bulk_batch_size() special-cases single field on SQLite according to outdated limit

[Jacob Walls]

On SQLite, bulk_batch_size() ​special-cases a field list of length 1 and applies the SQLITE_MAX_COMPOUND_SELECT limit to arrive at a value of 500.

I think this must date from before bulk inserts used VALUES syntax, which became available in SQLite in 2012, see ​discussion in Laravel.

When the list of fields exceeds 1, we go through the elif branch and arrive at much higher limits. I'm pretty sure this shows that the limit of 500 for fields=["pk"] is overly protective and can just be removed.

I don't have a unit test to provide, but you can play with changing the limit from 500 to 501 and see that test_large_delete still passes (no trouble selecting 501 objects).

(I found this while trying to ​refactor a different call site away from max_query_params in the hopes of just calling bulk_batch_size() until I saw how overly protective it was.)

I think this would be a good idea to resolve before anyone invests effort in ​reading the dynamic limit for this potentially irrelevant param.

[Jacob Walls]

Here's a rough test after all to demo, just adjust test_max_batch_size like this:

tests/bulk_create/tests.py

@@ -296 +296 @@
     @skipUnlessDBFeature("has_bulk_insert")
     def test_max_batch_size(self):
         objs = [Country(name=f"Country {i}") for i in range(1000)]
-        fields = ["name", "iso_two_letter", "description"]
+        fields = ["pk"]
         max_batch_size = connection.ops.bulk_batch_size(fields, objs)
         with self.assertNumQueries(ceil(len(objs) / max_batch_size)):
             Country.objects.bulk_create(objs)

---

The test fails as it was *more* efficient than expected:

======================================================================
FAIL: test_max_batch_size (bulk_create.tests.BulkCreateTests.test_max_batch_size)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/.../django/tests/bulk_create/tests.py", line 301, in test_max_batch_size
    with self.assertNumQueries(ceil(len(objs) / max_batch_size)):
         ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 1 != 2 : 1 queries executed, 2 expected
Captured queries were:
1. INSERT INTO "bulk_create_country" ("name", "iso_two_letter", "description") VALUES ('Country 0', '', ''), ('Country 1', '', ''), ('Country 2', '', ''), ('Country 3', '', ...

----------------------------------------------------------------------
Ran 2 tests in 0.025s

FAILED (failures=1)