Storage Engines

          When creating a table in MySQL, the storage engine that will be used has to be chosen.  If one is not chosen, MySQL uses the default storage engine chosen in the configuration of the server.  MySQL supports several storage engines that act as handlers for different table types. MySQL storage engines include both those that handle transaction-safe tables and those that handle non-transaction-safe tables. There are numerous storage engines that MySQL supports; however the most common storage types that are used with PHP are listed below: 

•  MyISAM -

                    This storage engine is supported in all MySQL configurations and is the default storage engine for all new tables created (when a specific storage engine is not assigned) in the majority of default installations.  This storage engine manages non-transactional tables.  It provides high-speed storage and retrieval, as well as fulltext searching capabilities.  MyISAM is a disk based storage engine.  Aiming for very low overhead, it does not support transactions. 

•  InnoDB -

                    InnoDB is also disk based, but offers versioned, fully ACID transactional capabilities. InnoDB requires more disk space than MyISAM to store its data, and this increased overhead is compensated by more aggressive use of memory caching, in order to attain high speeds.  InnoDB is included by default in all MySQL 5.0 binary distributions. 

•  Memory (formerly called "Heap") - 

                    This storage engine provides in-memory (utilizes only RAM) tables.  Special algorithms are used that make optimal use of this environment. It is very fast. Like MyISAM, the Memory storage engine handles non-transactional tables, and is also included in MySQL by default.  

•  NDB - 

                This is the MySQL Cluster Storage engine which connects to a cluster of nodes, offering high availability through redundancy, high performance through fragmentation (partitioning) of data across multiple node groups, and excellent scalability through the combination of these two. Prior to MySQL 5.1, NDB used main-memory only, with logging to disk. 

Table Limits 

                  The MySQL server has no limits overall on the number of tables that can be created in a database; however, there are limits within the storage engine type itself or on in the operating system that contains the MySQL server.  InnoDB, for example, limits the number of tables that can be created for the entire database to 2 million tables.  This means that one database server can not contain more the 2 million tables across one database or multiple databases.  This is because all InnoDB tables are stored in a central files in the operating system and not separated out by database.  MyISAM tables on the other hand do not have any such limitations; however, the operating system can oppose limits on the number of files that can be stored on disk or in an individual sub-directory.