Does SQLite optimize a query with multiple AND conditions in the WHERE clause?

Question

In SQL databases (I use Python+Sqlite), how to make sure that, if we have 1 million rows, the query

SELECT * FROM mytable WHERE myfunction(description) <          


        

Answer 1

Inspired by @GordThompson's answer, here is a benchmark between:

(1)  SELECT * FROM mytable WHERE col2 < 1000 AND myfunction(col1) < 500

vs.

(2)  SELECT * FROM mytable WHERE myfunction(col1) < 500 AND col2 < 1000

---

Test (1) (easy-to-test condition first): 1.02 seconds

import sqlite3, time, random

def myfunc(x):
    time.sleep(0.001) # wait 1 millisecond for each call of this function
    return x

# Create database
db = sqlite3.connect(':memory:')
db.create_function("myfunction", 1, myfunc)
c = db.cursor()
c.execute('CREATE TABLE mytable (col1 INTEGER, col2 INTEGER)');
for i in range(10*1000):
    a = random.randint(0,1000)
    c.execute('INSERT INTO mytable VALUES (?, ?)', (a, i));

# Do the evil query
t0 = time.time()
c.execute('SELECT * FROM mytable WHERE col2 < 1000 AND myfunction(col1) < 500')
for e in c.fetchall():
    print e
print "Elapsed time: %.2f" % (time.time() - t0)

Result: 1.02 seconds, it means that myfunc has been called max 1000 times, i.e. not for all the 10k rows.

---

Test (2) (Slow-to-compute condition first): 10.05 seconds

Idem with:

c.execute('SELECT * FROM mytable WHERE myfunction(col1) < 500 AND col2 < 1000')

instead.

Result: 10.05 seconds, it means that myfunc has been called ~ 10k times, i.e. for all the 10k rows, even those for which the condition col2 < 1000 is not True.

---

Global conclusion: Sqlite does lazy evaluation for AND, i.e. the easy condition has to be written first like this:

... WHERE  AND