In MySQL, tables have both a logical and physical structure. Logically, each table in a database consists of rows and columns. A table can be empty (it can have zero rows of data), but it must have at least one column. A table may also be indexed to improve query performance. Indexes enable MySQL to look up data values quickly rather than searching through the entire table. Indexes become increasingly important the larger a table becomes.
Physically, each table is associated with one or more files on disk. Every table has a format file in its database directory. The format file is created by the server and contains the definition, or structure, of the table. The format filename is the same as the table name, plus an .frm suffix. For example, the format file for a table named Country in the world database is named Country.frm and is located in the world database directory under the server's data directory.
MySQL manages tables using storage engines, each of which handles tables that have a given set of characteristics. Different storage engines have differing performance characteristics, and you can choose which engine most closely matches the characteristics that you need. For example, you might require transactional capabilities and guaranteed data integrity even if a crash occurs, or you might want a very fast lookup table stored in memory for which the contents can be lost in a crash and reloaded at the next server startup. With MySQL, you can make this choice on a per-table basis. Any given table is managed by a particular storage engine. In addition to the .frm file that the server creates, a table may be associated with one or more other files that the storage engine creates in which to store the table's contents. The number and types of files vary per storage engine, because each engine manages table storage differently. Here are some examples:
- The MyISAM engine creates a data file and index file for each table. If Country is a MyISAM table, the MyISAM storage engine creates data and index files named Country.MYD and Country.MYI to store data rows and indexes (respectively) for the table.
- By default, the InnoDB engine shares files for multiple tables. If Country is an InnoDB table, there will be a Country.frm format file created by the in the database directory, but the InnoDB storage engine itself stores the table data and index information elsewhere, in the InnoDB shared tablespace. The tablespace is used by multiple tables. That is, files for storing table contents are not per-table as for MyISAM but are shared among tables.
- The MEMORY engine does not use any disk storage at all for table contents. It manages table contents in memory.
The MySQL server places no limits on the number of tables in a database, although individual storage engines might have their own limits. For example, the InnoDB storage engine allows a maximum of two billion tables to exist within the InnoDB shared tablespace. This places a limit (albeit a rather high one) on the number of InnoDB tables that can be created among all databases combined. (The limit isn't enforced on a per-database basis because the InnoDB tablespace is shared among all databases.)
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