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id  select_type table   partitions  type    possible_keys   key key_len ref rows    filtered    Extra
1   SIMPLE  django_session  NULL    ALL NULL    NULL    NULL    NULL    24209837    33.33   Using where

id  select_type table   partitions  type    possible_keys   key key_len ref rows    filtered    Extra
1   SIMPLE  django_session  NULL    range   last_login  last_login  5   NULL    1   100.00  Using where; Using index

UPDATE v2: (after running ALTER TABLE django_session ADD KEY (last_login))

EXPLAIN SELECT COUNT(*) FROM django_session WHERE django_session.last_login >= '2019-03-04 13:00:37'

id  select_type table   partitions  type    possible_keys   key key_len ref rows    filtered    Extra
1   SIMPLE  django_session  NULL    ALL NULL    NULL    NULL    NULL    24209837    33.33   Using where