Shantanu's Blog

Corporate Consultant

September 22, 2007

 

MySQL FAQ - 2

1) How do I backup my tables using UNIX file system?

.frm holds the definition of a table. With MyISAM, the .MYD and MYI files hold the data and index information for the table.
If you are using InnoDB or another non-MyISAM engine for a table, you will not have .MYD and .MYI files. Therefore backing up the MyISAM tables is easy and fast if you have access to these files. You can create table of the same structure without keys, load data into it to get correct .MYD (data file). Create table with all keys defined and copy over .frm and .MYI files from it, followed by FLUSH TABLES. Now you can use REPAIR TABLE to rebuild all keys by sort, including UNIQUE keys.

2) What is the difference in MyISAM and InnoDB tables?

MyISAM InnoDB
Backup files Can't backup files
Doesn't support crash recovery Does support crash recovery
Table level locking Row level locking
fast full table scan Slow full tablescan
Full text index No full text index possible
No Foreign Key support Foreign Key support
No Transaction support Supports transactions


InnoDB is a storage engine, and so is MyISAM. When you create a table, you specify one of the types. When you add data and indexes to that table, the type of table determines the storage engine used. InnoDB does row level locking. This means that when you are updating a row, only that one row gets locked (which means that another connection to the database cannot modify that row). MyISAM locks the entire table. Only one connection / session at a time can update / insert / delete.

InnoDB uses the concept of a tablespace; MyISAM doesn't. A tablespace is where you store your data, and is made up of datafiles. You don't know where your data is stored in those data files. When you create a table in MyISAM, it crates a file of the same name as your table. Some queries are really bad on InnoDB compared to MyISAM, the most notorious is probably SELECT COUNT(*) A full table scan is also much faster on MyISAM than on InnoDB.

InnoDB doesn't support full text indexes on text columns so if you need to search text fields you may want to use MyISAM.

In addition to foreign keys, InnoDB offers transaction support, which is absolutely critical when dealing with larger applications. Sped does suffer though because all this Foreign Key / Transaction stuff takes lots of overhead. With InnoDB it becomes particularly important that you use good keys.

Inserting 50,000 records is something MyISAM is very comfortable with. In some cases it is faster to drop the indexes, insert the records and recreate the indexes (especially with full text indexes).

Use InnoDB in a situation where there are lots of inserts, updates and selects. Tests show that MyISAM is a lot faster when there are very few records (<50k), but the average execution time for a query increases almost linearly with the number of records while InnoDB shows almost constant query execution times for very small and large tables.

MyISAM does not support crash recovery while InnoDB does.


3) What should be the correct column definition?
VARCHAR(25) and VARCHAR(200) are practically the same for the purpose of storing data on the disk. If the user types only 5 characters in either columns, the bytes used will be the same. Select the value liberally but remember that VARCHAR(250) for
all text columns is not good idea either because it will not allow you to create composite indexes.

4) How does Primary and other keys affect the programming logic?

Composite primary indexes are great to avoid duplicate entries. But sometimes it can be a problem. For e.g. take a look at the following tables PRIMARY KEY definition.

CREATE TABLE `student` (
`enroll` varchar(13) NOT NULL default '',
`other` int(2) NOT NULL default '0',
`year` int(4) NOT NULL default '0',
`month` tinyint(2) unsigned default NULL,
`mode_read` int(2) NOT NULL default '0',
`mode_direct` int(2) NOT NULL default '0',
`sent_flag` int(11) NOT NULL default '0',
`system_date` datetime NOT NULL default '0000-00-00 00:00:00',
PRIMARY KEY (`enroll_no`,`other`,`year`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

Adding year as a part of primary key but not adding month column in it means there can be only 1 prospectus entry per year for each student. Now we can not use replace command that will insert a record if it does not exist in the primary key definition or update if it already does.

Labels:


Comments: Post a Comment

<< Home

Archives

June 2001   July 2001   January 2003   May 2003   September 2003   October 2003   December 2003   January 2004   February 2004   March 2004   April 2004   May 2004   June 2004   July 2004   August 2004   September 2004   October 2004   November 2004   December 2004   January 2005   February 2005   March 2005   April 2005   May 2005   June 2005   July 2005   August 2005   September 2005   October 2005   November 2005   December 2005   January 2006   February 2006   March 2006   April 2006   May 2006   June 2006   July 2006   August 2006   September 2006   October 2006   November 2006   December 2006   January 2007   February 2007   March 2007   April 2007   June 2007   July 2007   August 2007   September 2007   October 2007   November 2007   December 2007   January 2008   February 2008   March 2008   April 2008   July 2008   August 2008   September 2008   October 2008   November 2008   December 2008   January 2009   February 2009   March 2009   April 2009   May 2009   June 2009   July 2009   August 2009   September 2009   October 2009   November 2009   December 2009   January 2010   February 2010   March 2010   April 2010   May 2010   June 2010   July 2010   August 2010   September 2010   October 2010   November 2010   December 2010   January 2011   February 2011   March 2011   April 2011   May 2011   June 2011   July 2011   August 2011   September 2011   October 2011   November 2011   December 2011   January 2012   February 2012   March 2012   April 2012   May 2012   June 2012   July 2012   August 2012   October 2012   November 2012   December 2012   January 2013   February 2013   March 2013   April 2013   May 2013   June 2013   July 2013   September 2013   October 2013   January 2014   March 2014   April 2014   May 2014   July 2014   August 2014   September 2014   October 2014   November 2014   December 2014   January 2015   February 2015   March 2015   April 2015   May 2015   June 2015   July 2015   August 2015   September 2015   January 2016   February 2016   March 2016   April 2016   May 2016   June 2016   July 2016   August 2016   September 2016   October 2016   November 2016   December 2016   January 2017   February 2017   April 2017   May 2017   June 2017   July 2017   August 2017   September 2017   October 2017   November 2017   December 2017  

This page is powered by Blogger. Isn't yours?