Gets the friend pairs from table derived_friend_list and finds their mutual friends. The mutual friend counts are found by taking the size of the intersection of their friend set.

add_mutual_friends_count.py

import psycopg2
import psycopg2.extras
from collections import defaultdict


conn_dtu = psycopg2.connect(<connstring>)
cur_dtu = conn_dtu.cursor(cursor_factory=psycopg2.extras.DictCursor)

cur_dtu.execute("""SELECT user_a, user_b FROM derived_friend_list""")

friend_pairs = tuple(cur_dtu.fetchall())

# Start by finding the friend set of each user
friend_lookup = defaultdict(lambda: set())
for (user_a, user_b) in friend_pairs:
    friend_lookup[user_a].add(user_b)
    friend_lookup[user_b].add(user_a)

for i, (user_a, user_b) in enumerate(friend_pairs):

    # Calculate mutual friend count
    mutual_friends = len(friend_lookup[user_a].intersection(friend_lookup[user_b]))

    cur_dtu.execute('update derived_friend_features set mutual_friend_count=%s WHERE user_a=%s AND user_b=%s',
                    (mutual_friends, user_a, user_b,))

    if i % 500 == 0:
        conn_dtu.commit()
        print('Reached count: {0} '.format(str(i)))
        print('{0} finished'.format(str((i+1) / len(friend_pairs))))

conn_dtu.commit()