PostgreSQL, The cool stuff.

A collection of my favourite Postgres features. Rather than going into much detail I will link you to the relevant documentation (for now).


Create a column which can store key value data in it. This can then be queried by keys and values. As a data type this maps well to a Python dictionary.

Quick example - insert data, select one key and filter by one key.

CREATE TABLE example (
id serial PRIMARY KEY,
data hstore);

INSERT INTO example (data) VALUES
('name => "John Smith", age => 28, gender => "M"'),
('name => "Jane Smith", age => 24');

SELECT id, data->'name' FROM example;

SELECT id, data->'age' FROM example
WHERE data->'age' >= '25';


Use Python on the database. This allows you to reuse code, do faster python (as there is no round trip) and add database constraints with Python.

How to many a simple max function that given two numbers returns the biggest.


CREATE FUNCTION pymax (a integer, b integer)
  RETURNS integer
AS $$
  if a > b:
    return a
  return b
$$ LANGUAGE plpythonu;

SELECT pymax(1, 2);

Foreign Data Wrappers

Access data in other databases from Postgres. There are a large number of wrappers that allow you to query databases like Redis or MongoDB. This is particularly useful for doing reports and joins on a NoSQL data or migrating data from one database to Postgres.

Psycopg2 and Async

The Python psycopg2 driver supports async. This allows you to make queries and rather than wait for the result you can continue doing something while you wait. Typically this is used when working with something like Twisted but for slower or longer queries it would be useful.

Quick async example on an intentionally slow query.

import psycopg2
from psycopg2.extras import wait_select

aconn = psycopg2.connect(..., async=1)
acurs = aconn.cursor()
acurs.execute("SELECT pg_sleep(5); SELECT * FROM example;")

Table Inheritance

Rather than implementing 1-2-1 relationships between tables to fake inheritance you can actually extend tables via inheritance.

Inheriting from another table is very easy, just part of the child table definition.

CREATE TABLE detailed_example(
    more_data hstore
) INHERITS (example);

Transaction Savepoints

In a database transaction you can create savepoints and roll back a transaction to that point (rather than all or nothing.)

Nice example, borrowed from the docs.

UPDATE accounts SET balance = balance - 100.00
    WHERE name = 'Alice';
SAVEPOINT my_savepoint;
UPDATE accounts SET balance = balance + 100.00
    WHERE name = 'Bob';
~~ oops ... forget that and use Wally's account
ROLLBACK TO my_savepoint;
UPDATE accounts SET balance = balance + 100.00
    WHERE name = 'Wally';

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