MySql: Need to improve the performance for a query - php

Question

I need to pump up my query a bit for it's taking way too long on a large DB.
I have the following tables
vb_user
+++++++++++++++++++++++++++++++++
++ userid ++ username ++ posts ++
+++++++++++++++++++++++++++++++++
vb_post
++++++++++++++++++++++++
++ userid ++ dateline ++
++++++++++++++++++++++++
I use this query
SELECT VBU.userid AS USER_ID
, VBU.username AS USER_NAME
, COUNT(VBP.userid) AS NUMBER_OF_POSTS_FOR_30_DAYS
, FROM_UNIXTIME(VBU.joindate) as JOIN_DATE
FROM vb_user AS VBU
LEFT JOIN vb_post AS VBP
ON VBP.userid = VBU.userid
WHERE VBU.joindate BETWEEN '__START_DATE__' AND '__END_DATE__'
AND VBP.dateline BETWEEN VBU.joindate AND DATE_ADD(FROM_UNIXTIME(VBU.joindate), INTERVAL 30 DAY)
GROUP BY VBP.userid
ORDER BY NUMBER_OF_POSTS_FOR_30_DAYS DESC"
I have to select the users who have posted the most from when they joined till 30 days after..... and I can't figure out how to do it withouth the FROM_UNIXTIME function..
But it takes a lot of time. Any thoughts on how to improve the performance for the query?
Here is the output for explain
id,select_type,table,type,possible_keys,key,key_len,ref,rows,Extra
1,SIMPLE,VBP,index,userid,threadid_visible_dateline,18,NULL,2968000,"Using where; Using index; Using temporary; Using filesort"
1,SIMPLE,VBU,eq_ref,PRIMARY,PRIMARY,4,vb_copilul.VBP.userid,1,"Using where"
And here is the info about the tables
Table,"Create Table"
vb_user,"CREATE TABLE `vb_user` (
`userid` int(10) unsigned NOT NULL AUTO_INCREMENT,
`username` varchar(100) NOT NULL DEFAULT '',
`posts` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`userid`),
KEY `usergroupid` (`usergroupid`),
) ENGINE=MyISAM AUTO_INCREMENT=101076 DEFAULT CHARSET=latin1"
Table,"Create Table"
vb_post,"CREATE TABLE `vb_post` (
`postid` int(10) unsigned NOT NULL AUTO_INCREMENT,
`threadid` int(10) unsigned NOT NULL DEFAULT '0',
`parentid` int(10) unsigned NOT NULL DEFAULT '0',
`username` varchar(100) NOT NULL DEFAULT '',
`userid` int(10) unsigned NOT NULL DEFAULT '0',
`title` varchar(250) NOT NULL DEFAULT '',
`dateline` int(10) unsigned NOT NULL DEFAULT '0',
`pagetext` mediumtext,
`allowsmilie` smallint(6) NOT NULL DEFAULT '0',
`showsignature` smallint(6) NOT NULL DEFAULT '0',
`ipaddress` char(15) NOT NULL DEFAULT '',
`iconid` smallint(5) unsigned NOT NULL DEFAULT '0',
`visible` smallint(6) NOT NULL DEFAULT '0',
`attach` smallint(5) unsigned NOT NULL DEFAULT '0',
`infraction` smallint(5) unsigned NOT NULL DEFAULT '0',
`reportthreadid` int(10) unsigned NOT NULL DEFAULT '0',
PRIMARY KEY (`postid`),
KEY `userid` (`userid`),
KEY `threadid` (`threadid`,`userid`),
KEY `threadid_visible_dateline` (`threadid`,`visible`,`dateline`,`userid`,`postid`),
KEY `dateline` (`dateline`),
KEY `ipaddress` (`ipaddress`)
) ENGINE=MyISAM AUTO_INCREMENT=3009320 DEFAULT CHARSET=latin1"

Answer 1

Two things you can do to improve the query:
Do not convert VBP.datetime to unix time. Use the BETWEEN query with the dates directly. In your query the server has to convert all dates in the DB to compare them, instead of use the native types. If you are always using the datetime column as unix timestamp, then declare it as Double (I think?) instead of DATETIME (or TIMESTAMP - whatever you have chosen). This way you will speed up other operations too.
Add index to the datetime column to make sure the between query is fast enough.
Everything else looks OK