In Django, what is the most efficient way to check for an empty query set?

Question

I've heard suggestions to use the following:

if qs.exists():     ...  if qs.count():     ...  try:     qs[0] except IndexError:     ... 

Copied from comment below: "I'm looking for a statement like "In MySQL and PostgreSQL count() is faster for short queries, exists() is faster for long queries, and use QuerySet[0] when it's likely that you're going to need the first element and you want to check that it exists. However, when count() is faster it's only marginally faster so it's advisable to always use exists() when choosing between the two."

Answer 1

query.exists() is the most efficient way.

Especially on postgres count() can be very expensive, sometimes more expensive then a normal select query.

exists() runs a query with no select_related, field selections or sorting and only fetches a single record. This is much faster then counting the entire query with table joins and sorting.

qs[0] would still includes select_related, field selections and sorting; so it would be more expensive.

The Django source code is here (django/db/models/sql/query.py RawQuery.has_results):

https://github.com/django/django/blob/60e52a047e55bc4cd5a93a8bd4d07baed27e9a22/django/db/models/sql/query.py#L499

def has_results(self, using):     q = self.clone()     if not q.distinct:         q.clear_select_clause()     q.clear_ordering(True)     q.set_limits(high=1)     compiler = q.get_compiler(using=using)     return compiler.has_results() 

Another gotcha that got me the other day is invoking a QuerySet in an if statement. That executes and returns the whole query !

If the variable query_set may be None (unset argument to your function) then use:

if query_set is None:     #  

not:

if query_set:    # you just hit the database