title: How to Pull XCOM value in PostgresOperator
body: <p>Here I am pushing XCOM value:</p>
  
  <pre><code>task_get_username_bash = BashOperator(
                  task_id='execute_bash',
                  bash_command='whoami',
                  xcom_push=True)
  </code></pre>
  
  <p>So in XCOMs it stores like {'return_value' : '$USER'} (in my case $USER = 'airflow').</p>
  
  <p>Then I want to pull this <em>return_value</em> from XCOM:</p>
  
  <pre><code>task_insert_new_row = PostgresOperator(
                  task_id='insert_new_row',
                  trigger_rule=TriggerRule.ALL_DONE,
                  sql='''INSERT INTO table_name VALUES
                  (%s, %s, %s);''',
                  parameters=(uuid.uuid4().int % 123456789,
                              "{{ ti.xcom_pull(task_ids='execute_bash', key='return_value') }}",
                              datetime.now()))
  </code></pre>
  
  <p>But PostgresOperator interpret Macros reference as str. How to pull XCOM in PostgresOperator?</p>