Using multiple cursors in a nested loop in sqlite3 from python 2.7, RE: http://stackoverflow.com/questions/13237788/using-multiple-cursors-in-a-nested-loop-in-sqlite3-from-python-2-7

nested.py

#!/usr/bin/python -u
# encoding: UTF-8

import sqlite3
import argparse
from datetime import datetime

DBFILE = 'nested.sqlite'
MAX_A = 1000
MAX_B = 10000
          
parser = argparse.ArgumentParser(description='Nested SQLite cursors in Python')
parser.add_argument('step', type=int)
args = parser.parse_args()

connection = sqlite3.connect(DBFILE)
connection.row_factory = sqlite3.Row
t0 = datetime.now()

if args.step == 0:
    # set up test database
    cursor = connection.cursor()
    cursor.execute("""DROP TABLE IF EXISTS A""")
    cursor.execute("""DROP TABLE IF EXISTS B""")
    # intentionally omitting primary keys
    cursor.execute("""CREATE TABLE A ( K INTEGER )""")
    cursor.execute("""CREATE TABLE B ( K INTEGER, L INTEGER )""")
    cursor.executemany("""INSERT INTO A ( K ) VALUES ( ? )""", 
        [ (i,) for i in range(0, MAX_A) ])
    connection.commit()
    for row in cursor.execute("""SELECT COUNT(*) CNT FROM A"""):
        print row['CNT']

if args.step == 1:
    # do the nested SELECT and INSERT
    read = connection.cursor()
    write = connection.cursor()
    for row in read.execute("""SELECT * FROM A"""):
        bs = [ ( row['K'], i ) for i in range(0, MAX_B) ]
        for b in bs: # with .executemany() it would be twice as fast ;)
            write.execute("""INSERT INTO B ( K, L ) VALUES ( ?, ? )""", b)
    connection.commit()
    for row in connection.cursor().execute("""SELECT COUNT(*) CNT FROM B"""):
        print row['CNT']

elif args.step == 2:
    connection = sqlite3.connect(DBFILE)
    connection.row_factory = sqlite3.Row
    control = connection.cursor()
    ca = cb = 0 # will count along our expectation
    for row in control.execute("""SELECT * FROM B ORDER BY K ASC, L ASC"""):
        assert row['K'] == ca and row['L'] == cb
        cb += 1
        if cb == MAX_B:
            cb = 0
            ca += 1
    assert ca == MAX_A and cb == 0
    for row in connection.cursor().execute("""SELECT COUNT(*) CNT FROM B"""):
        print row['CNT']

print datetime.now() - t0