Question

I have 3 tables :

CREATE TABLE `t_event` (
  `id` int(10) NOT NULL auto_increment,
  `title` varchar(100) NOT NULL,
  `kind` int(10) NOT NULL,
  `type` int(10) NOT NULL,
  `short_desc` varchar(500) default NULL,
  `long_desc` varchar(1500) default NULL,
  `location` int(10) NOT NULL,
  `price` decimal(11,0) NOT NULL,
  `currency` int(11) NOT NULL default '1',
  `remark_price` varchar(250) default NULL,
  `remark_prerequisite` varchar(250) default NULL,
  `date_start` date NOT NULL,
  `date_end` date default NULL,
  `date_remark` varchar(300) default NULL,
  `time_start` time default NULL,
  `time_end` time default NULL,
  `remark_time` varchar(50) default NULL,
  `leader` int(50) NOT NULL,
  `leader2` int(100) NOT NULL,
  `eve_contact_name` varchar(50) default NULL,
  `eve_contact_phone` varchar(50) default NULL,
  `eve_contact_email` varchar(50) default NULL,
  `eve_contact_url` varchar(150) default NULL,
  `eve_image_path` varchar(250) default NULL,
  `provider` int(10) default NULL,
  `timestamp` datetime NOT NULL,
  `last_change` datetime NOT NULL default '0000-00-00 00:00:00',
  `quality` int(10) default NULL,
  `min_number` int(10) NOT NULL,
  `max_number` int(10) NOT NULL,
  `active_for_reservation` tinyint(1) NOT NULL,
  `cancellation_day1` int(10) NOT NULL,
  `cancellation_day2` int(10) NOT NULL,
  `cancellation_fee1` varchar(255) NOT NULL,
  `cancellation_fee2` varchar(255) NOT NULL,
  PRIMARY KEY  (`id`),
  KEY `FK_t_event_t_event_kind` (`kind`),
  KEY `FK_t_event_t_event_type` (`type`),
  KEY `FK_t_event_t_location` (`location`),
  KEY `FK_t_event_t_currency` (`currency`),
  KEY `FK_t_event_t_leader` (`leader`),
  KEY `FK_t_event_t_provider` (`provider`),
  KEY `FK_t_event_t_quality` (`quality`),
  CONSTRAINT `FK_t_event_t_currency` FOREIGN KEY (`currency`) REFERENCES `t_currency` (`id`),
  CONSTRAINT `FK_t_event_t_event_kind` FOREIGN KEY (`kind`) REFERENCES `t_event_kind` (`id`),
  CONSTRAINT `FK_t_event_t_event_type` FOREIGN KEY (`type`) REFERENCES `t_event_type` (`id`),
  CONSTRAINT `FK_t_event_t_leader` FOREIGN KEY (`leader`) REFERENCES `t_leader` (`id`),
  CONSTRAINT `FK_t_event_t_location` FOREIGN KEY (`location`) REFERENCES `t_location` (`id`),
  CONSTRAINT `FK_t_event_t_provider` FOREIGN KEY (`provider`) REFERENCES `t_provider` (`id`),
  CONSTRAINT `FK_t_event_t_quality` FOREIGN KEY (`quality`) REFERENCES `t_quality` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8432 DEFAULT CHARSET=latin1

CREATE TABLE `t_location` (
  `id` int(10) NOT NULL auto_increment,
  `loc_name` varchar(50) NOT NULL,
  `loc_detail` varchar(50) default NULL,
  `loc_adress1` varchar(50) NOT NULL,
  `loc_adress2` varchar(50) default NULL,
  `loc_country` int(50) NOT NULL default '1',
  `loc_zip` varchar(50) NOT NULL,
  `loc_loc` varchar(50) NOT NULL,
  `loc_shortdesc` varchar(250) default NULL,
  `loc_contact_name` varchar(250) default NULL,
  `loc_contact_gender` int(10) default NULL,
  `loc_contact_phone` varchar(250) default NULL,
  `loc_contact_email` varchar(250) default NULL,
  `loc_contact_url` varchar(250) default NULL,
  `loc_image_path` varchar(250) default NULL,
  `latitude` varchar(100) default NULL,
  `longitude` varchar(100) default NULL,
  `created` datetime NOT NULL,
  `last_change` datetime NOT NULL default '0000-00-00 00:00:00',
  `provider` int(10) NOT NULL default '1',
  PRIMARY KEY  (`id`),
  UNIQUE KEY `id` USING BTREE (`id`),
  KEY `FK_t_location_t_country` (`loc_country`),
  KEY `FK_t_location_t_gender` (`loc_contact_gender`),
  KEY `FK_t_location_t_provider` (`provider`),
  CONSTRAINT `FK_t_location_t_country` FOREIGN KEY (`loc_country`) REFERENCES `t_country`(`id`),
  CONSTRAINT `FK_t_location_t_provider` FOREIGN KEY (`provider`) REFERENCES `t_provider` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1287 DEFAULT CHARSET=latin1

CREATE TABLE `t_dates` (
  `id` int(10) NOT NULL auto_increment,
  `events_id` int(10) NOT NULL,
  `events_start_date` date NOT NULL,
  `events_end_date` date NOT NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `IND_id` (`id`),
  KEY `IND_events_id` (`events_id`),
  CONSTRAINT `t_dates_ibfk_1` FOREIGN KEY (`events_id`) REFERENCES `t_event` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=32048 DEFAULT CHARSET=latin1

---

My Query is :

SELECT e.*,I.* ,d.*
FROM t_event AS e
INNER JOIN t_location AS I ON   I.id = e.location
INNER JOIN t_dates  AS d ON  d.events_id  = e.id
;

this query take 90s to be executed and return = 27727

The PROFILE command show that section “sending data” take almost the time of execution.
The EXPLAIN command is the following :

+----+------------+------+------+----------------------------+--------------------+---------+-----------+-------+-------+
| id | select_type | table | type | possible_keys               | key                | key_len | ref       | rows | Extra |
+----+------------+------+------+----------------------------+--------------------+---------+-----------+-------+-------+
|  1 | SIMPLE     | I    | ALL | PRIMARY,id                 | NULL               | NULL    | NULL      |  1143 |       |
|  1 | SIMPLE     | e    | ref  | PRIMARY,FK_t_event_t_location | FK_t_event_t_location | 4       | wu_db.I.id |     4 |       |
|  1 | SIMPLE     | d    | ref  | IND_events_id               | IND_events_id       | 4       | wu_db.e.id |     3 |       |
+----+------------+------+------+----------------------------+--------------------+---------+-----------+-------+-------+

My point of view is that the big number of column is responsible of this slowdown but even when I write “SELECT e.id, I.events_id, d.id” it still take 16 s.

I think that I have to rewrite the query with LIMIT and OFFSET clause, what do you think?

number of records for each tables :

• t_event = 7991
• t_location = 1086
• t_dates = 27727

Answer 1

Broadly speaking, MySQL can only filter records using one index from each table in a query.

That is, whilst your t_event table has indexes defined on both id and location, only one of those indexes can be used to satisfy your query. You can see this in your EXPLAIN output, which indicates that both the PRIMARY and FK_t_event_t_location keys were identified as possibly useful (with the latter actually selected for use).

Therefore, your join with t_dates, which involves a test on the id column, is being fulfilled with a table scan rather than an index lookup. Again, you can see this from the first row in the EXPLAIN output which shows type = ALL (table scan) and key = NULL (no index being used).

You should create a composite index on (id, location) for your t_event table:

ALTER TABLE t_event ADD INDEX (id, location);