39ee7d953b4a Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Mon, 26 Jun 2023 17:46:34 +0100
Subject: [PATCH 10/38] WIP

---
 internal/handlers/sqlite/msg_delete.go | 27 +++++---------------------
 1 file changed, 5 insertions(+), 22 deletions(-)

diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 5afea5911426..75ce39bf9539 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -50,12 +50,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 
 	// process every delete filter
 	for i, deleteParams := range params.Deletes {
-		del, err := execDelete(ctx, &execDeleteParams{
-			coll,
-			deleteParams.Filter,
-			params.Collection,
-			deleteParams.Limited,
-		})
+		del, err := execDelete(ctx, coll, deleteParams.Filter, deleteParams.Limited)
 		if err == nil {
 			deleted += del
 			continue
@@ -86,26 +81,14 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	return &reply, nil
 }
 
-// execDeleteParams contains parameters for execDelete function.
-type execDeleteParams struct {
-	coll       backends.Collection
-	filter     *types.Document
-	collection string
-	limited    bool
-}
-
 // execDelete fetches documents, filters them out, limits them (if needed) and deletes them.
 // If limit is true, only the first matched document is chosen for deletion, otherwise all matched documents are chosen.
 // It returns the number of deleted documents or an error.
-func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
+func execDelete(ctx context.Context, coll backends.Collection, filter *types.Document, limited bool) (int32, error) {
 	var deleted int32
 
-	// filter is used to filter documents on the FerretDB side,
-	// qp.Filter is used to filter documents on the PostgreSQL side (query pushdown).
-	filter := dp.filter
-
 	// query documents here
-	res, err := dp.coll.Query(ctx, nil)
+	res, err := coll.Query(ctx, nil)
 	if err != nil {
 		return 0, err
 	}
@@ -138,7 +121,7 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		ids = append(ids, must.NotFail(doc.Get("_id")))
 
 		// if limit is set, no need to fetch all the documents
-		if dp.limited {
+		if limited {
 			res.Iter.Close()
 
 			break
@@ -150,7 +133,7 @@ func execDelete(ctx context.Context, dp *execDeleteParams) (int32, error) {
 		return 0, nil
 	}
 
-	deleteRes, err := dp.coll.Delete(ctx, &backends.DeleteParams{IDs: ids})
+	deleteRes, err := coll.Delete(ctx, &backends.DeleteParams{IDs: ids})
 	if err != nil {
 		return 0, err
 	}

From 7d847361404e240cff1347693e755eb129217a6a Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 10:33:45 +0100
Subject: [PATCH 11/38] WIP

---
 internal/backends/sqlite/collection.go | 4 ++--
 internal/handlers/sqlite/msg_delete.go | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index fb9ea5c11916..44c65e178720 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -154,7 +154,7 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 		id, _ := doc.Get("_id")
 		must.NotBeZero(id)
 		docArg := must.NotFail(sjson.Marshal(doc))
-		idArg := string(must.NotFail(sjson.MarshalSingleValue(id)))
+		idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
 
 		r, err := db.ExecContext(ctx, query, docArg, idArg)
 		if err != nil {
@@ -176,7 +176,7 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams) (*backends.DeleteResult, error) {
 	db := c.r.DatabaseGetExisting(ctx, c.dbName)
 	if db == nil {
-		return nil, lazyerrors.Errorf("no database %q", c.dbName)
+		return nil, nil
 	}
 
 	tableName := c.r.CollectionToTable(c.name)
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 75ce39bf9539..f9d3964311d9 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -95,7 +95,7 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 
 	defer res.Iter.Close()
 
-	ids := make([]any, 0, 16)
+	ids := make([]any, 0, 0)
 
 	for {
 		var doc *types.Document

From 164be8d1963fbe7f3904a4cc3bbfff012d97d94a Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 11:36:45 +0100
Subject: [PATCH 12/38] WIP

---
 internal/backends/sqlite/collection.go | 3 ++-
 internal/handlers/sqlite/msg_delete.go | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index 44c65e178720..d4d77ade8a2a 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -186,7 +186,8 @@ func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams)
 	var deleted int64
 
 	for _, id := range params.IDs {
-		idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
+		//idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
+		idArg := string(must.NotFail(sjson.MarshalSingleValue(id)))
 
 		res, err := db.ExecContext(ctx, query, idArg)
 		if err != nil {
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index f9d3964311d9..936a4e7c2ea0 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -95,7 +95,7 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 
 	defer res.Iter.Close()
 
-	ids := make([]any, 0, 0)
+	ids := make([]any, 0)
 
 	for {
 		var doc *types.Document

From f8da6424ccff649e8109f07ff441679e808004ed Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 11:37:07 +0100
Subject: [PATCH 13/38] WIP

---
 internal/backends/sqlite/collection.go | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index d4d77ade8a2a..44c65e178720 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -186,8 +186,7 @@ func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams)
 	var deleted int64
 
 	for _, id := range params.IDs {
-		//idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
-		idArg := string(must.NotFail(sjson.MarshalSingleValue(id)))
+		idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
 
 		res, err := db.ExecContext(ctx, query, idArg)
 		if err != nil {

From ac4e4731f3efc1b790bd895cf5c8847e39220175 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 11:46:57 +0100
Subject: [PATCH 14/38] WIP

---
 internal/backends/collection.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/internal/backends/collection.go b/internal/backends/collection.go
index 957261b98c31..5de821cd0ceb 100644
--- a/internal/backends/collection.go
+++ b/internal/backends/collection.go
@@ -68,6 +68,8 @@ type QueryResult struct {
 
 // Query executes a query against the collection.
 //
+// If the collection is not exist it returns empty iterator.
+//
 // The passed context should be used for canceling the initial query.
 // It also can be used to close the returned iterator and free underlying resources,
 // but doing so is not necessary - the handler will do that anyway.

From a95761ebb78258badc97e0e25136805a88f7bb55 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 11:48:34 +0100
Subject: [PATCH 15/38] WIP

---
 internal/backends/sqlite/collection.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index 44c65e178720..7c0cd47751b6 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -176,7 +176,7 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams) (*backends.DeleteResult, error) {
 	db := c.r.DatabaseGetExisting(ctx, c.dbName)
 	if db == nil {
-		return nil, nil
+		return &backends.DeleteResult{Deleted: 0}, nil
 	}
 
 	tableName := c.r.CollectionToTable(c.name)

From 73ec4959eaacc1ef0a841e1e427acf3d4ab4106f Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 11:49:43 +0100
Subject: [PATCH 16/38] WIP

---
 internal/backends/collection.go | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/internal/backends/collection.go b/internal/backends/collection.go
index 5de821cd0ceb..392b4b0c3c7a 100644
--- a/internal/backends/collection.go
+++ b/internal/backends/collection.go
@@ -134,6 +134,8 @@ type DeleteResult struct {
 }
 
 // Delete deletes documents in collection.
+//
+// If request database is not exist it returns 0 deleted documents.
 func (cc *collectionContract) Delete(ctx context.Context, params *DeleteParams) (res *DeleteResult, err error) {
 	defer observability.FuncCall(ctx)()
 	defer checkError(err)

From 81b95957b78627e1a85676a068363235a31ddfe7 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 14:47:32 +0100
Subject: [PATCH 17/38] WIP

---
 internal/handlers/sqlite/msg_delete.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 936a4e7c2ea0..c077c5e5fa34 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -95,7 +95,7 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 
 	defer res.Iter.Close()
 
-	ids := make([]any, 0)
+	var ids []any
 
 	for {
 		var doc *types.Document

From 3788becca4af36934cbca1a01f83c092000a423d Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Tue, 27 Jun 2023 17:55:04 +0100
Subject: [PATCH 18/38] Apply suggestions from code review

Co-authored-by: Elena Grahovac <elena@grahovac.me>
---
 internal/backends/collection.go | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/internal/backends/collection.go b/internal/backends/collection.go
index 392b4b0c3c7a..e8cb1e9f8824 100644
--- a/internal/backends/collection.go
+++ b/internal/backends/collection.go
@@ -68,7 +68,7 @@ type QueryResult struct {
 
 // Query executes a query against the collection.
 //
-// If the collection is not exist it returns empty iterator.
+// If the collection does not exist it returns empty iterator.
 //
 // The passed context should be used for canceling the initial query.
 // It also can be used to close the returned iterator and free underlying resources,
@@ -135,7 +135,7 @@ type DeleteResult struct {
 
 // Delete deletes documents in collection.
 //
-// If request database is not exist it returns 0 deleted documents.
+// If request database does not exist it returns 0 deleted documents.
 func (cc *collectionContract) Delete(ctx context.Context, params *DeleteParams) (res *DeleteResult, err error) {
 	defer observability.FuncCall(ctx)()
 	defer checkError(err)

From 3b713728850bdda50fc6a3c0a28a0c2dc8744e9d Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 10:21:33 +0100
Subject: [PATCH 19/38] WIP

---
 internal/backends/sqlite/collection.go | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/internal/backends/sqlite/collection.go b/internal/backends/sqlite/collection.go
index 7c0cd47751b6..3dacc3b953f4 100644
--- a/internal/backends/sqlite/collection.go
+++ b/internal/backends/sqlite/collection.go
@@ -18,7 +18,6 @@ import (
 	"context"
 	"errors"
 	"fmt"
-	"strings"
 
 	"modernc.org/sqlite"
 	sqlitelib "modernc.org/sqlite/lib"
@@ -129,7 +128,7 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 
 	tableName := c.r.CollectionToTable(c.name)
 
-	query := fmt.Sprintf(`UPDATE %q SET _ferretdb_sjson = ? WHERE json_extract(_ferretdb_sjson, '$._id') = ?`, tableName)
+	query := fmt.Sprintf(`UPDATE %q SET _ferretdb_sjson = ? WHERE _ferretdb_sjson -> '$._id' = ?`, tableName)
 
 	var res backends.UpdateResult
 
@@ -154,7 +153,7 @@ func (c *collection) Update(ctx context.Context, params *backends.UpdateParams)
 		id, _ := doc.Get("_id")
 		must.NotBeZero(id)
 		docArg := must.NotFail(sjson.Marshal(doc))
-		idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
+		idArg := string(must.NotFail(sjson.MarshalSingleValue(id)))
 
 		r, err := db.ExecContext(ctx, query, docArg, idArg)
 		if err != nil {
@@ -181,12 +180,12 @@ func (c *collection) Delete(ctx context.Context, params *backends.DeleteParams)
 
 	tableName := c.r.CollectionToTable(c.name)
 
-	query := fmt.Sprintf(`DELETE FROM %q WHERE json_extract(_ferretdb_sjson, '$._id') = ?`, tableName)
+	query := fmt.Sprintf(`DELETE FROM %q WHERE _ferretdb_sjson -> '$._id' = ?`, tableName)
 
 	var deleted int64
 
 	for _, id := range params.IDs {
-		idArg := strings.ReplaceAll(string(must.NotFail(sjson.MarshalSingleValue(id))), "\"", "")
+		idArg := string(must.NotFail(sjson.MarshalSingleValue(id)))
 
 		res, err := db.ExecContext(ctx, query, idArg)
 		if err != nil {

From 3d03875319f2550dc82081bcbf71c4411a9fc23e Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 10:40:32 +0100
Subject: [PATCH 20/38] WIP

---
 integration/delete_test.go | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/integration/delete_test.go b/integration/delete_test.go
index a45e6af3edd8..c20fa5b946fb 100644
--- a/integration/delete_test.go
+++ b/integration/delete_test.go
@@ -138,3 +138,21 @@ func TestDelete(t *testing.T) {
 		})
 	}
 }
+
+func TestDeleteNotExistingDatabase(t *testing.T) {
+	t.Parallel()
+
+	ctx, collection := setup.Setup(t)
+
+	db := collection.Database().Client().Database("doesnotexist")
+
+	var res bson.D
+	err := db.RunCommand(ctx, bson.D{
+		{"delete", "test"},
+		{"deletes", bson.A{bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}}}},
+	}).Decode(&res)
+
+	assert.NoError(t, err)
+
+	assert.Equal(t, int32(0), res.Map()["n"])
+}

From 73fed900c2a12b1fe6ad430b2b04356bcd2bf701 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 11:55:18 +0100
Subject: [PATCH 21/38] WIP

---
 integration/basic_test.go | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/integration/basic_test.go b/integration/basic_test.go
index 7906d97f4dc0..a1df4ec00975 100644
--- a/integration/basic_test.go
+++ b/integration/basic_test.go
@@ -15,6 +15,7 @@
 package integration
 
 import (
+	"errors"
 	"fmt"
 	"math"
 	"strings"
@@ -25,7 +26,6 @@ import (
 	"go.mongodb.org/mongo-driver/bson"
 	"go.mongodb.org/mongo-driver/mongo"
 	"go.mongodb.org/mongo-driver/mongo/options"
-	"go.mongodb.org/mongo-driver/x/mongo/driver"
 
 	"github.com/FerretDB/FerretDB/integration/setup"
 	"github.com/FerretDB/FerretDB/integration/shareddata"
@@ -524,8 +524,7 @@ func TestDatabaseName(t *testing.T) {
 		ctx, collection := setup.Setup(t)
 
 		err := collection.Database().Client().Database("").CreateCollection(ctx, collection.Name())
-		expectedErr := driver.InvalidOperationError(driver.InvalidOperationError{MissingField: "Database"})
-		assert.Equal(t, expectedErr, err)
+		assert.Equal(t, errors.New("database name cannot be empty"), err)
 	})
 
 	t.Run("63ok", func(t *testing.T) {

From 98de1cff801162bf9eb439aa42cd8da8a555b7d1 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 12:03:10 +0100
Subject: [PATCH 22/38] WIP

---
 integration/delete_test.go | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/integration/delete_test.go b/integration/delete_test.go
index c20fa5b946fb..218207b99d5a 100644
--- a/integration/delete_test.go
+++ b/integration/delete_test.go
@@ -154,5 +154,7 @@ func TestDeleteNotExistingDatabase(t *testing.T) {
 
 	assert.NoError(t, err)
 
-	assert.Equal(t, int32(0), res.Map()["n"])
+	expectedRes := bson.D{{"ok", float64(1)}, {"n", int32(0)}}
+
+	assert.Equal(t, expectedRes, res)
 }

From bebad9b9baa75599473631b07528f7706eb7d347 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 12:22:00 +0100
Subject: [PATCH 23/38] WIP

---
 integration/delete_compat_test.go      | 30 ++++++++++++++++++++++++++
 integration/delete_test.go             |  5 ++++-
 internal/handlers/sqlite/msg_delete.go | 23 +++++++++-----------
 3 files changed, 44 insertions(+), 14 deletions(-)

diff --git a/integration/delete_compat_test.go b/integration/delete_compat_test.go
index 6b9b74a9054b..7324c3cfa85c 100644
--- a/integration/delete_compat_test.go
+++ b/integration/delete_compat_test.go
@@ -73,6 +73,36 @@ func TestDeleteCompat(t *testing.T) {
 			},
 			ordered: true,
 		},
+		"TwoDuplicateFilter": {
+			filters: []bson.D{
+				{{"v", int32(42)}},
+				{{"v", int32(42)}},
+			},
+		},
+
+		{v: 42},
+		{v: 42},
+		{v: 42},
+
+		/*
+
+			db.runCommand(
+			    {
+			      delete: "test",
+			      deletes: [
+			         {
+			           q : {v: 42},
+			           limit : 1
+			         },
+			         {
+			           q : {v: 42},
+			           limit : 1
+			         },
+			      ]
+			   }
+			)
+
+		*/
 
 		"OrderedError": {
 			filters: []bson.D{
diff --git a/integration/delete_test.go b/integration/delete_test.go
index 218207b99d5a..de4f01dd25a8 100644
--- a/integration/delete_test.go
+++ b/integration/delete_test.go
@@ -149,7 +149,10 @@ func TestDeleteNotExistingDatabase(t *testing.T) {
 	var res bson.D
 	err := db.RunCommand(ctx, bson.D{
 		{"delete", "test"},
-		{"deletes", bson.A{bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}}}},
+		{"deletes", bson.A{
+			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+		}},
 	}).Decode(&res)
 
 	assert.NoError(t, err)
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index c077c5e5fa34..6745a1bead1b 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -82,36 +82,33 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 }
 
 // execDelete fetches documents, filters them out, limits them (if needed) and deletes them.
-// If limit is true, only the first matched document is chosen for deletion, otherwise all matched documents are chosen.
+// If limited is true, only the first matched document is chosen for deletion, otherwise all matched documents are chosen.
 // It returns the number of deleted documents or an error.
 func execDelete(ctx context.Context, coll backends.Collection, filter *types.Document, limited bool) (int32, error) {
-	var deleted int32
-
 	// query documents here
 	res, err := coll.Query(ctx, nil)
 	if err != nil {
-		return 0, err
+		return 0, lazyerrors.Error(err)
 	}
 
 	defer res.Iter.Close()
 
 	var ids []any
+	var deleted int32
 
 	for {
-		var doc *types.Document
-
-		if _, doc, err = res.Iter.Next(); err != nil {
+		_, doc, err := res.Iter.Next()
+		if err != nil {
 			if errors.Is(err, iterator.ErrIteratorDone) {
 				break
 			}
 
-			return 0, err
+			return 0, lazyerrors.Error(err)
 		}
 
-		var matches bool
-
-		if matches, err = common.FilterDocument(doc, filter); err != nil {
-			return 0, err
+		matches, err := common.FilterDocument(doc, filter)
+		if err != nil {
+			return 0, lazyerrors.Error(err)
 		}
 
 		if !matches {
@@ -122,7 +119,7 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 
 		// if limit is set, no need to fetch all the documents
 		if limited {
-			res.Iter.Close()
+			res.Iter.Close() // good comment
 
 			break
 		}

From 1c2ca9a2095b80476b88fa367eca61f5d6aefa5d Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 13:54:07 +0100
Subject: [PATCH 24/38] WIP

---
 integration/delete_command_compat_test.go | 113 ++++++++++++++++++++++
 integration/delete_compat_test.go         |  30 ------
 integration/delete_test.go                |   1 -
 internal/handlers/pg/msg_delete.go        |   6 +-
 internal/handlers/sqlite/msg_delete.go    |   8 +-
 5 files changed, 121 insertions(+), 37 deletions(-)
 create mode 100644 integration/delete_command_compat_test.go

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
new file mode 100644
index 000000000000..d011641c37a6
--- /dev/null
+++ b/integration/delete_command_compat_test.go
@@ -0,0 +1,113 @@
+// Copyright 2021 FerretDB Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package integration
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"go.mongodb.org/mongo-driver/bson"
+
+	"github.com/FerretDB/FerretDB/integration/setup"
+)
+
+// deleteCommandCompatTestCase describes delete compatibility test case.
+type deleteCommandCompatTestCase struct {
+	deletes    bson.A                   // required
+	resultType compatTestCaseResultType // defaults to nonEmptyResult
+
+	skip string // optional, skip test with a specified reason
+}
+
+func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCompatTestCase) {
+	t.Helper()
+
+	for name, tc := range testCases {
+		name, tc := name, tc
+		t.Run(name, func(t *testing.T) {
+			t.Helper()
+
+			t.Parallel()
+
+			// Use per-test setup because deletes modify data set.
+			ctx, targetCollections, compatCollections := setup.SetupCompat(t)
+
+			var nonEmptyResults bool
+			for i := range targetCollections {
+				targetCollection := targetCollections[i]
+				compatCollection := compatCollections[i]
+				t.Run(targetCollection.Name(), func(t *testing.T) {
+					t.Helper()
+
+					var targetRes, compatRes bson.D
+					targetErr := targetCollection.Database().RunCommand(ctx,
+						bson.D{
+							{"delete", targetCollection.Name()},
+							{"deletes", tc.deletes},
+						}).Decode(&targetRes)
+					compatErr := compatCollection.Database().RunCommand(ctx,
+						bson.D{
+							{"delete", compatCollection.Name()},
+							{"deletes", tc.deletes},
+						}).Decode(&compatRes)
+
+					if targetErr != nil {
+						t.Logf("Target error: %v", targetErr)
+						targetErr = UnsetRaw(t, targetErr)
+						compatErr = UnsetRaw(t, compatErr)
+						assert.Equal(t, compatErr, targetErr)
+					} else {
+						require.NoError(t, compatErr, "compat error; target returned no error")
+					}
+
+					assert.Equal(t, compatRes, targetRes)
+
+					targetDocs := FindAll(t, ctx, targetCollection)
+					compatDocs := FindAll(t, ctx, compatCollection)
+
+					t.Logf("Compat (expected) IDs: %v", CollectIDs(t, compatDocs))
+					t.Logf("Target (actual)   IDs: %v", CollectIDs(t, targetDocs))
+					AssertEqualDocumentsSlice(t, compatDocs, targetDocs)
+				})
+			}
+
+			switch tc.resultType {
+			case nonEmptyResult:
+				assert.True(t, nonEmptyResults, "expected non-empty results (some documents should be deleted)")
+			case emptyResult:
+				assert.False(t, nonEmptyResults, "expected empty results (no documents should be deleted)")
+			default:
+				t.Fatalf("unknown result type %v", tc.resultType)
+			}
+		})
+	}
+}
+
+func TestDeleteCompatCommand(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]deleteCommandCompatTestCase{
+		"DuplicateFilter": {
+			deletes: bson.A{
+				bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+				bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+			},
+			skip: "https://github.com/FerretDB/FerretDB/issues/2935",
+		},
+	}
+
+	testDeleteCommandCompat(t, testCases)
+}
diff --git a/integration/delete_compat_test.go b/integration/delete_compat_test.go
index 7324c3cfa85c..6b9b74a9054b 100644
--- a/integration/delete_compat_test.go
+++ b/integration/delete_compat_test.go
@@ -73,36 +73,6 @@ func TestDeleteCompat(t *testing.T) {
 			},
 			ordered: true,
 		},
-		"TwoDuplicateFilter": {
-			filters: []bson.D{
-				{{"v", int32(42)}},
-				{{"v", int32(42)}},
-			},
-		},
-
-		{v: 42},
-		{v: 42},
-		{v: 42},
-
-		/*
-
-			db.runCommand(
-			    {
-			      delete: "test",
-			      deletes: [
-			         {
-			           q : {v: 42},
-			           limit : 1
-			         },
-			         {
-			           q : {v: 42},
-			           limit : 1
-			         },
-			      ]
-			   }
-			)
-
-		*/
 
 		"OrderedError": {
 			filters: []bson.D{
diff --git a/integration/delete_test.go b/integration/delete_test.go
index de4f01dd25a8..113dac07abea 100644
--- a/integration/delete_test.go
+++ b/integration/delete_test.go
@@ -151,7 +151,6 @@ func TestDeleteNotExistingDatabase(t *testing.T) {
 		{"delete", "test"},
 		{"deletes", bson.A{
 			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
-			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
 		}},
 	}).Decode(&res)
 
diff --git a/internal/handlers/pg/msg_delete.go b/internal/handlers/pg/msg_delete.go
index 0b61ae78f91e..1d32def49729 100644
--- a/internal/handlers/pg/msg_delete.go
+++ b/internal/handlers/pg/msg_delete.go
@@ -81,15 +81,15 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"ok", float64(1),
+		"n", deleted,
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
+	} else {
+		replyDoc.Set("ok", float64(1))
 	}
 
-	replyDoc.Set("n", deleted)
-
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
 		Documents: []*types.Document{replyDoc},
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 6745a1bead1b..e3597aea49b3 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -64,14 +64,16 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"ok", float64(1),
+		"n", deleted,
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
+	} else {
+		replyDoc.Set("ok", float64(1))
 	}
 
-	replyDoc.Set("n", deleted)
+	//replyDoc.Set("n", deleted)
 
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
@@ -119,7 +121,7 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 
 		// if limit is set, no need to fetch all the documents
 		if limited {
-			res.Iter.Close() // good comment
+			res.Iter.Close() // call Close() to release the underlying connection early
 
 			break
 		}

From 70e68a8cec550b3c5209ac0d9b3b1708f6507e51 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 14:11:50 +0100
Subject: [PATCH 25/38] WIP

---
 integration/delete_command_compat_test.go |  2 +-
 internal/handlers/sqlite/msg_delete.go    | 15 +++++----------
 2 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index d011641c37a6..54fbd2c94a87 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -25,7 +25,7 @@ import (
 )
 
 // deleteCommandCompatTestCase describes delete compatibility test case.
-type deleteCommandCompatTestCase struct {
+type deleteCommandCompatTestCase struct { //nolint:vet // for readability
 	deletes    bson.A                   // required
 	resultType compatTestCaseResultType // defaults to nonEmptyResult
 
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index e3597aea49b3..badbaa9e2f95 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -73,8 +73,6 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 		replyDoc.Set("ok", float64(1))
 	}
 
-	//replyDoc.Set("n", deleted)
-
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
 		Documents: []*types.Document{replyDoc},
@@ -96,11 +94,11 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 	defer res.Iter.Close()
 
 	var ids []any
-	var deleted int32
+	var doc *types.Document
+	var matches bool
 
 	for {
-		_, doc, err := res.Iter.Next()
-		if err != nil {
+		if _, doc, err = res.Iter.Next(); err != nil {
 			if errors.Is(err, iterator.ErrIteratorDone) {
 				break
 			}
@@ -108,8 +106,7 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 			return 0, lazyerrors.Error(err)
 		}
 
-		matches, err := common.FilterDocument(doc, filter)
-		if err != nil {
+		if matches, err = common.FilterDocument(doc, filter); err != nil {
 			return 0, lazyerrors.Error(err)
 		}
 
@@ -137,7 +134,5 @@ func execDelete(ctx context.Context, coll backends.Collection, filter *types.Doc
 		return 0, err
 	}
 
-	deleted = int32(deleteRes.Deleted)
-
-	return deleted, nil
+	return int32(deleteRes.Deleted), nil
 }

From d13635b2f1b91f7cccdcbf5f9e5382425de17b9c Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 14:19:32 +0100
Subject: [PATCH 26/38] WIP

---
 internal/handlers/tigris/msg_delete.go | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/internal/handlers/tigris/msg_delete.go b/internal/handlers/tigris/msg_delete.go
index 2b13a158c251..24b5cf9e5fea 100644
--- a/internal/handlers/tigris/msg_delete.go
+++ b/internal/handlers/tigris/msg_delete.go
@@ -80,15 +80,15 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"ok", float64(1),
+		"n", deleted,
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
+	} else {
+		replyDoc.Set("ok", float64(1))
 	}
 
-	replyDoc.Set("n", deleted)
-
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
 		Documents: []*types.Document{replyDoc},

From 6353c9331de0509b0bc08e7b333e426cd4d99d93 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 14:20:16 +0100
Subject: [PATCH 27/38] WIP

---
 integration/basic_test.go | 1 -
 1 file changed, 1 deletion(-)

diff --git a/integration/basic_test.go b/integration/basic_test.go
index 7815db13b7ea..e86a4cad95fc 100644
--- a/integration/basic_test.go
+++ b/integration/basic_test.go
@@ -15,7 +15,6 @@
 package integration
 
 import (
-	"errors"
 	"fmt"
 	"math"
 	"strings"

From 19bb6087b1db9abd2016165ea74c752f700ae97d Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 14:31:31 +0100
Subject: [PATCH 28/38] WIP

---
 integration/delete_command_compat_test.go | 34 +++++++++++++----------
 1 file changed, 19 insertions(+), 15 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index 54fbd2c94a87..50ed841d7bea 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -25,11 +25,9 @@ import (
 )
 
 // deleteCommandCompatTestCase describes delete compatibility test case.
-type deleteCommandCompatTestCase struct { //nolint:vet // for readability
-	deletes    bson.A                   // required
-	resultType compatTestCaseResultType // defaults to nonEmptyResult
-
-	skip string // optional, skip test with a specified reason
+type deleteCommandCompatTestCase struct {
+	skip    string // optional, skip test with a specified reason
+	deletes bson.A // required
 }
 
 func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCompatTestCase) {
@@ -40,12 +38,15 @@ func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCom
 		t.Run(name, func(t *testing.T) {
 			t.Helper()
 
+			if tc.skip != "" {
+				t.Skip(tc.skip)
+			}
+
 			t.Parallel()
 
 			// Use per-test setup because deletes modify data set.
 			ctx, targetCollections, compatCollections := setup.SetupCompat(t)
 
-			var nonEmptyResults bool
 			for i := range targetCollections {
 				targetCollection := targetCollections[i]
 				compatCollection := compatCollections[i]
@@ -83,15 +84,6 @@ func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCom
 					AssertEqualDocumentsSlice(t, compatDocs, targetDocs)
 				})
 			}
-
-			switch tc.resultType {
-			case nonEmptyResult:
-				assert.True(t, nonEmptyResults, "expected non-empty results (some documents should be deleted)")
-			case emptyResult:
-				assert.False(t, nonEmptyResults, "expected empty results (no documents should be deleted)")
-			default:
-				t.Fatalf("unknown result type %v", tc.resultType)
-			}
 		})
 	}
 }
@@ -100,6 +92,18 @@ func TestDeleteCompatCommand(t *testing.T) {
 	t.Parallel()
 
 	testCases := map[string]deleteCommandCompatTestCase{
+		"OneLimited": {
+			deletes: bson.A{
+				bson.D{{"q", bson.D{{"v", int32(0)}}}, {"limit", 1}},
+			},
+		},
+		"TwoLimited": {
+			deletes: bson.A{
+				bson.D{{"q", bson.D{{"v", int32(42)}}}, {"limit", 1}},
+				bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+			},
+			skip: "https://github.com/FerretDB/FerretDB/issues/2935",
+		},
 		"DuplicateFilter": {
 			deletes: bson.A{
 				bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},

From 398d51aeefe5fe2c74a50c65a71f159da54315c6 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 14:53:15 +0100
Subject: [PATCH 29/38] WIP

---
 integration/delete_test.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/integration/delete_test.go b/integration/delete_test.go
index 113dac07abea..af231ef80739 100644
--- a/integration/delete_test.go
+++ b/integration/delete_test.go
@@ -156,7 +156,7 @@ func TestDeleteNotExistingDatabase(t *testing.T) {
 
 	assert.NoError(t, err)
 
-	expectedRes := bson.D{{"ok", float64(1)}, {"n", int32(0)}}
+	expectedRes := bson.D{{"n", int32(0)}, {"ok", float64(1)}}
 
 	assert.Equal(t, expectedRes, res)
 }

From d8dc923bc76cc4a625eae5bb5c16b9863e969311 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 17:12:47 +0100
Subject: [PATCH 30/38] WIP

---
 integration/delete_command_compat_test.go | 2 +-
 internal/handlers/pg/msg_delete.go        | 6 +++---
 internal/handlers/sqlite/msg_delete.go    | 6 +++---
 internal/handlers/tigris/msg_delete.go    | 6 +++---
 4 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index 50ed841d7bea..1e20ce48b73e 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -88,7 +88,7 @@ func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCom
 	}
 }
 
-func TestDeleteCompatCommand(t *testing.T) {
+func TestDeleteCommandCompat(t *testing.T) {
 	t.Parallel()
 
 	testCases := map[string]deleteCommandCompatTestCase{
diff --git a/internal/handlers/pg/msg_delete.go b/internal/handlers/pg/msg_delete.go
index 1d32def49729..0b61ae78f91e 100644
--- a/internal/handlers/pg/msg_delete.go
+++ b/internal/handlers/pg/msg_delete.go
@@ -81,15 +81,15 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"n", deleted,
+		"ok", float64(1),
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
-	} else {
-		replyDoc.Set("ok", float64(1))
 	}
 
+	replyDoc.Set("n", deleted)
+
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
 		Documents: []*types.Document{replyDoc},
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index badbaa9e2f95..390a7fdfbd1e 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -64,15 +64,15 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"n", deleted,
+		"ok", float64(1),
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
-	} else {
-		replyDoc.Set("ok", float64(1))
 	}
 
+	replyDoc.Set("n", deleted)
+
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
 		Documents: []*types.Document{replyDoc},
diff --git a/internal/handlers/tigris/msg_delete.go b/internal/handlers/tigris/msg_delete.go
index 24b5cf9e5fea..2b13a158c251 100644
--- a/internal/handlers/tigris/msg_delete.go
+++ b/internal/handlers/tigris/msg_delete.go
@@ -80,15 +80,15 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"n", deleted,
+		"ok", float64(1),
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
-	} else {
-		replyDoc.Set("ok", float64(1))
 	}
 
+	replyDoc.Set("n", deleted)
+
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
 		Documents: []*types.Document{replyDoc},

From 2f929a3d6400add22d20ca7479f767c34f751893 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 17:35:25 +0100
Subject: [PATCH 31/38] WIP

---
 integration/delete_command_compat_test.go | 33 +++++++++++++++++++++++
 integration/delete_test.go                | 22 ---------------
 2 files changed, 33 insertions(+), 22 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index 1e20ce48b73e..44fa1417025b 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -22,6 +22,7 @@ import (
 	"go.mongodb.org/mongo-driver/bson"
 
 	"github.com/FerretDB/FerretDB/integration/setup"
+	"github.com/FerretDB/FerretDB/integration/shareddata"
 )
 
 // deleteCommandCompatTestCase describes delete compatibility test case.
@@ -115,3 +116,35 @@ func TestDeleteCommandCompat(t *testing.T) {
 
 	testDeleteCommandCompat(t, testCases)
 }
+
+func TestDeleteCommandCompatNotExistingDatabase(t *testing.T) {
+	t.Parallel()
+
+	res := setup.SetupCompatWithOpts(t, &setup.SetupCompatOpts{
+		Providers: []shareddata.Provider{shareddata.Int32s},
+	})
+	ctx, targetColl, compatColl := res.Ctx, res.TargetCollections[0], res.CompatCollections[0]
+
+	targetDB := targetColl.Database().Client().Database("doesnotexist")
+	compatDB := compatColl.Database().Client().Database("doesnotexist")
+
+	var targetRes, compatRes bson.D
+
+	targetErr := targetDB.RunCommand(ctx, bson.D{
+		{"delete", "test"},
+		{"deletes", bson.A{
+			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+		}},
+	}).Decode(&targetRes)
+	compatErr := compatDB.RunCommand(ctx, bson.D{
+		{"delete", "test"},
+		{"deletes", bson.A{
+			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
+		}},
+	}).Decode(&compatRes)
+
+	assert.NoError(t, targetErr)
+	assert.NoError(t, compatErr)
+
+	assert.Equal(t, compatRes, targetRes)
+}
diff --git a/integration/delete_test.go b/integration/delete_test.go
index af231ef80739..a45e6af3edd8 100644
--- a/integration/delete_test.go
+++ b/integration/delete_test.go
@@ -138,25 +138,3 @@ func TestDelete(t *testing.T) {
 		})
 	}
 }
-
-func TestDeleteNotExistingDatabase(t *testing.T) {
-	t.Parallel()
-
-	ctx, collection := setup.Setup(t)
-
-	db := collection.Database().Client().Database("doesnotexist")
-
-	var res bson.D
-	err := db.RunCommand(ctx, bson.D{
-		{"delete", "test"},
-		{"deletes", bson.A{
-			bson.D{{"q", bson.D{{"v", "foo"}}}, {"limit", 1}},
-		}},
-	}).Decode(&res)
-
-	assert.NoError(t, err)
-
-	expectedRes := bson.D{{"n", int32(0)}, {"ok", float64(1)}}
-
-	assert.Equal(t, expectedRes, res)
-}

From cab52dab2ac746fe96305fb4f27b6e729027505d Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 18:00:55 +0100
Subject: [PATCH 32/38] WIP

---
 internal/handlers/common/delete.go     | 1 +
 internal/handlers/pg/msg_delete.go     | 4 ++--
 internal/handlers/sqlite/msg_delete.go | 4 ++--
 internal/handlers/tigris/msg_delete.go | 4 ++--
 4 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/internal/handlers/common/delete.go b/internal/handlers/common/delete.go
index ba8f1faf5370..47cbb95e5abb 100644
--- a/internal/handlers/common/delete.go
+++ b/internal/handlers/common/delete.go
@@ -33,6 +33,7 @@ type DeleteParams struct {
 	Let *types.Document `ferretdb:"let,unimplemented"`
 
 	WriteConcern *types.Document `ferretdb:"writeConcern,ignored"`
+	LSID         any             `ferretdb:"lsid,ignored"`
 }
 
 // Delete represents single delete operation parameters.
diff --git a/internal/handlers/pg/msg_delete.go b/internal/handlers/pg/msg_delete.go
index 0b61ae78f91e..40c3e7142aaa 100644
--- a/internal/handlers/pg/msg_delete.go
+++ b/internal/handlers/pg/msg_delete.go
@@ -81,14 +81,14 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"ok", float64(1),
+		"n", deleted,
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
 	}
 
-	replyDoc.Set("n", deleted)
+	replyDoc.Set("ok", float64(1))
 
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 390a7fdfbd1e..5252f8e79071 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -64,14 +64,14 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"ok", float64(1),
+		"n", deleted,
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
 	}
 
-	replyDoc.Set("n", deleted)
+	replyDoc.Set("ok", float64(1))
 
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{
diff --git a/internal/handlers/tigris/msg_delete.go b/internal/handlers/tigris/msg_delete.go
index 2b13a158c251..4d0b96ec9e16 100644
--- a/internal/handlers/tigris/msg_delete.go
+++ b/internal/handlers/tigris/msg_delete.go
@@ -80,14 +80,14 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	}
 
 	replyDoc := must.NotFail(types.NewDocument(
-		"ok", float64(1),
+		"n", deleted,
 	))
 
 	if delErrors.Len() > 0 {
 		replyDoc = delErrors.Document()
 	}
 
-	replyDoc.Set("n", deleted)
+	replyDoc.Set("ok", float64(1))
 
 	var reply wire.OpMsg
 	must.NoError(reply.SetSections(wire.OpMsgSection{

From f2b14ce5164c43f8fbdc3eed20f8dae3910f450e Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 18:24:28 +0100
Subject: [PATCH 33/38] WIP

---
 internal/handlers/pg/msg_delete.go     | 2 +-
 internal/handlers/sqlite/msg_delete.go | 2 +-
 internal/handlers/tigris/msg_delete.go | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/internal/handlers/pg/msg_delete.go b/internal/handlers/pg/msg_delete.go
index 40c3e7142aaa..5bf39c5f5b84 100644
--- a/internal/handlers/pg/msg_delete.go
+++ b/internal/handlers/pg/msg_delete.go
@@ -85,7 +85,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	))
 
 	if delErrors.Len() > 0 {
-		replyDoc = delErrors.Document()
+		replyDoc.Set("writeErrors", must.NotFail(delErrors.Document().Get("writeErrors")))
 	}
 
 	replyDoc.Set("ok", float64(1))
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 5252f8e79071..640a2310f283 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -68,7 +68,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	))
 
 	if delErrors.Len() > 0 {
-		replyDoc = delErrors.Document()
+		replyDoc.Set("writeErrors", must.NotFail(delErrors.Document().Get("writeErrors")))
 	}
 
 	replyDoc.Set("ok", float64(1))
diff --git a/internal/handlers/tigris/msg_delete.go b/internal/handlers/tigris/msg_delete.go
index 4d0b96ec9e16..b4b3ec1a10ce 100644
--- a/internal/handlers/tigris/msg_delete.go
+++ b/internal/handlers/tigris/msg_delete.go
@@ -84,7 +84,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	))
 
 	if delErrors.Len() > 0 {
-		replyDoc = delErrors.Document()
+		replyDoc.Set("writeErrors", must.NotFail(delErrors.Document().Get("writeErrors")))
 	}
 
 	replyDoc.Set("ok", float64(1))

From 311d6fcfa546e680ce90b2b6bbb5f9ae4fd98970 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Wed, 28 Jun 2023 19:20:54 +0100
Subject: [PATCH 34/38] WIP

---
 internal/handlers/pg/msg_delete.go     | 1 +
 internal/handlers/sqlite/msg_delete.go | 1 +
 internal/handlers/tigris/msg_delete.go | 1 +
 3 files changed, 3 insertions(+)

diff --git a/internal/handlers/pg/msg_delete.go b/internal/handlers/pg/msg_delete.go
index 5bf39c5f5b84..4d3d7babf157 100644
--- a/internal/handlers/pg/msg_delete.go
+++ b/internal/handlers/pg/msg_delete.go
@@ -85,6 +85,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	))
 
 	if delErrors.Len() > 0 {
+		// "writeErrors" should be after "n" field
 		replyDoc.Set("writeErrors", must.NotFail(delErrors.Document().Get("writeErrors")))
 	}
 
diff --git a/internal/handlers/sqlite/msg_delete.go b/internal/handlers/sqlite/msg_delete.go
index 640a2310f283..ad4cf4df2707 100644
--- a/internal/handlers/sqlite/msg_delete.go
+++ b/internal/handlers/sqlite/msg_delete.go
@@ -68,6 +68,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	))
 
 	if delErrors.Len() > 0 {
+		// "writeErrors" should be after "n" field
 		replyDoc.Set("writeErrors", must.NotFail(delErrors.Document().Get("writeErrors")))
 	}
 
diff --git a/internal/handlers/tigris/msg_delete.go b/internal/handlers/tigris/msg_delete.go
index b4b3ec1a10ce..2ce511ce37fa 100644
--- a/internal/handlers/tigris/msg_delete.go
+++ b/internal/handlers/tigris/msg_delete.go
@@ -84,6 +84,7 @@ func (h *Handler) MsgDelete(ctx context.Context, msg *wire.OpMsg) (*wire.OpMsg,
 	))
 
 	if delErrors.Len() > 0 {
+		// "writeErrors" should be after "n" field
 		replyDoc.Set("writeErrors", must.NotFail(delErrors.Document().Get("writeErrors")))
 	}
 

From 28fffde1539652bf10fe86436adcfc9b45f9bfc2 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Thu, 29 Jun 2023 09:27:32 +0100
Subject: [PATCH 35/38] WIP

---
 integration/delete_command_compat_test.go | 14 +++++++++-----
 integration/delete_compat_test.go         | 11 +++++++----
 internal/backends/collection.go           |  2 +-
 3 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index 44fa1417025b..a9dea51611a9 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -68,13 +68,17 @@ func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCom
 
 					if targetErr != nil {
 						t.Logf("Target error: %v", targetErr)
-						targetErr = UnsetRaw(t, targetErr)
-						compatErr = UnsetRaw(t, compatErr)
-						assert.Equal(t, compatErr, targetErr)
-					} else {
-						require.NoError(t, compatErr, "compat error; target returned no error")
+						t.Logf("Compat error: %v", compatErr)
+
+						// error messages are intentionally not compared
+						AssertMatchesCommandError(t, compatErr, targetErr)
+
+						return
 					}
+					require.NoError(t, compatErr, "compat error; target returned no error")
 
+					t.Logf("Compat (expected) result: %v", compatRes)
+					t.Logf("Target (actual)   result: %v", targetRes)
 					assert.Equal(t, compatRes, targetRes)
 
 					targetDocs := FindAll(t, ctx, targetCollection)
diff --git a/integration/delete_compat_test.go b/integration/delete_compat_test.go
index 6b9b74a9054b..8a8833f28f29 100644
--- a/integration/delete_compat_test.go
+++ b/integration/delete_compat_test.go
@@ -166,10 +166,11 @@ func testDeleteCompat(t *testing.T, testCases map[string]deleteCompatTestCase) {
 
 					if targetErr != nil {
 						t.Logf("Target error: %v", targetErr)
-						targetErr = UnsetRaw(t, targetErr)
-						compatErr = UnsetRaw(t, compatErr)
-						assert.Equal(t, compatErr, targetErr)
-					} else {
+						t.Logf("Compat error: %v", compatErr)
+
+						// error messages are intentionally not compared
+						AssertMatchesCommandError(t, compatErr, targetErr)
+					} else { // we have to check the results in case of error because some documents may be deleted
 						require.NoError(t, compatErr, "compat error; target returned no error")
 					}
 
@@ -177,6 +178,8 @@ func testDeleteCompat(t *testing.T, testCases map[string]deleteCompatTestCase) {
 						nonEmptyResults = true
 					}
 
+					t.Logf("Compat (expected) result: %v", compatRes)
+					t.Logf("Target (actual)   result: %v", targetRes)
 					assert.Equal(t, compatRes, targetRes)
 
 					targetDocs := FindAll(t, ctx, targetCollection)
diff --git a/internal/backends/collection.go b/internal/backends/collection.go
index e8cb1e9f8824..33de8d5ecd22 100644
--- a/internal/backends/collection.go
+++ b/internal/backends/collection.go
@@ -135,7 +135,7 @@ type DeleteResult struct {
 
 // Delete deletes documents in collection.
 //
-// If request database does not exist it returns 0 deleted documents.
+// If requested database or collection does not exist it returns 0 deleted documents.
 func (cc *collectionContract) Delete(ctx context.Context, params *DeleteParams) (res *DeleteResult, err error) {
 	defer observability.FuncCall(ctx)()
 	defer checkError(err)

From 296029dcc2e6c2610e3bf5f13546bfc8b0474858 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Thu, 29 Jun 2023 10:45:55 +0100
Subject: [PATCH 36/38] WIP

---
 integration/delete_command_compat_test.go | 2 +-
 integration/delete_compat_test.go         | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index a9dea51611a9..dd8e24a2d377 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -71,7 +71,7 @@ func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCom
 						t.Logf("Compat error: %v", compatErr)
 
 						// error messages are intentionally not compared
-						AssertMatchesCommandError(t, compatErr, targetErr)
+						AssertMatchesWriteError(t, compatErr, targetErr)
 
 						return
 					}
diff --git a/integration/delete_compat_test.go b/integration/delete_compat_test.go
index 8a8833f28f29..02e8e9575756 100644
--- a/integration/delete_compat_test.go
+++ b/integration/delete_compat_test.go
@@ -169,7 +169,7 @@ func testDeleteCompat(t *testing.T, testCases map[string]deleteCompatTestCase) {
 						t.Logf("Compat error: %v", compatErr)
 
 						// error messages are intentionally not compared
-						AssertMatchesCommandError(t, compatErr, targetErr)
+						AssertMatchesWriteError(t, compatErr, targetErr)
 					} else { // we have to check the results in case of error because some documents may be deleted
 						require.NoError(t, compatErr, "compat error; target returned no error")
 					}

From f527db43416688aa9d8749bce84671696c75f2c3 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Thu, 29 Jun 2023 11:08:18 +0100
Subject: [PATCH 37/38] WIP

---
 integration/delete_command_compat_test.go | 2 +-
 integration/delete_compat_test.go         | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index dd8e24a2d377..aaa9ec33695b 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -71,7 +71,7 @@ func testDeleteCommandCompat(t *testing.T, testCases map[string]deleteCommandCom
 						t.Logf("Compat error: %v", compatErr)
 
 						// error messages are intentionally not compared
-						AssertMatchesWriteError(t, compatErr, targetErr)
+						AssertMatchesBulkException(t, compatErr, targetErr)
 
 						return
 					}
diff --git a/integration/delete_compat_test.go b/integration/delete_compat_test.go
index 02e8e9575756..b40ad213464e 100644
--- a/integration/delete_compat_test.go
+++ b/integration/delete_compat_test.go
@@ -169,7 +169,7 @@ func testDeleteCompat(t *testing.T, testCases map[string]deleteCompatTestCase) {
 						t.Logf("Compat error: %v", compatErr)
 
 						// error messages are intentionally not compared
-						AssertMatchesWriteError(t, compatErr, targetErr)
+						AssertMatchesBulkException(t, compatErr, targetErr)
 					} else { // we have to check the results in case of error because some documents may be deleted
 						require.NoError(t, compatErr, "compat error; target returned no error")
 					}

From fe07ff2f35d2e32993127e01a5132de62fc09812 Mon Sep 17 00:00:00 2001
From: Dmitry <dmitry.eremenko@ferretdb.io>
Date: Thu, 29 Jun 2023 11:17:06 +0100
Subject: [PATCH 38/38] WIP

---
 integration/delete_command_compat_test.go | 1 +
 integration/delete_compat_test.go         | 5 -----
 2 files changed, 1 insertion(+), 5 deletions(-)

diff --git a/integration/delete_command_compat_test.go b/integration/delete_command_compat_test.go
index aaa9ec33695b..6a5e8f6a79b1 100644
--- a/integration/delete_command_compat_test.go
+++ b/integration/delete_command_compat_test.go
@@ -101,6 +101,7 @@ func TestDeleteCommandCompat(t *testing.T) {
 			deletes: bson.A{
 				bson.D{{"q", bson.D{{"v", int32(0)}}}, {"limit", 1}},
 			},
+			skip: "https://github.com/FerretDB/FerretDB/issues/2935",
 		},
 		"TwoLimited": {
 			deletes: bson.A{
diff --git a/integration/delete_compat_test.go b/integration/delete_compat_test.go
index b40ad213464e..752b50f06450 100644
--- a/integration/delete_compat_test.go
+++ b/integration/delete_compat_test.go
@@ -38,11 +38,6 @@ func TestDeleteCompat(t *testing.T) {
 	t.Parallel()
 
 	testCases := map[string]deleteCompatTestCase{
-		"Empty": {
-			filters:    []bson.D{},
-			resultType: emptyResult,
-		},
-
 		"One": {
 			filters: []bson.D{
 				{{"v", int32(42)}},