I seem to remember someone saying that if a query would end up returning more than x% of a table (I think it was either 30% or 50%), then mysql just ends up doing a full scan regardless...
> -----Original Message----- > From: Eric Anderson [mailto:[EMAIL PROTECTED] > Sent: Thursday, November 06, 2003 2:08 PM > To: [EMAIL PROTECTED] > Subject: This is confusing..? > > > > Given the following table: > > CREATE TABLE campaign_t ( > acct_id int(11) unsigned NOT NULL default '0', > site_id tinyint(3) unsigned NOT NULL default '0', > ref_id int(11) unsigned NOT NULL default '0', > datestamp char(10) NOT NULL default '', > raws int(11) unsigned NOT NULL default '0', > uniques int(11) unsigned NOT NULL default '0', > trial_signups int(11) NOT NULL default '0', > full_signups int(11) NOT NULL default '0', > annual_signups int(11) unsigned NOT NULL default '0', > PRIMARY KEY (acct_id,site_id,ref_id,datestamp), > KEY acct_id (acct_id), > KEY site_id (site_id), > KEY ref_id (ref_id), > KEY datestamp (datestamp) > ) TYPE=MyISAM; > > How come it doesn't use the 'datestamp' index on this query: > > mysql> explain SELECT * FROM campaign_t WHERE datestamp < 20041105\g > +------------+------+---------------+------+---------+------+- > -------+------------+ > | table | type | possible_keys | key | key_len | ref | > rows | Extra | > +------------+------+---------------+------+---------+------+- > -------+------------+ > | campaign_t | ALL | datestamp | NULL | NULL | NULL | > 438166 | where used | > +------------+------+---------------+------+---------+------+- > -------+------------+ > 1 row in set (0.00 sec) > > -- > MySQL General Mailing List > For list archives: http://lists.mysql.com/mysql > To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED] -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
