When we’re looking at benchmarks we typically run some stable workload and we run it in isolation – nothing else is happening on the system. This is not however how things happen in real world when we have significant variance in the load and many things can be happening concurrently. It is very typical to [...]
Reasons for MySQL Replication Lag
July 29, 2011 By 9 Comments
One common theme in the questions our MySQL Support customers ask is MySQL Replication Lag. The story is typically along the lines everything is same as before and for some unknown reason the slave is started to lag and not catching up any more. I always smile at “nothing has changed” claim as it usually [...]
How to debug long-running transactions in MySQL
March 8, 2011 By 6 Comments
Among the many things that can cause a “server stall” is a long-running transaction. If a transaction remains open for a very long time without committing, and has modified data, then other transactions could block and fail with a lock wait timeout. The problem is, it can be very difficult to find the offending code [...]
Innodb undo segment size and transaction isolation
January 12, 2011 By 3 Comments
You might know if you have long running transactions you’re risking having a lot of “garbage” accumulated in undo segment size which can cause performance degradation as well as increased disk space usage. Long transactions can also be bad for other reasons such as taking row level locks which will prevent other transactions for execution, [...]
Estimating Replication Capacity
July 20, 2010 By 4 Comments
It is easy for MySQL replication to become bottleneck when Master server is not seriously loaded and the more cores and hard drives the get the larger the difference becomes, as long as replication remains single thread process. At the same time it is a lot easier to optimize your system when your replication runs [...]
InnoDB TABLE/INDEX stats
March 20, 2010 By 7 Comments
In Released and new coming features I did not mentioned two additional INFORMATION_SCHEMA tables available in XtraDB: It is INNODB_TABLE_STATS INNODB_INDEX_STATS These table show statistics about InnoDB tables ( taken from InnoDB data dictionary). INNODB_TABLE_STATS is | table_name | table name in InnoDB internal style (‘database/table’) | | rows | estimated number of all rows [...]
How (not) to find unused indexes
October 16, 2009 By 2 Comments
I’ve seen a few people link to an INFORMATION_SCHEMA query to be able to find any indexes that have low cardinality, in an effort to find out what indexes should be removed. This method is flawed – here’s the first reason why:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 | CREATE TABLE `sales` ( `id` int(11) NOT NULL AUTO_INCREMENT, `customer_id` int(11) DEFAULT NULL, `status` enum('archived','active') DEFAULT NULL, PRIMARY KEY (`id`), KEY `status` (`status`) ) ENGINE=MyISAM AUTO_INCREMENT=65691 DEFAULT CHARSET=latin1; mysql> SELECT count(*), status FROM sales GROUP by status; +----------+---------+ | count(*) | status | +----------+---------+ |   65536 | archived | |     154 | active | +----------+---------+ 2 rows in set (0.17 sec) mysql> EXPLAIN SELECT * FROM sales WHERE status='active'; # query 1 +----+-------------+-------+------+---------------+--------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key   | key_len | ref  | rows | Extra      | +----+-------------+-------+------+---------------+--------+---------+-------+------+-------------+ | 1 | SIMPLE     | sales | ref | status       | status | 2      | const | 196 | Using where | +----+-------------+-------+------+---------------+--------+---------+-------+------+-------------+ 1 row in set (0.06 sec) mysql> EXPLAIN SELECT * FROM sales WHERE status='archived'; # query 2 +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra      | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ | 1 | SIMPLE     | sales | ALL | status       | NULL | NULL   | NULL | 65690 | Using where | +----+-------------+-------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.01 sec) |
The cardinality of status index is woeful, but provided that the application [...]
Mass killing of MySQL Connections
May 21, 2009 By 30 Comments
Every so often I run into situation when I need to kill a lot of connections on MySQL server – for example hundreds of instances of some bad query is running making server unusable. Many people have special scripts which can take the user, source host or query as a parameter and perform the action. [...]
Dropping unused indexes
January 15, 2009 By 6 Comments
Vadim wrote some time ago about how to find unused indexes with single query. I was working on the system today and found hundreds of unused indexes on dozens of tables so just dropping indexes manually did not look fun. So I extended Vadim’s query to generate ALTER TABLE statements automatically. I also made it [...]
How much space does empty Innodb table take ?
December 16, 2008 By 7 Comments
How much space would empty MyISAM table take ? Probably 8K for .frm file, 1KB for .MYI file and 0 for MYD file. .MYI file can be larger if you have many indexes. How much space will Innodb take:
