Question

EXPLAIN 
    SELECT * FROM test_askcomment WHERE cid IN(1,2,3) ORDER BY dateline DESC;


id  select_type table   type    possible_keys   key key_len ref rows    Extra
1   SIMPLE  test_askcomment ALL PRIMARY NULL    NULL    NULL    3   Using where; Using filesort

Answer 1

ALTER TABLE `test_askcomment ` ADD INDEX `index1` (`cid`);

ALTER TABLE `test_askcomment ` ADD INDEX `index2` (`dateline `);

try to index your coloumns on which you are using where clause or selecting the coloumns