Storage Layout Tips In InnoDB , having a long PRIMARY KEY wastes a lot of disk space because its value must be stored with every secondary index record. (See Section 14.2.10, “ InnoDB Table and Index Structures” .) Create an AUTO_INCREMENT column as the primary key if your primary key is long. Use the VARCHAR data type instead of CHAR if you are storing variable-length strings or if the column may contain many NULL values. A CHAR( N ) column always takes N characters to store data, even if the string is shorter or its value is NULL . Smaller tables fit better in the buffer pool and reduce disk I/O. When using COMPACT row format (the default InnoDB format in MySQL 5.0) and variable-length character sets, such as utf8 or sjis , CHAR( N ) will occupy a variable amount of space, at least N bytes. Transaction Managemen...
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