mysql - What would the effect of using indices on those columns whose default value in null or contain null -

mysql - What would the effect of using indices on those columns whose default value in null or contain null -

explain extended select * application email = 'abcd@gmail.com';

this explain extended produces below result have applied proper index on email filed , default value null.i want know why scanning 3 rows instead of 1.any 1 can explain??

id, select_type, table, type, possible_keys, key, key_len, ref,rows, filtered, '1', 'simple', 'application', 'ref', 'idx_email', 'idx_email', '258', 'const', '3', '100.00', 'using pushed condition(`db`.`application`.`email` = \'abcd@gmail.com\')'

explain extended

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