On Wed, 21 Aug 2002, Brian Moon wrote:
> If I have a query like: > select field1 from table where field2=0 order by field3 desc limit 30; > and I have keys: > key field2 (field2) > key field3 (field3) > > Will MySQL use one key to select and one to order? Or do I need a: > > key field2_field3 (field2, field3) > > This is a big table and i don't want to have to alter it unless I need > too. Hi Brian, In reading this: http://www.mysql.com/doc/en/ORDER_BY_optimisation.html Two points stick out re: your situation: "The following queries will use the index to resolve the ORDER BY / GROUP BY part: .. .. SELECT * FROM t1 WHERE key_part1=constant ORDER BY key_part2 " and.. "Some cases where MySQL can not use indexes to resolve the ORDER BY: (Note that MySQL will still use indexes to find the rows that matches the WHERE clause): .. The key used to fetch the rows are not the same one that is used to do the ORDER BY: SELECT * FROM t1 WHERE key2=constant ORDER BY key1 " Sorry -- no direct answer, but just in case you haven't read the above, I hope it helps. According to that second quote, your ORDER BY will not use the key on field3 for a speedy order. Regards, Neil Mansilla whatUseek.com --------------------------------------------------------------------- Before posting, please check: http://www.mysql.com/manual.php (the manual) http://lists.mysql.com/ (the list archive) To request this thread, e-mail <[EMAIL PROTECTED]> To unsubscribe, e-mail <[EMAIL PROTECTED]> Trouble unsubscribing? Try: http://lists.mysql.com/php/unsubscribe.php
