On Sat, Apr 27, 2002 at 08:25:26AM -0400, Richard F. Rebel wrote: > > Hello, > > Perhaps I don't understand indexes but I was wondering how to improve teh > performance of a query on an indexed column that uses MAX(). > > mysql> explain select max(master_mtime) from redirects where uri_scheme = > 'http'; > >+-----------+------+-----------------------------+----------------+---------+-------+---------+------------+| > table | type | possible_keys | key | key_len > | ref | rows | Extra | > >+-----------+------+-----------------------------+----------------+---------+-------+---------+------------+| > redirects | ref | full_uri_idx,uri_scheme_idx | uri_scheme_idx | 16 > | const | 7694050 | where used | > >+-----------+------+-----------------------------+----------------+---------+-------+---------+------------+1 > row in set (0.01 sec) > > There is an index on master_mtime but it's not listed by explain???
Indexes typically help out in the WHERE clause. You'll notice that MySQL selected the uri_scheme_idx index because that's the column on which you've asked it to filter. You might get better performance out of a compund index on (uri_scheme, master_time), but you'll have to be the judge of that. Jeremy -- Jeremy D. Zawodny, <[EMAIL PROTECTED]> Technical Yahoo - Yahoo Finance Desk: (408) 349-7878 Fax: (408) 349-5454 Cell: (408) 685-5936 MySQL 3.23.47-max: up 79 days, processed 2,065,294,319 queries (302/sec. avg) --------------------------------------------------------------------- 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
