What are the main differences between InnoDB and MyISAM?

Question

What are the main differences between InnoDB and MyISAM?

Answer 1

One more thing: you can backup InnoDB tables just by taking a snapshot of the filesystem. Backing up MyISAM requires using mysqldump and is not guaranteed to be consistent (e.g. if you insert into a parent and a child table, you might find only the child table's row in your backup).

Basically, if you have another copy of the data and are only caching it in MySQL e.g. to allow a standard means of accessing it from a PHP website, then MyISAM is fine (i.e. it's better than a flat CSV file or a logfile for querying and concurrent access). If the database is the actual "master copy" of the data, if you are doing INSERT and UPDATE using real data from users, then it is foolish to use anything other than InnoDB, at any sort of scale MyISAM is unreliable and hard to manage, you'll be doing myisamchk half the time, negating any performance gains...

(My personal experience: a 2 terabyte DB in MyISAM).

Answer 2

First major difference i see is that InnoDB implements row-level lock while MyISAM can do only a table-level lock. You will find better crash recovery in InnoDB. However, it doesn't have FULLTEXT search indexes, as does MyISAM. InnoDB also implements transactions, foreign keys and relationship constraints while MyISAM does not.

The list can go a bit further. Yet, they both have their unique advantages in their favor and disadvantages against each other. Each of them is more suitable in some scenarios than the other.

Answer 3

Another major difference not as yet mentioned is how caching for each storage engine is done.

MYISAM

The main mechanism used is the key cache. It only caches index pages from .MYI files. To size your key cache, run the following query:

SELECT CONCAT(ROUND(KBS/POWER(1024,
IF(PowerOf1024<0,0,IF(PowerOf1024>3,0,PowerOf1024)))+0.4999),
SUBSTR(' KMG',IF(PowerOf1024<0,0,
IF(PowerOf1024>3,0,PowerOf1024))+1,1))
recommended_key_buffer_size FROM
(SELECT LEAST(POWER(2,32),KBS1) KBS
FROM (SELECT SUM(index_length) KBS1
FROM information_schema.tables
WHERE engine='MyISAM' AND
table_schema NOT IN ('information_schema','mysql')) AA ) A,
(SELECT 2 PowerOf1024) B;

This will give the Recommended Setting for MyISAM Key Cache (key_buffer_size) given your current data set (the query will cap the recommendation at 4G (4096M). For 32-bit OS, 4GB is the limit. For 64-bit, 8GB.

InnoDB

The main mechanism used is the InnoDB Buffer Pool. It caches data and index pages from InnoDB tables accessed. To size your InnoDB Buffer Pool, run the following query:

SELECT CONCAT(ROUND(KBS/POWER(1024,
IF(PowerOf1024<0,0,IF(PowerOf1024>3,0,PowerOf1024)))+0.49999),
SUBSTR(' KMG',IF(PowerOf1024<0,0,
IF(PowerOf1024>3,0,PowerOf1024))+1,1)) recommended_innodb_buffer_pool_size
FROM (SELECT SUM(data_length+index_length) KBS FROM information_schema.tables
WHERE engine='InnoDB') A,
(SELECT 2 PowerOf1024) B;

This will give the Recommended Setting for the size of the InnoDB Buffer Pool (innodb_buffer_pool_size) given your current data set.

Don't forget to resize the InnoDB Log Files (ib_logfile0 and ib_logfile1). MySQL Source Code places a cap of the combined sizes of all InnoDB Log Files must be < 4G (4096M). For the sake of simplicity, given just two log files, here is how you can size them:

Step 1) add innodb_log_file_size=NNN to /etc/my.cnf
(NNN should be 25% of innodb_buffer_pool_size or 2047M, whichever is smaller)
Step 2) service mysql stop
Step 3) rm /var/log/mysql/ib_logfile[01]
Step 4) service mysql start (ib_logfile0 and ib_logfile1 are recreated)

CAVEAT

At the End of both queries is a an Inline Query (SELECT 2 PowerOfTwo) B

(SELECT 0 PowerOf1024) gives the Setting in Bytes
(SELECT 1 PowerOf1024) gives the Setting in Kilobytes
(SELECT 2 PowerOf1024) gives the Setting in Megabytes
(SELECT 3 PowerOf1024) gives the Setting in Gigabytes
No powers less that 0 or greater than 3 is accepted

FINAL NOTE

There is no substitute for common sense. If you have limited memory, a mixture of storage engines, or a combination thereof, you will have to adjust for different scenarios.

If you have 2GB RAM and 16GB of InnoDB, allocate 512M as innodb_buffer_pool.

If you have 2GB RAM and 4GB of MyISAM Indexes, allocate 512M as key_buffer_size.

If you have 2GB RAM and 4GB of MyISAM Indexes and 16GB InnoDB, allocate 512M as key_buffer_size and 512M as innodb_buffer_pool.

Possible scenarios are endless !!!

Remember, whatever you allocate for, leave enough RAM for DB Connections and the Operating System.

Answer 4

InnoDB offers:

• ACID transactions
• row-level locking
• foreign key constraints
• automatic crash recovery
• table compression (read/write)

In InnoDB all data in a row except for TEXT and BLOB can occupy 8,000 bytes at most. No full text indexing is available for InnoDB. In InnoDB the COUNT(*)s execute slower than in MyISAM because the row count is not stored internally. InnoDB stores both data and indexes in one file. InnoDB uses a buffer pool to cache both data and indexes.

MyISAM offers:

• fast COUNT(*)s
• full text indexing
• smaller disk footprint
• very high table compression (read only)

MyISAM has table-level locking, but no row-level locking. No transactions. No automatic crash recovery, but it does offer repair table functionality. No foreign key constraints. MyISAM tables are more compact in size on disk when compared to InnoDB tables. MyISAM tables could be further highly reduced in size by compressing with myisampack if needed, but become read-only. MyISAM stores indexes in one file and data in another. MyISAM uses key buffers for caching indexes and leaves the data caching management to the operating system.

Overall I would recommend InnoDB for most purposes and MyISAM for specialized uses only. InnoDB is now the default engine in new MySQL versions.

Answer 5

I tend to view MyISAM as the 'default' table choice for MySQL, so I'll point out the differences for most users of InnoDB

• Row level-locking
• Foreign key enforcement
• Transaction support
• Performance hit on high-use systems
• There is No FULLTEXT search available for InnoDB tables http://dev.mysql.com/doc/refman/5.6/en/fulltext-search.html

Answer 6

In my experience, the most significant difference is the way each engine handles locking. InnoDB uses row locking while MyISAM uses table locking. As a rule of thumb, I use InnoDB for write heavy tables and MyISAM for read heavy tables.

Other important differences include:

1. InnoDB support transactions and foreign keys. MyISAM does not.
2. MyISAM uses full text indexing.
3. MyISAM does a poor job of enforcing data integrity.