Sorry, my previous e-mail was a test on MySQL-5.5.28 on an empty table. Here is the MySQL-5.1 Percona testing table:
mysql> select count(*) from send_sms_test; +----------+ | count(*) | +----------+ | 143879 | +----------+ 1 row in set (0.03 sec) Without LIMIT: mysql> desc select * from send_sms_test FORCE INDEX (time_priority) where time<=UNIX_TIMESTAMP(NOW()) order by priority; +----+-------------+---------------+-------+---------------+---------------+---------+------+-------+-----------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------------+-------+---------------+---------------+---------+------+-------+-----------------------------+ | 1 | SIMPLE | send_sms_test | range | time_priority | time_priority | 8 | NULL | 73920 | Using where; Using filesort | +----+-------------+---------------+-------+---------------+---------------+---------+------+-------+-----------------------------+ 1 row in set (0.00 sec) mysql> desc select * from send_sms_test FORCE INDEX (priority_time) where time<=UNIX_TIMESTAMP(NOW()) order by priority; +----+-------------+---------------+-------+---------------+---------------+---------+------+--------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------------+-------+---------------+---------------+---------+------+--------+-------------+ | 1 | SIMPLE | send_sms_test | index | NULL | priority_time | 12 | NULL | 147840 | Using where | +----+-------------+---------------+-------+---------------+---------------+---------+------+--------+-------------+ 1 row in set (0.00 sec) But I actually need to use LIMIT, because client uses this to limit the number of records returned to process. mysql> select * from send_sms_test FORCE INDEX (priority_time) where time<=UNIX_TIMESTAMP(NOW()) order by priority LIMIT 0,100; 100 rows in set (0.00 sec) mysql> show profile; +--------------------+----------+ | Status | Duration | +--------------------+----------+ | starting | 0.000053 | | Opening tables | 0.000009 | | System lock | 0.000005 | | Table lock | 0.000004 | | init | 0.000037 | | optimizing | 0.000005 | | statistics | 0.000007 | | preparing | 0.000005 | | executing | 0.000001 | | Sorting result | 0.000003 | | Sending data | 0.000856 | | end | 0.000003 | | query end | 0.000001 | | freeing items | 0.000015 | | logging slow query | 0.000001 | | logging slow query | 0.000047 | | cleaning up | 0.000002 | +--------------------+----------+ 17 rows in set (0.00 sec) mysql> select * from send_sms_test FORCE INDEX (time_priority) where time<=UNIX_TIMESTAMP(NOW()) order by priority LIMIT 0,100; 100 rows in set (0.08 sec) mysql> show profile; +--------------------+----------+ | Status | Duration | +--------------------+----------+ | starting | 0.000048 | | Opening tables | 0.000009 | | System lock | 0.000002 | | Table lock | 0.000004 | | init | 0.000047 | | optimizing | 0.000006 | | statistics | 0.000043 | | preparing | 0.000018 | | executing | 0.000001 | | Sorting result | 0.076725 | | Sending data | 0.001406 | | end | 0.000003 | | query end | 0.000001 | | freeing items | 0.000012 | | logging slow query | 0.000001 | | cleaning up | 0.000002 | +--------------------+----------+ 16 rows in set (0.00 sec) As you can see latter query takes more time, because it's using filesort as well. Now, handler: mysql> SHOW SESSION STATUS LIKE 'Handler_read%';select * from send_sms_test FORCE INDEX (priority_time) where time<=UNIX_TIMESTAMP(NOW()) order by priority LIMIT 0,100;SHOW SESSION STATUS LIKE 'Handler_read%'; +-----------------------+--------+ | Variable_name | Value | +-----------------------+--------+ | Handler_read_first | 18 | | Handler_read_key | 244 | | Handler_read_next | 719969 | | Handler_read_prev | 0 | | Handler_read_rnd | 226 | | Handler_read_rnd_next | 223 | +-----------------------+--------+ 6 rows in set (0.00 sec) .... 100 rows in set (0.00 sec) +-----------------------+--------+ | Variable_name | Value | +-----------------------+--------+ | Handler_read_first | 19 | | Handler_read_key | 245 | | Handler_read_next | 720068 | | Handler_read_prev | 0 | | Handler_read_rnd | 226 | | Handler_read_rnd_next | 223 | +-----------------------+--------+ 6 rows in set (0.00 sec) mysql> SHOW SESSION STATUS LIKE 'Handler_read%'; +-----------------------+--------+ | Variable_name | Value | +-----------------------+--------+ | Handler_read_first | 17 | | Handler_read_key | 143 | | Handler_read_next | 576090 | | Handler_read_prev | 0 | | Handler_read_rnd | 126 | | Handler_read_rnd_next | 223 | +-----------------------+--------+ 6 rows in set (0.00 sec) mysql> select * from send_sms_test FORCE INDEX (time_priority) where time<=UNIX_TIMESTAMP(NOW()) order by priority LIMIT 0,100; 100 rows in set (0.09 sec) mysql> SHOW SESSION STATUS LIKE 'Handler_read%'; +-----------------------+--------+ | Variable_name | Value | +-----------------------+--------+ | Handler_read_first | 18 | | Handler_read_key | 244 | | Handler_read_next | 719969 | | Handler_read_prev | 0 | | Handler_read_rnd | 226 | | Handler_read_rnd_next | 223 | +-----------------------+--------+ 6 rows in set (0.00 sec) I don't understand much in Handler thing, could you please explain more, based on the results I've posted ? In which case it works better and how it uses the index? About BIGINT(20) and INT(3) I will look further into this later, I understand it might be oversized, but my main question is about index why it's using it so weird. Many thanks for your quick answer! 2012/10/16 Rick James <rja...@yahoo-inc.com> > * Rows = 11 / 22 -- don't take the numbers too seriously; they are crude > approximations based on estimated cardinality. > > * The 11 comes from the LIMIT -- therefore useless in judging the > efficiency. (The 22 may be 2*11; I don't know.) > > * Run the EXPLAINs without LIMIT -- that will avoid the bogus 11/22. > > * If the CREATE INDEX took only 0.67 sec, I surmise that you have very few > rows in the table?? So this discussion is not necessarily valid in general > cases. > > * What percentage of time values meet the WHERE? This has a big impact on > the choice of explain plan and performance. > > * Set long_query_time = 0; to get it in the slowlog even if it is fast. > Then look at the various extra values (such as filesort, on disk, temp > table used, etc). > > * Do this (with each index): > SHOW SESSION STATUS LIKE 'Handler_read%'; > SELECT ... FORCE INDEX(...) ...; > SHOW SESSION STATUS LIKE 'Handler_read%'; > Then take the diffs of the handler counts. This will give you a pretty > detailed idea of what is going on; better than the SlowLog. > > * INT(3) is not a 3-digit integer, it is a full 32-bit integer (4 bytes). > Perhaps you should have SMALLINT UNSIGNED (2 bytes). > > * BIGINT takes 8 bytes -- usually over-sized. > > > > -----Original Message----- > > From: spameden [mailto:spame...@gmail.com] > > Sent: Monday, October 15, 2012 1:42 PM > > To: mysql@lists.mysql.com > > Subject: mysql logs query with indexes used to the slow-log and not > > logging if there is index in reverse order > > > > Hi, list. > > > > Sorry for the long subject, but I'm really interested in solving this > > and need a help: > > > > I've got a table: > > > > mysql> show create table send_sms_test; > > +---------------+------------------------------------------------------ > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > -------------------------------+ > > | Table | Create > > Table > > | > > +---------------+------------------------------------------------------ > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > ----------------------------------------------------------------------- > > -------------------------------+ > > | send_sms_test | CREATE TABLE `send_sms_test` ( > > `sql_id` bigint(20) NOT NULL AUTO_INCREMENT, > > `momt` enum('MO','MT') DEFAULT NULL, > > `sender` varchar(20) DEFAULT NULL, > > `receiver` varchar(20) DEFAULT NULL, > > `udhdata` blob, > > `msgdata` text, > > `time` bigint(20) NOT NULL, > > `smsc_id` varchar(255) DEFAULT 'main', > > `service` varchar(255) DEFAULT NULL, > > `account` varchar(255) DEFAULT NULL, > > `id` bigint(20) DEFAULT NULL, > > `sms_type` tinyint(1) DEFAULT '2', > > `mclass` bigint(20) DEFAULT NULL, > > `mwi` bigint(20) DEFAULT NULL, > > `coding` bigint(20) DEFAULT NULL, > > `compress` bigint(20) DEFAULT NULL, > > `validity` bigint(20) DEFAULT NULL, > > `deferred` bigint(20) DEFAULT NULL, > > `dlr_mask` bigint(20) DEFAULT NULL, > > `dlr_url` varchar(255) DEFAULT NULL, > > `pid` bigint(20) DEFAULT NULL, > > `alt_dcs` bigint(20) DEFAULT NULL, > > `rpi` bigint(20) DEFAULT NULL, > > `charset` varchar(255) DEFAULT NULL, > > `boxc_id` varchar(255) DEFAULT NULL, > > `binfo` varchar(255) DEFAULT NULL, > > `meta_data` text, > > `task_id` bigint(20) DEFAULT NULL, > > `msgid` bigint(20) DEFAULT NULL, > > `priority` int(3) unsigned NOT NULL DEFAULT '500', > > PRIMARY KEY (`sql_id`), > > KEY `task_id` (`task_id`), > > KEY `receiver` (`receiver`), > > KEY `msgid` (`msgid`), > > KEY `priority_time` (`priority`,`time`) > > ) ENGINE=InnoDB AUTO_INCREMENT=7806318 DEFAULT CHARSET=utf8 > > > > Slow-queries turned on with an option: > > | log_queries_not_using_indexes | ON | > > > > mysqld --version > > mysqld Ver 5.1.65-rel14.0 for debian-linux-gnu on x86_64 ((Percona > > Server (GPL), 14.0, Revision 475)) > > > > If I check with EXPLAIN MySQL says it would use the index: > > mysql> *desc select * from send_sms_test where > > mysql> time<=UNIX_TIMESTAMP(NOW()) > > order by priority limit 0,11;* > > +----+-------------+---------------+-------+---------------+----------- > > ----+---------+------+------+-------------+ > > | id | select_type | table | type | possible_keys | key > > | key_len | ref | rows | Extra | > > +----+-------------+---------------+-------+---------------+----------- > > ----+---------+------+------+-------------+ > > | 1 | SIMPLE | send_sms_test | index | NULL | > > priority_time > > | 12 | NULL | * 11* | Using where | > > +----+-------------+---------------+-------+---------------+----------- > > ----+---------+------+------+-------------+ > > 1 row in set (0.00 sec) > > > > But If I issue the query I see in the mysql-slow.log: > > select * from send_sms_test where time<=UNIX_TIMESTAMP(NOW()) order by > > priority limit 0,11; > > > > If I do create INDEX time,priority (in reverse order instead of > > priority,time) I get still the same usage of priority_time key with the > > same length, but rows now are doubled): > > mysql> *create index time_priority ON send_sms_test (time,priority);* > > Query OK, 0 rows affected (0.67 sec) > > Records: 0 Duplicates: 0 Warnings: 0 > > > > mysql> *desc select * from send_sms_test where > > mysql> time<=UNIX_TIMESTAMP(NOW()) > > order by priority limit 0,11;* > > +----+-------------+---------------+-------+---------------+----------- > > ----+---------+------+------+-------------+ > > | id | select_type | table | type | possible_keys | key > > | key_len | ref | rows | Extra | > > +----+-------------+---------------+-------+---------------+----------- > > ----+---------+------+------+-------------+ > > | 1 | SIMPLE | send_sms_test | index | time_priority | > > priority_time > > | 12 | NULL | *22* | Using where | > > +----+-------------+---------------+-------+---------------+----------- > > ----+---------+------+------+-------------+ > > > > And if both indexes created I do not have anymore this query in the > > slow-log. > > > > Of course If I disable log_queries_not_using_indexes I get none of the > > queries. > > > > So is it a bug inside Percona's implementation or it's generally MySQL > > behavior? > > > > Thanks >
