Speed test: `exists()` vs. `get() + try/except

From Try2Explore.com by Abraham

问题描述

I’m writing tests for a django application and I want to check if an object has been saved to the database. Which is the most efficient/correct way to do it?

User.objects.filter(username=testusername).exists()

or

try:
    User.objects.get(username=testusername)
except User.DoesNotExist:

解决方案

Speed test: exists() vs get() + try/except

Test functions in test.py:

from testapp.models import User

def exists(x):
    return User.objects.filter(pk=x).exists()

def get(x):
    try:
        User.objects.get(pk=x)
        return True
    except User.DoesNotExist:
        return False

Using timeit in shell:

In [1]: from testapp import test
In [2]: %timeit for x in range(100): test.exists(x)
10 loops, best of 3: 88.4 ms per loop
In [3]: %timeit for x in range(100): test.get(x)
10 loops, best of 3: 105 ms per loop
In [4]: timeit for x in range(1000): test.exists(x)
1 loops, best of 3: 880 ms per loop
In [5]: timeit for x in range(1000): test.get(x)
1 loops, best of 3: 1.02 s per loop

Conclusion: exists() is over 10% faster for checking if an object has been saved in the database.

说明

在只要求判断数据库是否存在某条数据的情况，使用 exists()有更高的效率。

在需要获得实例并作检查等情况下，get() + try/except有着更广的应用场景。