Annotating a count of distinct ForeignKey (a) of a reverse foreignkey (b) doesn't work on oracle if (a) has any TextFields

[kimvais@…]

If there is a model X that has foreignkey to model Y and X has foreign key to Z and has TextFields the query for an instance of Y fails.

E.g.
y.xys.all().defer(list_of_all_text_fields).annotate(Count('z__pk', distinct=True) causes "DatabaseError: ORA-00932: inconsistent datatypes: expected - got NCLOB"

This is probably(?) because the "GROUP BY" clause will contain all the deferred fields.

As a workaround I have to do sum(len(set(x)) for x in y.xys.all().values_list('z__pk'))
Works flawlessly on PostgreSQL.

[kimvais@…]

Looks related to #17144

[Shai Berger]

This seems to be a correctness problem on Oracle, but may cause performance issues elsewhere (other backends will group over deferred fields, in particular text-fields, which may indeed bring back the problems of #17144).

What this needs now is a test-case. Note that if the test checks the generated SQL (this may be done with qset.query.sql_with_params()) it can be made to fail on non-oracle backends.

[Shai Berger]

#19259 appears related.

[Shai Berger]

#24096 is a more general (and simple) description of the problem, and has a test-case.