from __future__ import print_function
import sqlite3
import time

cnxn = sqlite3.connect(':memory:')
with cnxn:
    cnxn.execute('CREATE TABLE tbl (col1 PRIMARY KEY, col2)')
    cnxn.executemany('INSERT INTO tbl (col2) VALUES (?)', zip(range(2000000)))


sql_fast = """
UPDATE tbl
SET col2 = NULL
WHERE col1 = ?
"""

sql_slow = """
UPDATE
  tbl
SET col2 = NULL
WHERE col1 = ?
"""

half_ids = list(zip(range(1000000)))

start_time = time.clock()
with cnxn:
    cnxn.executemany(sql_fast, half_ids)
mid_time = time.clock()

print('First step: {s}'.format(s=mid_time - start_time))

with cnxn:
    cnxn.executemany(sql_slow, half_ids)
end_time = time.clock()

print('Second step: {s}'.format(s=end_time - mid_time))