How to optimize MySQL query for a large database - mysql

Question

I've noticed a serious problem recently, when my database increased to over 620000 records. Following query:
SELECT *,UNIX_TIMESTAMP(`time`) AS `time` FROM `log` WHERE (`projectname`="test" OR `projectname` IS NULL) ORDER BY `time` DESC LIMIT 0, 20
has an execution time about 2,5s on a local database. I was wondering how can I speed it up?
The EXPLAIN commands produces following output:
ID: 1
select type: SIMPLE
TABLE: log
type: ref_or_null
possible_keys: projectname
key: projectname
key_len: 387
ref: const
rows: 310661
Extra: Using where; using filesort
I've got indexes set on projectname, time columns.
Any help?
EDIT: Thanks to ypercube response, I was able to decrease query execution time. But when I only add another condition to WHERE clause (AND severity="changes") it lasts 2s again. Is it a good solution to include all of the possible "WHERE" columns to my merged-index?
ID: 1
select type: SIMPLE
TABLE: log
type: ref_or_null
possible_keys: projectname
key: projectname
key_len: 419
ref: const, const
rows: 315554
Extra: Using where; using filesort
Table structure:
CREATE TABLE `log` (
`id` INT(10) UNSIGNED NOT NULL AUTO_INCREMENT,
`projectname` VARCHAR(128) DEFAULT NULL,
`time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
`master` VARCHAR(128) NOT NULL,
`itemName` VARCHAR(128) NOT NULL,
`severity` VARCHAR(10) NOT NULL DEFAULT 'info',
`message` VARCHAR(255) NOT NULL,
`more` TEXT NOT NULL,
PRIMARY KEY (`id`),
KEY `projectname` (`severity`,`projectname`,`time`)
) ENGINE=INNODB AUTO_INCREMENT=621691 DEFAULT CHARSET=utf8

Answer 1

Add an index on (projectname, time):
ALTER TABLE log
ADD INDEX projectname_time_IX -- choose a name for the index
(projectname, time) ;
And then use the original column for the ORDER BY
SELECT *, UNIX_TIMESTAMP(time) AS unix_time
FROM log
WHERE (projectname = 'test' OR projectname IS NULL)
ORDER BY time DESC
LIMIT 0, 20 ;
or this variation - to make sure that the index is used effectively:
( SELECT *, UNIX_TIMESTAMP(time) AS unix_time
FROM log
WHERE projectname = 'test'
ORDER BY time DESC
LIMIT 20
)
UNION ALL
( SELECT *, UNIX_TIMESTAMP(time) AS unix_time
FROM log
WHERE projectname IS NULL
ORDER BY time DESC
LIMIT 20
)
ORDER BY time DESC
LIMIT 20 ;