add foreign key as part of set statement when reserved connection is needed

go/test/endtoend/vtgate/foreignkey/fk_test.go

@@ -1207,3 +1207,31 @@ func TestReplaceWithFK(t *testing.T) {
 	utils.AssertMatches(t, conn, `select * from u_t1`, `[[INT64(1) INT64(1)] [INT64(2) INT64(2)]]`)
 	utils.AssertMatches(t, conn, `select * from u_t2`, `[[INT64(1) NULL] [INT64(2) NULL]]`)
 }
+
+// TestDDLFk tests that table is created with fk constraint when foreign_key_checks is off.
+func TestDDLFk(t *testing.T) {
+	mcmp, closer := start(t)
+	defer closer()
+
+	utils.Exec(t, mcmp.VtConn, `use uks`)
+
+	createTableDDLTemp1 := `
+create table temp1(id bigint auto_increment primary key, col varchar(20) not null,
+foreign key (col) references temp2(col))
+`
+	mcmp.Exec(`set foreign_key_checks = off`)
+	// should be able to create `temp1` table without a `temp2`
+	mcmp.Exec(createTableDDLTemp1)
+
+	createTableDDLTemp2 := `
+create table temp2(id bigint auto_increment primary key, col varchar(20) not null, key (col))
+`
+	// now create `temp2`
+	mcmp.Exec(createTableDDLTemp2)
+
+	// inserting some data with fk constraints on.
+	mcmp.Exec(`set foreign_key_checks = on`)
+	mcmp.Exec(`insert into temp2(col) values('a'), ('b'), ('c') `)
+	mcmp.Exec(`insert into temp1(col) values('a') `)
+	mcmp.ExecAllowAndCompareError(`insert into temp1(col) values('d') `)
+}

go/vt/vtgate/safe_session.go

@@ -571,6 +571,22 @@ func (session *SafeSession) TimeZone() *time.Location {
 	loc, _ := datetime.ParseTimeZone(tz)
 	return loc
 }
+func (session *SafeSession) ForeignKeyChecks() *string {
+	session.mu.Lock()
+	fkVal, ok := session.SystemVariables["foreign_key_checks"]
+	session.mu.Unlock()
+
+	if !ok {
+		return nil
+	}
+	fkCheck := "0"
+	switch strings.ToLower(fkVal) {
+	case "off", "0":
+		return &fkCheck
+	}
+	fkCheck = "1"
+	return &fkCheck
+}
 
 // SetOptions sets the options
 func (session *SafeSession) SetOptions(options *querypb.ExecuteOptions) {
@@ -610,6 +626,13 @@ func (session *SafeSession) SetPreQueries() []string {
 		sysVars[k] = v
 	})
 
+	fkVal := session.ForeignKeyChecks()
+	if fkVal != nil {
+		k := "foreign_key_checks"
+		keys = append(keys, k)
+		sysVars[k] = *fkVal
+	}
+
 	// if not system variables to set, return
 	if len(keys) == 0 {
 		return nil