PL/Python allows users to write PostgreSQL functions and procedures using Python. It enables accessing PostgreSQL data and running Python code from within SQL queries. For example, a function could query a database table, process the results in Python by accessing modules, and return a value to the SQL query. This opens up possibilities to leverage Python's extensive libraries and expressiveness to expose data and perform complex validation from PostgreSQL.

1. PostgreSQL and
Python
henning.jacobs@zalando.de

2. Well known...
import psycopg2
conn = psycopg2.connect('dbname=mydb user=myusr')
cur = conn.cursor()
cur.execute('SELECT * FROM mytable')
row = cur.fetchone()

3. PostgreSQL speaks Python!
CREATE FUNCTION greet(name TEXT) RETURNS TEXT AS
$$
import random
attr = random.choice([
'great', 'fabulous', 'bright', 'brave'])
return 'Hello {} {}!'.format(attr, name)
$$
LANGUAGE plpythonu;
SELECT greet('Python fan');

4. PostgreSQL speaks Python
● PL/Python
– “just” another procedural language
● SQL script that contains a string of Python
● Allows using any Python module

5. Real World Examples...
mydb=# SELECT * FROM get_server_connection_stats() LIMIT 5;
server_addr | server_port | client_addr | client_port
---------------+-------------+--------------+-------------
10.160.26.128 | 5435 | 10.111.74.16 | 40524
10.160.26.128 | 5435 | 10.160.26.1 | 49681
10.160.26.128 | 5435 | 10.160.26.1 | 15456
10.160.26.128 | 5435 | 10.160.26.2 | 64136
10.160.26.128 | 5435 | 10.160.26.1 | 1938
(5 rows)

6. Expose Stats via SQL with Python
CREATE FUNCTION get_server_connection_stats()
RETURNS SETOF server_connection AS
$$
# …
try:
with open('/proc/net/tcp') as fp:
lines = fp.read().splitlines()
except:
plpy.error('Unable to read /proc/net/tcp')
# …
return result
$$
LANGUAGE plpythonu;

7. Custom JSON Type Validation
CREATE TRIGGER validation AFTER INSERT OR UPDATE
ON config_value FOR EACH …
CREATE FUNCTION validate(value JSON, type_id INT)
RETURNS VOID AS
$$
import json
# … complex type checking here
# … using all of Python's expressiveness :-)
raise ValueError('{} not valid'.format(value))
$$
LANGUAGE plpythonu;

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