vtorc: enable read_uncommitted when in sqlite shared-cache mode

go/vt/vtorc/config/config.go

@@ -307,6 +307,11 @@ func GetSQLiteDataFile() string {
 	return sqliteDataFile.Get()
 }
 
+// SetSQLiteDataFile is a Setter function.
+func SetSQLiteDataFile(dataFile string) {
+	sqliteDataFile.Set(dataFile)
+}
+
 // GetReasonableReplicationLagSeconds gets the reasonable replication lag but in seconds.
 func GetReasonableReplicationLagSeconds() int64 {
 	return int64(reasonableReplicationLag.Get() / time.Second)

go/vt/vtorc/db/db.go

@@ -18,6 +18,7 @@ package db
 
 import (
 	"database/sql"
+	"strings"
 
 	"vitess.io/vitess/go/vt/external/golib/sqlutils"
 	"vitess.io/vitess/go/vt/log"
@@ -99,6 +100,25 @@ func ClearVTOrcDatabase() {
 	}
 }
 
+// getSqlitePragmaSQLs returns a slice of SQL statements to set sqlite PRAGMAs.
+func getSqlitePragmaSQLs() []string {
+	// PRAGMAs: https://www.sqlite.org/pragma.html
+	pragmaSQLs := []string{
+		`PRAGMA journal_mode = WAL`,
+		`PRAGMA synchronous = NORMAL`,
+	}
+
+	// Enable read_uncommitted transaction isolation (no read locks) when in shared cache mode.
+	// Dirty reads should not be a concern for VTOrc's use of sqlite/transactions. No read
+	// locks allows updates to tables to not be blocked by reads.
+	// https://www.sqlite.org/pragma.html#pragma_read_uncommitted
+	if strings.Contains(config.GetSQLiteDataFile(), "cache=shared") {
+		pragmaSQLs = append(pragmaSQLs, `PRAGMA read_uncommitted = 1`)
+	}
+
+	return pragmaSQLs
+}
+
 // initVTOrcDB attempts to create/upgrade the vtorc backend database. It is created once in the
 // application's lifetime.
 func initVTOrcDB(db *sql.DB) error {
@@ -110,11 +130,12 @@ func initVTOrcDB(db *sql.DB) error {
 	if err := registerVTOrcDeployment(db); err != nil {
 		return err
 	}
-	if _, err := ExecVTOrc(`PRAGMA journal_mode = WAL`); err != nil {
-		return err
-	}
-	if _, err := ExecVTOrc(`PRAGMA synchronous = NORMAL`); err != nil {
-		return err
+
+	for _, pragmaSQL := range getSqlitePragmaSQLs() {
+		if _, err := ExecVTOrc(pragmaSQL); err != nil {
+			log.Errorf("Failed to execute PRAGMA query %q: %+v", pragmaSQL, err)
+			return err
+		}
 	}
 	return nil
 }

go/vt/vtorc/db/db_test.go

@@ -0,0 +1,45 @@
+/*
+Copyright 2025 The Vitess Authors.
+
+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 db
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"vitess.io/vitess/go/vt/vtorc/config"
+)
+
+func TestGetSqlitePragmaSQLs(t *testing.T) {
+	origSqliteDataPath := config.GetSQLiteDataFile()
+	defer config.SetSQLiteDataFile(origSqliteDataPath)
+
+	// no shared-cache mode
+	config.SetSQLiteDataFile("file::memory:?mode=memory")
+	require.Equal(t, []string{
+		"PRAGMA journal_mode = WAL",
+		"PRAGMA synchronous = NORMAL",
+	}, getSqlitePragmaSQLs())
+
+	// shared-cache mode
+	config.SetSQLiteDataFile("file::memory:?mode=memory&cache=shared")
+	require.Equal(t, []string{
+		"PRAGMA journal_mode = WAL",
+		"PRAGMA synchronous = NORMAL",
+		"PRAGMA read_uncommitted = 1",
+	}, getSqlitePragmaSQLs())
+}