connect spark to postgres; connect spark to database
spark = SparkSession.builder.config(conf=spark_conf).getOrCreate()
#make sure the necessary jar files are loaded
#@ opt/spark/jars folder
df = spark.read \
.format("jdbc") \
.option("url", "jdbc:postgresql://<host>:<port>/<db_name>") \
.option("dbtable", "<schema>.<tablename>") \
.option("user", "<username>") \
.option("password", "<password>") \
.option("driver", "org.postgresql.Driver") \
.load()