Improve speed of MySQL query with 5 left joins

Question

Working on a support ticketing system with not a lot of tickets (~3,000). To get a summary grid of ticket information, there are five LEFT JOIN statements on custom field table (j25_field_value) containing about 10,000 records. The query runs too long (~10 seconds) and in cases with a WHERE clause, it runs even longer (up to ~30 seconds or more).

Any suggestions for improving the query to reduce the time to run?

Four tables:

• j25_support_tickets

   CREATE TABLE `j25_support_tickets` ( `id` int(11) NOT NULL AUTO_INCREMENT, `category_id` int(11) NOT NULL DEFAULT '0', `user_id` int(11) DEFAULT NULL, `email` varchar(50) DEFAULT NULL, `subject` varchar(255) DEFAULT NULL, `message` text, `modified_date` datetime DEFAULT NULL, `priority_id` tinyint(3) unsigned DEFAULT NULL, `status_id` tinyint(3) unsigned DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `id` (`id`) ) ENGINE=MyISAM AUTO_INCREMENT=3868 DEFAULT CHARSET=utf8 

• j25_support_priorities

   CREATE TABLE `j25_support_priorities` ( `id` int(11) NOT NULL AUTO_INCREMENT, `title` varchar(100) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `id` (`id`) ) ENGINE=MyISAM AUTO_INCREMENT=14 DEFAULT CHARSET=utf8 

• j25_support_statuses

   CREATE TABLE `j25_support_statuses` ( `id` int(11) NOT NULL AUTO_INCREMENT, `title` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`), UNIQUE KEY `id` (`id`) ) ENGINE=MyISAM AUTO_INCREMENT=7 DEFAULT CHARSET=utf8 

• j25_field_value (id, ticket_id, field_id, field_value)

   CREATE TABLE `j25_support_field_value` ( `id` int(10) unsigned NOT NULL AUTO_INCREMENT, `ticket_id` int(11) DEFAULT NULL, `field_id` int(11) DEFAULT NULL, `field_value` tinytext, PRIMARY KEY (`id`) ) ENGINE=MyISAM AUTO_INCREMENT=10889 DEFAULT CHARSET=utf8 

Also, ran this:

    SELECT LENGTH(field_value) len FROM j25_support_field_value ORDER BY len DESC LIMIT 1
    note:  the result = 38

The query:

SELECT DISTINCT t.id as ID
, (select p.title from j25_support_priorities p where p.id = t.priority_id) as Priority
, (select s.title from j25_support_statuses   s where s.id = t.status_id)   as Status
,     t.subject       as Subject
,     t.email         as SubmittedByEmail
,  type.field_value   AS IssueType
,   ver.field_value   AS Version
, utype.field_value   AS UserType
,  cust.field_value   AS Company
, refno.field_value   AS RefNo
, t.modified_date     as Modified
    FROM j25_support_tickets AS t
LEFT JOIN j25_support_field_value AS type  ON t.id = type.ticket_id  AND  type.field_id =1
LEFT JOIN j25_support_field_value AS ver   ON t.id = ver.ticket_id   AND   ver.field_id =2
LEFT JOIN j25_support_field_value AS utype ON t.id = utype.ticket_id AND utype.field_id =3
LEFT JOIN j25_support_field_value AS cust  ON t.id = cust.ticket_id  AND  cust.field_id =4
LEFT JOIN j25_support_field_value AS refno ON t.id = refno.ticket_id AND refno.field_id =5

Answer 1

ALTER TABLE j25_support_field_value
ADD INDEX (`ticket_id`,`field_id`,`field_value`(50))

This index will work as a covering index for your query. It will allow the joins to use only this index to look up the values. It should perform massively faster than without this index, since currently your query would have to read every row in the table to find what matches each combination of ticket_id and field_id .

I would also suggest converting your tables to InnoDB engine, unless you have a very explicit reason for using MyISAM.

ALTER TABLE tablename ENGINE=InnoDB

Answer 2

As above - a better index would help. You could probably then simplify your query into something like this too (join to the table only once):

SELECT t.id as ID
, p.title  as Priority
, s.title as Status
,     t.subject       as Subject
,     t.email         as SubmittedByEmail
, case when v.field_id=1 then v.field_value else null end as IssueType
, case when v.field_id=2 then v.field_value else null end as Version
, case when v.field_id=3 then v.field_value else null end as UserType
, case when v.field_id=4 then v.field_value else null end as Company
, case when v.field_id=5 then v.field_value else null end as RefNo
 , t.modified_date     as Modified
    FROM  j25_support_tickets AS t
LEFT JOIN j25_support_field_value v ON t.id = v.ticket_id
LEFT JOIN j25_support_priorities p ON p.id = t.priority_id
LEFT JOIN j25_support_statuses  s ON s.id = t.status_id;

Answer 3

You can do away with the subqueries for starters and just get them from another join. You can add an index to j25_support_field_value

alter table j25_support_field_value add key(id, field_type);

I assume there is an index on id in j25_support_tickets - if not and if they are unique, add a unique index alter table j25_support_tickets add unique key(id); If they're not unique, remove the word unique from that statement.

In MySQL, a join usually requires an index on the field(s) that you are using to join on. This will hold up and produce very reasonable results with huge tables (100m+), if you follow that rule, you will not go wrong.

are the ids in j25_support_tickets unique? If they are you can do away with the distinct - if not, or if you are getting exact dupicates in each row, still do away with the distinct and add a group by t.id to the end of this:

SELECT t.id as ID
, p.title  as Priority
, s.title as Status
,     t.subject       as Subject
,     t.email         as SubmittedByEmail
,  type.field_value   AS IssueType
,   ver.field_value   AS Version
, utype.field_value   AS UserType
,  cust.field_value   AS Company
, refno.field_value   AS RefNo
, t.modified_date     as Modified
    FROM  j25_support_tickets AS t
LEFT JOIN j25_support_field_value AS type  ON t.id = type.ticket_id  AND  type.field_id =1
LEFT JOIN j25_support_field_value AS ver   ON t.id = ver.ticket_id   AND   ver.field_id =2
LEFT JOIN j25_support_field_value AS utype ON t.id = utype.ticket_id AND utype.field_id =3
LEFT JOIN j25_support_field_value AS cust  ON t.id = cust.ticket_id  AND  cust.field_id =4
LEFT JOIN j25_support_field_value AS refno ON t.id = refno.ticket_id AND refno.field_id =5
LEFT JOIN j25_support_priorities p ON p.id = t.priority_id
LEFT JOIN j25_support_statuses  s ON s.id = t.status_id;

Answer 4

• Switch to InnoDB.
• After switching to InnoDB, make the PRIMARY KEY for j25_support_field_value be (ticket_id, field_id) (and get rid if id ). (Tacking on field_value(50) will hurt, not help.)
• A PRIMARY KEY is a UNIQUE KEY , so don't have both.
• Use VARCHAR(255) instead of the nearly-equivalent TINYTEXT .
• EAV schema sucks. My ran on EAV .