import psycopg2
import random
from concurrent.futures import ThreadPoolExecutor, as_completed

def write_sim_to_db(all_ids2):
    if all_ids1[i] != all_ids2:
        c.execute("""SELECT count(*) FROM similarity WHERE prod_id1 = %s AND prod_id2 = %s""", (all_ids1[i], all_ids2,))
        count = c.fetchone()
        if count[0] == 0:
            sim_sum = random.random()
            c.execute("""INSERT INTO similarity(prod_id1, prod_id2, sim_sum)
                    VALUES(%s, %s, %s)""", (all_ids1[i], all_ids2, sim_sum,))
            conn.commit()

conn = psycopg2.connect("dbname='db' user='user' host='localhost' password='pass'")
c = conn.cursor()

all_ids1 = list(n for n in range(1000))
all_ids2_list = list(n for n in range(1000))

for i in range(len(all_ids1)):
    with ThreadPoolExecutor(max_workers=5) as pool:
        results = [pool.submit(write_sim_to_db, i) for i in all_ids2_list]

Segmentation fault (core dumped)

*** Error in `python3': double free or corruption (out): 0x00007fe574002270 ***
Aborted (core dumped)

with ThreadPoolExecutor(max_workers=1) as pool: