add calls in db

Observable.create(subscriber -> {
            List<HistoryDbHelper.History> items = new ArrayList<>();
            for (int i = 0; i < 20000; i++) {
                String n;
                if (i < 10)
                    n = "0000" + i;
                else if (i < 100)
                    n = "000" + i;
                else if (i < 1000)
                    n = "00" + i;
                else if (i < 10000)
                    n = "0" + i;
                else
                    n = "" + i;
                String number = "791534" + n;
                HistoryDbHelper.History row = new HistoryDbHelper.History();

                String cleanUpNumber = MyPhoneNumberUtil.getInstance().cleanUpNumber(number);

                if (!TextUtils.isEmpty(cleanUpNumber) && MyPhoneNumberUtil.isValidNumber(cleanUpNumber)) {
                    row.setNumber(cleanUpNumber);
                } else {
                    row.setNumber(PersonManager.UNDEFINED_PHONE_NUMBER);
                }

                row.setName("test-" + n);
                row.setType(HistoryDbHelper.OUTGOING_TYPE);
                row.setDuration(10);
                row.setInternalId(i);
                items.add(row);
            }
            HistoryDbHelper.getInstance().add(items);
        }).subscribeOn(Schedulers.io()).observeOn(AndroidSchedulers.mainThread()).subscribe();