Lots of temporary tables in MySQL - mysql

I am running website which i have below two queries which are very much repetitive. And in MySQL innoDB processes in can see they are taking lots of time and whenever i see processes those are there they keep on creating temporary tables and takes long to execute taking lots of memory and CPU.
They were really bad i somehow manage to optimised these.. but i am not able to do beyond that.
$getmoddetails = "SELECT a.id, a.name, a.defvar, a.description, a.icon, a.thumb, a.average_rating, a.total_rating, c.rating, a.group_access, d.long_name, a.editor_id, e.users_count
FROM dir_cat_item AS b
INNER JOIN dir_item AS a ON a.id = b.item_id
AND a.status = 'O'
LEFT JOIN dir_item_notation_user_map AS c ON a.id = c.item_id
AND c.user_id =%u
LEFT JOIN users AS d ON d.id = a.editor_id
LEFT JOIN (SELECT item_id, COUNT(*) AS users_count
FROM module
GROUP BY item_id) AS e ON e.item_id = b.item_id
WHERE a.id=%u";
$getnbModules_by_col = "SELECT
posx,COUNT(posx) as nb
FROM module WHERE
user_id = %u
AND profile_id = %u
GROUP BY posx
ORDER BY posx ASC";
Table index on Module
- item_id
- user_id
- profile_id
- uniq
For USERS Table
- id
- username
Any suggestion please...
Update :-
CREATE TABLE IF NOT EXISTS `module` (
`item_id` mediumint(8) unsigned NOT NULL DEFAULT '0',
`user_id` int(10) unsigned NOT NULL DEFAULT '0',
`profile_id` int(3) unsigned NOT NULL DEFAULT '0',
`posx` tinyint(3) unsigned NOT NULL DEFAULT '0',
`posy` tinyint(3) unsigned NOT NULL DEFAULT '0',
`posj` tinyint(3) unsigned NOT NULL DEFAULT '0',
`x` smallint(5) unsigned NOT NULL DEFAULT '0',
`y` smallint(5) unsigned NOT NULL DEFAULT '0',
`typ` char(1) CHARACTER SET utf8 NOT NULL DEFAULT 'D',
`variables` text COLLATE utf8_unicode_ci,
`uniq` smallint(5) unsigned NOT NULL DEFAULT '1',
`blocked` tinyint(1) unsigned NOT NULL DEFAULT '0',
`minimized` tinyint(1) unsigned NOT NULL DEFAULT '0',
`old_id` tinyint(3) unsigned DEFAULT NULL,
`feed_id` mediumint(8) unsigned NOT NULL DEFAULT '0',
`shared` varchar(33) COLLATE utf8_unicode_ci DEFAULT NULL,
`currentview` varchar(255) COLLATE utf8_unicode_ci DEFAULT NULL,
KEY `item_id` (`item_id`,`user_id`,`profile_id`,`uniq`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;
item_id 18 A No
user_id 393467 A No
profile_id 393467 A No
uniq 393467 A No
Thank you in advance

For best performance of that second query, you'd need an appropriate index on the module table, e.g.
... ON module (user_id, profile_id, posx)
For the first query, you'd likely benefit from a different index on the module table:
... ON module (item_id)
But without the table definitions, datatype and cardinality of the columns, it's really impossible to make definitive suggestions.
In your first query, I'd suggest you add a predicate in the inline view (derived table) aliased as e. I don't think MySQL is pushing the predicate from the outer query into the inline view.
( SELECT item_id
, COUNT(*) AS users_count
FROM module
WHERE item_id = %u
GROUP BY item_id
) AS e
You're going to need to supply the same value in that WHERE clause as you are providing in the WHERE clause on the outer query. From what I'm reading there...
e.item_id = b.item_id = a.id = %u
By adding that WHERE clause in that inline view, that should cut down the number or rows retrieved from the module table, and that derived table will have just one row. An index with a leading column of item_id would be a covering index. The EXPLAIN PLAN should show Using index and not show Using filesort.
If your first query is pulling back a relatively small number of rows, you might consider a correlated subquery on the module table, in place of a join to the derived table (aliased as e) in your first query (to avoid materializing a large derived table). (In general, however, correlated subqueries can be real performance killers. But in some cases, where the outer query is pulling back a small number of rows, the repeated execution of the subquery can actually perform better than generating a large derived table, which you only need a few rows from.)
SELECT a.id
, a.name
, a.defvar
, a.description
, a.icon
, a.thumb
, a.average_rating
, a.total_rating
, c.rating
, a.group_access
, d.long_name
, a.editor_id
, ( SELECT SUM(1)
FROM module e
WHERE e.item_id = b.item_id
AND e.item_id = %u
) AS users_count
FROM dir_cat_item b
JOIN dir_item a
ON a.id = b.item_id
AND b.item_id = %u
AND a.status = 'O'
LEFT
JOIN dir_item_notation_user_map c
ON c.item_id = a.id
AND c.item_id = %u
AND c.user_id = %u
LEFT
JOIN users d
ON d.id = a.editor_id
WHERE a.id = %u

Related

Mysql query with multiple selects results in high CPU load

I'm trying to do a link exchange script and run into a bit of trouble.
Each link can be visited by an IP address a number of x times (frequency in links table). Each visit costs a number of credits (spend limit given in limit in links table)
I've got the following tables:
CREATE TABLE IF NOT EXISTS `contor` (
`key` varchar(25) NOT NULL,
`uniqueHandler` varchar(30) DEFAULT NULL,
`uniqueLink` varchar(30) DEFAULT NULL,
`uniqueUser` varchar(30) DEFAULT NULL,
`owner` varchar(50) NOT NULL,
`ip` varchar(15) DEFAULT NULL,
`credits` float NOT NULL,
`tstamp` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`key`),
KEY `uniqueLink` (`uniqueLink`),
KEY `uniqueHandler` (`uniqueHandler`),
KEY `uniqueUser` (`uniqueUser`),
KEY `owner` (`owner`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
CREATE TABLE IF NOT EXISTS `links` (
`unique` varchar(30) NOT NULL DEFAULT '',
`url` varchar(1000) DEFAULT NULL,
`frequency` varchar(5) DEFAULT NULL,
`limit` float NOT NULL DEFAULT '0',
PRIMARY KEY (`unique`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
I've got the following query:
$link = MYSQL_QUERY("
SELECT *
FROM `links`
WHERE (SELECT count(key) FROM contor WHERE ip = '$ip' AND contor.uniqueLink = links.unique) <= `frequency`
AND (SELECT sum(credits) as cost FROM contor WHERE contor.uniqueLink = links.unique) <= `limit`")
There are 20 rows in the table links.
The problem is that whenever there are about 200k rows in the table contor the CPU load is huge.
After applying the solution provided by #Barmar:
Added composite index on (uniqueLink, ip) and droping all other indexes except PRIMARY, EXPLAIN gives me this:
id select_type table type possible_keys key key_len ref rows Extra
1 PRIMARY l ALL NULL NULL NULL NULL 18
1 PRIMARY <derived2> ALL NULL NULL NULL NULL 15
2 DERIVED pop_contor index NULL contor_IX1 141 NULL 206122
Try using a join rather than a correlated subquery.
SELECT l.*
FROM links AS l
LEFT JOIN (
SELECT uniqueLink, SUM(ip = '$ip') AS ip_visits, SUM(credits) AS total_credits
FROM contor
GROUP BY uniqueLink
) AS c
ON c.uniqueLink = l.unique AND ip_visits <= frequency AND total_credits <= limit
If this doesn't help, try adding an index on contor.ip.
The current query is of the form:
SELECT l.*
FROM `links` l
WHERE l.frequency >= ( SELECT COUNT(ck.key)
FROM contor ck
WHERE ck.uniqueLink = l.unique
AND ck.ip = '$ip'
)
AND l.limit >= ( SELECT SUM(sc.credits)
FROM contor sc
WHERE sc.uniqueLink = l.unique
)
Those correlated subqueries are going to each your lunch. And your lunchbox too.
I'd suggest testing an inline view that performs both of the aggregations from contor in one pass, and then join the result from that to the links table.
Something like this:
SELECT l.*
FROM ( SELECT c.uniqueLink
, SUM(c.ip = '$ip' AND c.key IS NOT NULL) AS count_key
, SUM(c.credits) AS sum_credits
FROM `contor` c
GROUP
BY c.uniqueLink
) d
JOIN `links` l
ON l.unique = d.uniqueLink
AND l.frequency >= d.count_key
AND l.limit >= d.sum_credits
For optimal performance of the aggregation inline view query, provide a covering index that MySQL can use to optimize the GROUP BY (avoiding a Using filesort operation)
CREATE INDEX `contor_IX1` ON `contor` (`uniqueLink`, `credits`, `ip`) ;
Adding that index renders the uniqueLink index redundant, so also...
DROP INDEX `uniqueLink` ON `contor` ;
EDIT
Since we have a guarantee that contor.key column is non-NULL (i.e. the NOT NULL constraint), this part of the query above is unneeded AND c.key IS NOT NULL, and can be removed. (I also removed the key column from the covering index definition above.)
SELECT l.*
FROM ( SELECT c.uniqueLink
, SUM(c.ip = '$ip') AS count_key
, SUM(c.credits) AS sum_credits
FROM `contor` c
GROUP
BY c.uniqueLink
) d
JOIN `links` l
ON l.unique = d.uniqueLink
AND l.frequency >= d.count_key
AND l.limit >= d.sum_credits

Improve speed of MySQL query with 5 left joins

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
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
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;
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;
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.

MySQL with JOINS is very slow

I have this query:
select
b.user as user1, b.timestamp as ts1,
c.user as user2, c.timestamp as ts2,
d.user as user3, d.timestamp as ts3,
e.user as user4, e.timestamp as ts4,
f.user as user5, f.timestamp as ts5,
g.user as user6, g.timestamp as ts6,
h.user as user7, h.timestamp as ts7,
i.user as user8, i.timestamp as ts8,
j.user as user9, j.timestamp as ts9,
k.user as user10, k.timestamp as ts10,
a.beschreibung, a.auftragsnummer, a.faellig, a.subkunde,
(SELECT firma from kunden where id=a.kunde limit 0,1) as kunde,
(SELECT kommision from kommision where id=a.kommision limit 0,1) as kommision
from auftrag a
left join details b on (b.beschreibung='Step1' AND b.auftrags_id=a.id)
left join details c on (c.beschreibung='Step2' AND c.auftrags_id=a.id)
left join details d on (d.beschreibung='Step3' AND d.auftrags_id=a.id)
left join details e on (e.beschreibung='Step4' AND e.auftrags_id=a.id)
left join details f on (f.beschreibung='Step5' AND f.auftrags_id=a.id)
left join details g on (g.beschreibung='Step6' AND g.auftrags_id=a.id)
left join details h on (h.beschreibung='Step7' AND h.auftrags_id=a.id)
left join details i on (i.beschreibung='Step8' AND i.auftrags_id=a.id)
left join details j on (j.beschreibung='Step9' AND j.auftrags_id=a.id)
left join details k on (k.beschreibung='Step10' AND k.auftrags_id=a.id)
where a.erledigt='1'
It runs very, very slow and take about 1 Minute to get a resultset.
Table "auftrag" has 820 rows and table "details" about 7000 rows.
What am I doing wrong?
Thanks
Patrick
CREATE TABLE auftrag
(
id INT(10) NOT NULL AUTO_INCREMENT,
beschreibung VARCHAR(255) NULL DEFAULT '0',
auftragsnummer VARCHAR(50) NULL DEFAULT '0',
faellig VARCHAR(50) NULL DEFAULT NULL,
kunde INT(11) NULL DEFAULT NULL,
subkunde VARCHAR(50) NULL DEFAULT NULL,
kommision VARCHAR(50) NULL DEFAULT NULL,
notiz TEXT NULL,
werbeanbringung TEXT NULL,
erledigt INT(1) NULL DEFAULT NULL,
INDEX Schlüssel 1 (id),
INDEX Schlüssel 2 (id, auftragsnummer, kunde, subkunde, beschreibung)
) COLLATE='latin1_swedish_ci' ENGINE=MyISAM AUTO_INCREMENT=850 ;
CREATE TABLE details
(
id INT(11) NOT NULL AUTO_INCREMENT,
auftrags_id VARCHAR(50) NULL DEFAULT '0',
beschreibung VARCHAR(50) NULL DEFAULT '0',
user VARCHAR(50) NULL DEFAULT '0',
timestamp VARCHAR(50) NULL DEFAULT '0',
notiz VARCHAR(255) NULL DEFAULT NULL,
INDEX Schlüssel 2 (user, timestamp, beschreibung, auftrags_id),
INDEX Schlüssel 1 (id, user, timestamp, beschreibung)
) COLLATE='latin1_swedish_ci' ENGINE=MyISAM AUTO_INCREMENT=7260 ;
You have no indexes on your tables that are useful for this query, and you are joining one table against another repeatedly.
On the auftrag table add an index on the erledigt column.
On the details table add an index covering both the beschreibungand the auftrags_id columns
It may also be possible to avoid most of the joins if you are prepared to process the returned data afterwards (ie, split up a returned field into an array). But a bit messy:-
SELECT
GROUP_CONCAT(CONCAT_WS('##', b.beschreibung, b.user, b.timestamp)),
a.beschreibung, a.auftragsnummer, a.faellig, a.subkunde,
(SELECT firma from kunden where id=a.kunde limit 0,1) as kunde,
(SELECT kommision from kommision where id=a.kommision limit 0,1) as kommision
FROM auftrag a
LEFT OUTER JOIN details b on b.auftrags_id=a.id AND b.beschreibung IN ('Step1', 'Step2', 'Step3', 'Step4', 'Step5', 'Step6', 'Step7', 'Step8', 'Step9', 'Step10')
WHERE a.erledigt='1'
GROUP BY a.id
EDIT - I have just noticed that the id on the auftrag table is an INT field (as I would expect) but the auftrags_id column on the details table is a VARCHAR(50) field. This will dramatically affect performance as it needs to convert the values for every single comparison.
Note that Gordons comment above on the order of the fields on the index does apply (how much will depend on the actual data - for example how many other values of beschreibung there are), but without fixing this problem with the mismatched data types changing the index will almost certainly not help.
For your query, you want the following indexes:
auftrag(erledigt, id)
details(auftrags_id, beschreibung)
kunden(kunde, firma)
You can also rewrite the query as a conditional aggregation, but the right indexes will be a big help.
Note: the order of the columns in the composite indexes is important.
This is probably a case where InnoDB will be a clear winner over MyISAM. In details, have PRIMARY KEY(auftrags_id, beschreibung).
Meanwhile, in auftrag, PRIMARY KEY(id). And the indexes Gordon suggests for auftrag and kunden.

How to join two tables without messing up the query

I have this query for example (good, it works how I want it to)
SELECT `discusComments`.`memberID`, COUNT( `discusComments`.`memberID`) AS postcount
FROM `discusComments`
GROUP BY `discusComments`.`memberID` ORDER BY postcount DESC
Example Results:
memberid postcount
3 283
6 230
9 198
Now I want to join the memberid of the discusComments table with that of the discusTopic table (because what I really want to do is only get my results from a specific GROUP, and the group id is only in the topic table and not in the comment one hence the join.
SELECT `discusComments`.`memberID`, COUNT( `discusComments`.`memberID`) AS postcount
FROM `discusComments`
LEFT JOIN `discusTopics` ON `discusComments`.`memberID` = `discusTopics`.`memberID`
GROUP BY `discusComments`.`memberID` ORDER BY postcount DESC
Example Results:
memberid postcount
3 14789
6 8678
9 6987
How can I stop this huge increase happening in the postcount? I need to preserve it as before.
Once I have this sorted I want to have some kind of line which says WHERE discusTopics.groupID = 6, for example
CREATE TABLE IF NOT EXISTS `discusComments` (
`id` bigint(255) NOT NULL auto_increment,
`topicID` bigint(255) NOT NULL,
`comment` text NOT NULL,
`timeStamp` bigint(12) NOT NULL,
`memberID` bigint(255) NOT NULL,
`thumbsUp` int(15) NOT NULL default '0',
`thumbsDown` int(15) NOT NULL default '0',
`status` int(1) NOT NULL default '1',
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=7190 ;
.
CREATE TABLE IF NOT EXISTS `discusTopics` (
`id` bigint(255) NOT NULL auto_increment,
`groupID` bigint(255) NOT NULL,
`memberID` bigint(255) NOT NULL,
`name` varchar(255) NOT NULL,
`views` bigint(255) NOT NULL default '0',
`lastUpdated` bigint(10) NOT NULL,
PRIMARY KEY (`id`),
KEY `groupID` (`groupID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 AUTO_INCREMENT=913 ;
SELECT `discusComments`.`memberID`, COUNT( `discusComments`.`memberID`) AS postcount
FROM `discusComments`
JOIN `discusTopics` ON `discusComments`.`topicID` = `discusTopics`.`id`
GROUP BY `discusComments`.`memberID` ORDER BY postcount DESC
Joining the topicid in both tables solved the memberID issue. Thanks #Andiry M
You need to use just JOIN not LEFT JOIN and you can add AND discusTopics.memberID = 6 after ON discusComments.memberID = discusTopics.memberID
You can use subqueries lik this
SELECT `discusComments`.`memberID`, COUNT( `discusComments`.`memberID`) AS postcount
FROM `discusComments` where `discusComments`.`memberID` in
(select distinct memberid from `discusTopics` WHERE GROUPID = 6)
If i understand your question right you do not need to use JOIN here at all. JOINs are needed in case when you have many to many relationships and you need for each value in one table select all corresponding values in another table.
But here you have many to one relationship if i got it right. Then you can simply do select from two tables like this
SELECT a.*, b.id FROM a, b WHERE a.pid = b.id
This is simple request and won't create a giant overhead as JOIN does
PS: In the future try to experiment with your queries, try to avoid JOINs especially in MySQL. They are slow and dangerous in their complexity. For 90% of cases when you want to use JOIN there is simple and much faster solution.

MySQL Group, Join and Select Max

First of all I know similar questions has been asked here, I've checked but could not find a proper solution to my issue.
I have these two tables (trimming to only required parts for simplicity):
CREATE TABLE messages(
message_id MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
message_conversation_id MEDIUMINT(8) UNSIGNED NOT NULL DEFAULT '0',
message_from MEDIUMINT(8) UNSIGNED NOT NULL DEFAULT '0',
message_to MEDIUMINT(8) UNSIGNED NOT NULL DEFAULT '0',
message_text TEXT NOT NULL DEFAULT '',
PRIMARY KEY (message_id),
KEY message_conversation_id (message_conversation_id),
KEY message_from (message_from),
KEY message_to (message_to)
) ENGINE=INNODB DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
CREATE TABLE users (
user_id MEDIUMINT(8) UNSIGNED NOT NULL AUTO_INCREMENT,
user_name VARCHAR(400) NOT NULL DEFAULT '',
PRIMARY KEY (user_id)
) ENGINE=INNODB DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
I want make a query, which shows maximum message_id's message_text, and user's info (which may be both stored at message_from or message_to, and filtered with a where clause (simply: message will come to me, and provide me the other user's user info, because in that conversation last message may both be from me or the other guy in conversation) (which made me stuck on solving)).
This query is what I came up with so far:
SELECT `m`.*, `u`.*, `u2`.*
FROM (`messages` AS m)
INNER JOIN `users` AS u ON `u`.`user_id`=`m`.`message_from`
INNER JOIN `users` AS u2 ON `u2`.`user_id`=`m`.`message_to`
WHERE (m.message_from="1" OR m.message_to="1")
AND `u`.`user_id` != '1'
AND `u2`.`user_id` != '1'
GROUP BY `m`.`message_conversation_id`
ORDER BY `m`.`message_id` desc
This query shows exactly what I need (I guess), except it provides lowest value from messages table.
How can I do this ? Where am I doing wrong ?
Thanks,
Here is an alternative way to get latest message of each conversation.
SELECT m.*, u.*, u2.*
FROM (`messages` AS m)
LEFT JOIN users AS u ON u.user_id=m.message_from
LEFT JOIN users AS u2 ON u2.user_id=m.message_to
WHERE (m.`message_from` != 1 AND m.`message_to` != 1) AND m.`message_id` IN (
SELECT MAX(`message_id`)
FROM `messages`
GROUP BY `message_conversation_id`
HAVING `message_id` = m.`message_id`)
ORDER BY m.message_id DESC;
EXPLAIN this, the result is "Using where; Using index", means "mission accomplished".
I got what I needed this way, What I needed is at the message_body result column. Is this healthy, or are there any other (possibly better) ways?
SELECT DISTINCT(m.message_conversation_id),max(m.message_id), m.*,u.*, u2.*,
(SELECT m2.message_text FROM messages m2 WHERE m2.message_id = max(m.message_id)) AS 'message_body'
FROM (`messages` AS m)
LEFT JOIN users AS u ON u.user_id=m.message_from
LEFT JOIN users AS u2 ON u2.user_id=m.message_to
WHERE (m.message_from='1' OR m.message_to='1')
GROUP BY m.message_conversation_id
ORDER BY m.message_id DESC