Why is django ORM so much slower than raw SQL

问题:

I have the following two pieces of code:

First, in SQL:

self.cursor.execute('SELECT apple_id FROM main_catalog WHERE apple_id=%s', apple_id)
if self.cursor.fetchone():
    print '##' 

Next, in Django:

if Catalog.objects.filter(apple_id=apple_id).exists():
    print '>>>'

Doing it the first way is about 4x faster than the second way in a loop of 100k entries. What accounts for Django being so much slower?

回答1:

Typically ORMs go to the trouble of instantiating a complete object for each row and returning it. Your raw SQL doesn't do that, so it won't take the penalty that incurs. For large result sets where you don't intend to use the object, it is better to bypass the ORM.