[mysql] how do u optimize Select Newest N result from BIG range

[Xuefer]

TABLE thread (
threadid INT,
forumid INT,
ctime INT, (creation time)
...
)
PRIMARY KEY threadid, auto_increment
INDEX(forumid,ctime)

query:
select .. from thread where forumid=1 and ctime>$one_month_ago order by ctime DESC
LIMIT 10

EXPLAIN gives "use index" and NO filesort, thus mysql will find by index as stop as soon as LIMIT exceed

but i now modify "forumid=1" to "forumid IN($idlist)", and EXPLAN gives "use index, filesort", which will get ALL match result from table into tmpfile, and sort as soon as LIMIT of rows is found.

btw: $idlist is not too much, but "thread"s in it is too many rows

any idea?

[jatar_k]

not sure but I'll bump it