Airflow: How to push xcom value from PostgreOperator?

I'm using Airflow 1.8.1 and I want to push the result of a sql request from PostgreOperator.

Here's my tasks:

check_task = PostgresOperator(
    task_id='check_task',
    postgres_conn_id='conx',
    sql="check_task.sql",
    xcom_push=True,
    dag=dag)

def py_is_first_execution(**kwargs):
    value = kwargs['ti'].xcom_pull(task_ids='check_task')
    print 'count ----> ', value
    if value == 0:
       return 'next_task'
    else:
       return 'end-flow'

check_branch = BranchPythonOperator(
    task_id='is-first-execution',
    python_callable=py_is_first_execution,
    provide_context=True,
    dag=dag)

and here is my sql script:

select count(1) from table

when i check the xcom value from check_task it retrieves none value.