Wednesday, March 31, 2010

vBulletin session table and Hash index in MEMORY Engine


In vBulletin Board System there is session table contains online user information and tracking, in which forum now, what the current URL now, So it is have huge concurrent update statement, By default this table is MEMORY engine because its data not important in case MySQL restart.

I notice early loaded in MySQL and server, So I do full tuning for the server then analyze slow query using mysqlsla and notice this query in the top:
SELECT user.username, (user.options & 512) AS invisible, user.usergroupid,
   session.userid, session.inforum, session.lastactivity, session.badlocation,
   IF(displaygroupid=0, user.usergroupid, displaygroupid) AS displaygroupid, infractiongroupid
  FROM session AS session force index(lastact_user)
  LEFT JOIN user AS user ON(user.userid = session.userid)
  WHERE session.lastactivity > 1269376225
  ORDER BY lastactivity DESC
and this is create table statement
mysql> show create table session\G
*************************** 1. row ***************************
       Table: session
Create Table: CREATE TABLE `session` (
  `sessionhash` char(32) NOT NULL DEFAULT '',
  `userid` int(10) unsigned NOT NULL DEFAULT '0',
  `host` char(15) NOT NULL DEFAULT '',
  `idhash` char(32) NOT NULL DEFAULT '',
  `lastactivity` int(10) unsigned NOT NULL DEFAULT '0',
  `location` char(255) NOT NULL DEFAULT '',
  `useragent` char(100) NOT NULL DEFAULT '',
  `styleid` smallint(5) unsigned NOT NULL DEFAULT '0',
  `languageid` smallint(5) unsigned NOT NULL DEFAULT '0',
  `loggedin` smallint(5) unsigned NOT NULL DEFAULT '0',
  `inforum` smallint(5) unsigned NOT NULL DEFAULT '0',
  `inthread` int(10) unsigned NOT NULL DEFAULT '0',
  `incalendar` smallint(5) unsigned NOT NULL DEFAULT '0',
  `badlocation` smallint(5) unsigned NOT NULL DEFAULT '0',
  `bypass` tinyint(4) NOT NULL DEFAULT '0',
  `profileupdate` smallint(5) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`sessionhash`),
  KEY `userid` (`userid`),
  KEY `lastact_user` (`lastactivity`,`userid`),
  KEY `idx_host_user` (`host`,`userid`),
  KEY `idhash_idx` (`idhash`),
  KEY `idx_lastactivity` (`lastactivity`)
) ENGINE=MEMORY DEFAULT CHARSET=latin1 PACK_KEYS=0


so for first impression this query use index for where and order by, so its optimal!!!!!

Unfortunately, we see it not using the index even when force it.
mysql> explain select * from session where lastactivity > 1269509561;
+----+-------------+---------+------+-------------------------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys                 | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+-------------------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | session | ALL  | lastact_user,idx_lastactivity | NULL | NULL    | NULL | 3199 | Using where |
+----+-------------+---------+------+-------------------------------+------+---------+------+------+-------------+

mysql> explain select * from session force index (idx_lastactivity) where lastactivity > 1269509561;
+----+-------------+---------+------+------------------+------+---------+------+------+-------------+
| id | select_type | table   | type | possible_keys    | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------+------+------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | session | ALL  | idx_lastactivity | NULL | NULL    | NULL | 3199 | Using where |
+----+-------------+---------+------+------------------+------+---------+------+------+-------------+
Why?

MEMORY engine support B-tree and Hash indexes, but by default use Hash algorithm:
    *      They are used only for equality comparisons that use the = or <=> operators (but are very fast). They are not used for comparison operators such as < that find a range of values.
    *      The optimizer cannot use a hash index to speed up ORDER BY operations. (This type of index cannot be used to search for the next entry in order.)
    *      MySQL cannot determine approximately how many rows there are between two values (this is used by the range optimizer to decide which index to use). This may affect some queries if you change a MyISAM table to a hash-indexed MEMORY table.
    *      Only whole keys can be used to search for a row. (With a B-tree index, any leftmost prefix of the key can be used to find rows.)

so the optimizer will not use this index.

when we alter index algorithm by:

alter table session drop index idx_lastactivity, add index `idx_lastactivity` (`lastactivity`) USING BTREE;

we notice:
mysql> explain select * from session where lastactivity > 1269509561;
+----+-------------+---------+-------+-------------------------------+------------------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys                 | key              | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+-------------------------------+------------------+---------+------+------+-------------+
|  1 | SIMPLE      | session | range | lastact_user,idx_lastactivity | idx_lastactivity | 4       | NULL | 1919 | Using where |
+----+-------------+---------+-------+-------------------------------+------------------+---------+------+------+-------------+

mysql> explain select * from session force index (idx_lastactivity) where lastactivity > 1269509561;
+----+-------------+---------+-------+------------------+------------------+---------+------+------+-------------+
| id | select_type | table   | type  | possible_keys    | key              | key_len | ref  | rows | Extra       |
+----+-------------+---------+-------+------------------+------------------+---------+------+------+-------------+
|  1 | SIMPLE      | session | range | idx_lastactivity | idx_lastactivity | 4       | NULL | 1919 | Using where |
+----+-------------+---------+-------+------------------+------------------+---------+------+------+-------------+

So we should be careful when use MEMORY table with indexes

3 comments:

  1. it 's really good and easy in use....

    ReplyDelete