Unnecessary column in a GROUP BY clause with QuerySet.exists()

[Marc Perrin]

I've got a question about the interaction between exists() and group by.

For example something like:

Manager.values('field').annotate(cnt=Count('id')).filter(cnt__gt=1).exists()

The corresp. query with PostgreSQL looks like this:

SELECT (1) AS "a"
FROM "app_model"
GROUP BY "app_model"."field", (1)
HAVING COUNT("app_model"."id") > 1
LIMIT 1

exists() (see ​https://github.com/django/django/blob/470708f50d8c13a50770893b8d7181f5218bf3ac/django/db/models/sql/query.py#L563) clears the SELECT clause and replaces it by (if I understand correctly) a hardcoded value 1 (as "a"), along with a limit of 1, which makes sense to me.

But get_group_by() (see ​https://github.com/django/django/blob/6b53114dd862ec97c282fdfdc83579cbd6d1560d/django/db/models/sql/compiler.py#L79) pushes this hardcoded value to the GROUP BY clause and we end up with the query above.

Now, on PostgreSQL, that works, but to me it sounds like it works by luck/lucky robustness... and certainly the same query without the , (1) in the GROUP BY clause yields the same result.
The problem is that outside of PostgreSQL that GROUP BY clause can be downright invalid...
Note that if I alter exists() to use {'a': 2} instead of {'a': 1}, it does not work anymore (on PostgreSQL), because (2) in the SELECT clause means the hardcoded number 2 while (2) in the GROUP BY clause presumably (??) refers to the 2nd expr of the SELECT clause, but we only got one...

My feeling is that get_group_by() should avoid adding extra/raw sql (or at least extra/raw pure values, if we can detect that...) to the group by expressions?

NB: the existing/old ticket that seems most related to my question would probably be: https://code.djangoproject.com/ticket/24835

[Mariusz Felisiak]

Thanks for the report. I agree that (1) is unnecessary in the GROUP BY clause unfortunately we cannot always remove constants from it because folks can use them as an alias for n-th column (as you already noticed). Maybe we could select only the first column from the GROUP BY clause instead:

django/db/models/sql/query.py

@@ -582 +582 @@
         q.clear_ordering(force=True)
         if limit:
             q.set_limits(high=1)
-        q.add_extra({"a": 1}, None, None, None, None, None)
-        q.set_extra_mask(["a"])
+        if q.group_by and not q.select:
+            q.add_select_col(q.group_by[0], "a")
+        else:
+            q.add_extra({"a": 1}, None , None, None, None, None)
+            q.set_extra_mask(["a"])
         return q
 
     def has_results(self, using):

Tentatively accepted for future investigation.

[Marc Perrin]

Thanks for the quick answer.

Your suggested workaround seems interesting indeed (provided the value of said column does not affect the exists() return value - for example if it's 0?)

NB: I didn't mean removing all constants from GROUP BY, rather: removing the automatic transmission of constants from the SELECT clause to the GROUP BY clause, since their meaning automatically switches from "constant number n" to "nth element of the select clause" which imo can never be intended.

[Mariusz Felisiak]

provided the value of said column does not affect the exists() return value - for example if it's 0?

No, it's not. It's enough for the query to return any rows.

[Simon Charette]

Maybe we should stop relying on extra and use Value(1) instead ​which already disables grouping.

django/db/models/sql/query.py

@@ -27 +27 @@
     OuterRef,
     Ref,
     ResolvedOuterRef,
+    Value,
 )
 from django.db.models.fields import Field
 from django.db.models.fields.related_lookups import MultiColSource
@@ -582 +583 @@
         q.clear_ordering(force=True)
         if limit:
             q.set_limits(high=1)
-        q.add_extra({"a": 1}, None, None, None, None, None)
-        q.set_extra_mask(["a"])
+        q.add_annotation(Value(1), "a")
         return q
 
     def has_results(self, using):

Anything that relies on extra is a black box to the compiler as it could be an aggregate function or not.

[Mariusz Felisiak]

Replying to Simon Charette:

Maybe we should stop relying on extra and use Value(1) instead ​which already disables grouping.

Great idea!

[Marc Perrin]

Sounds good indeed!

Replying to Mariusz Felisiak:

Replying to Simon Charette:

Maybe we should stop relying on extra and use Value(1) instead ​which already disables grouping.

Great idea!

[Mariusz Felisiak]

Marc, Would you like to prepare a patch?

[Marc Perrin]

Yeah, I guess I can do that, with Simon's suggestion the whole thing should be straightforward enough.
I'll take the ticket - I'll have to look up the contribution guide etc. / especially about the tests, as I haven't really contributed before.

[Mariusz Felisiak]

Replying to Marc Perrin:

Yeah, I guess I can do that, with Simon's suggestion the whole thing should be straightforward enough.
I'll take the ticket - I'll have to look up the contribution guide etc. / especially about the tests, as I haven't really contributed before.

Great, thanks! In this ticket we are forced to check a generated SQL, IMO it's fine to add assertions to an existing test e.g. aggregation.tests.AggregateTestCase.test_aggregation_subquery_annotation_exists:

        with self.assertNumQueries(1) as ctx:
            self.assertTrue(publisher_qs.exists())
        sql = ctx[0]["sql"]
        self.assertIn("SELECT 1 ", sql)
        self.assertNotIn("(1)", sql)
        # There is just one GROUP BY clause (zero commas means at most one
        # clause).
        self.assertEqual(sql[sql.index("GROUP BY") :].count(", "), 0)

[Marc Perrin]

Yeah, looks like the most relevant existing test case indeed, thanks for the pointer

[Marc Perrin]

Though after debugging this test, it might be a bit more complex than we want/need (e.g. there are 4 fields in the GROUP BY anyway...) and I might just add a new test e.g. "test_aggregation_annotation_exists" that gets close to what I mentioned originally (e.g.

Book.objects.values('publisher').annotate(cnt=Count('isbn')).filter(cnt__gt=1).exists()

)

[Marc Perrin]

But anyway I've started the PR: ​https://github.com/django/django/pull/15630 se we can discuss the details in there

[Mariusz Felisiak <felisiak.mariusz@…>]

In 4282fd4:

Fixed #33655 -- Removed unnecessary constant from GROUP BY clause for QuerySet.exists().