Use result from one operator inside another

Question

I would like in get_birth_date use result from get_all_pets. How can i access it inside get_birth_date? Moreover i would like to print result from get_all_pets, where could i deifne such print()?

Where in my code could i do this?

import datetime

from airflow import DAG
from airflow.providers.postgres.operators.postgres import PostgresOperator

# create_pet_table, populate_pet_table, get_all_pets, and get_birth_date are examples of tasks created by
# instantiating the Postgres Operator

with DAG(
    dag_id="postgres_operator_dag",
    start_date=datetime.datetime(2020, 2, 2),
    schedule_interval="@once",
    catchup=False,
) as dag:
    create_pet_table = PostgresOperator(
        task_id="create_pet_table",
        postgres_conn_id="postgres_robert",
        sql="""
            CREATE TABLE IF NOT EXISTS pet (
            pet_id SERIAL PRIMARY KEY,
            name VARCHAR NOT NULL,
            pet_type VARCHAR NOT NULL,
            birth_date DATE NOT NULL,
            OWNER VARCHAR NOT NULL);
          """,
    )
    populate_pet_table = PostgresOperator(
        task_id="populate_pet_table",
        postgres_conn_id="postgres_robert",
        sql="""
            INSERT INTO pet (name, pet_type, birth_date, OWNER)
            VALUES ( 'Max', 'Dog', '2018-07-05', 'Jane');
            INSERT INTO pet (name, pet_type, birth_date, OWNER)
            VALUES ( 'Susie', 'Cat', '2019-05-01', 'Phil');
            INSERT INTO pet (name, pet_type, birth_date, OWNER)
            VALUES ( 'Lester', 'Hamster', '2020-06-23', 'Lily');
            INSERT INTO pet (name, pet_type, birth_date, OWNER)
            VALUES ( 'Quincy', 'Parrot', '2013-08-11', 'Anne');
            """,
    )
    get_all_pets = PostgresOperator(task_id="get_all_pets",postgres_conn_id="postgres_robert", sql="SELECT * FROM pet;")

    get_birth_date = PostgresOperator(
        task_id="get_birth_date",
        postgres_conn_id="postgres_robert",
        sql="SELECT * FROM pet WHERE birth_date BETWEEN SYMMETRIC %(begin_date)s AND %(end_date)s",
        parameters={"begin_date": "2020-01-01", "end_date": "2020-12-31"},
        runtime_parameters={'statement_timeout': '3000ms'},
    )

    create_pet_table >> populate_pet_table >> get_all_pets >> get_birth_date

Answer 1

PostgresOperator is not suitable for running SELECT statements. SELECT statements are more suitable for transfer operators or using hooks directly.

In your case you should use the PostgresHook:

from airflow.decorators import task
from airflow.providers.postgres.hooks.postgres import PostgresHook

@task()
def get_all_pets(**kwargs):
    hook = PostgresHook(postgres_conn_id="postgres_robert")
    df = hook.get_pandas_df(sql="SELECT * FROM pet;")
    print(df)