Question

Table:

CREATE TABLE `table1` (
  `f1` int(11) NOT NULL default '0',
  `f2` int(4) NOT NULL default '0',
  `f3` bigint(20) NOT NULL default '0',
  PRIMARY KEY  (`f1`)
) TYPE=MyISAM

Query:

select `f1` from table1 where `f2`=123 order by `f3` desc;

I want create a “covering index” for this query

ALTER TABLE `table1` ADD INDEX (`f2`,`f3`,`f1`);

or

ALTER TABLE `table1` ADD INDEX (`f2`,`f1`,`f3`);

which should I choose?

Answer 1

The first one. MySQL can use either index to obtain the result set without needing to read from the actual table. The first index is slightly more efficient because it is not necessary to perform the extra step of re-ordering the rows.