Data Type Storage Requirements

The storage requirements for each of the data types supported by MySQL are listed here by category.

The maximum size of a row in a MyISAM table is 65,535 bytes. Each BLOB and TEXT column accounts for only nine to twelve bytes toward this size. This limitation may be shared by other storage engines as well.

In addition, when calculating storage requirements for Cluster tables, you must remember that every table using the NDBCluster storage engine requires a primary key; if no primary key is defined by the user, then a “hidden” primary key will be created by NDB. This hidden primary key consumes 31-35 bytes per table record.

You may find the ndb_size.pl utility to be useful for estimating NDB storage requirements. This Perl script connects to a current MySQL (non-Cluster) database and creates a report on how much space that database would require if it used the NDBCluster storage engine. See Section 16.11.15, “ndb_size.pl — NDBCluster Size Requirement Estimator”, for more information.

Storage Requirements for Numeric Types

Values for DECIMAL (and NUMERIC) columns are represented using a binary format that packs nine decimal (base 10) digits into four bytes. Storage for the integer and fractional parts of each value are determined separately. Each multiple of nine digits requires four bytes, and the “leftover” digits require some fraction of four bytes. The storage required for excess digits is given by the following table:

Storage Requirements for Date and Time Types

The storage requirements shown in the table arise from the way that MySQL represents temporal values:

Storage Requirements for String Types

In the following table, M represents the declared column length in characters for non-binary string types and bytes for binary string types. L represents the actual length in bytes of a given string value.

Variable-length string types are stored using a length prefix plus data. The length prefix requires from one to four bytes depending on the data type, and the value of the prefix is L (the byte length of the string). For example, storage for a MEDIUMTEXT value requires L bytes to store the value plus three bytes to store the length of the value.

To calculate the number of bytes used to store a particular CHAR, VARCHAR, or TEXT column value, you must take into account the character set used for that column and whether the value contains multi-byte characters. In particular, when using the utf8 Unicode character set, you must keep in mind that not all utf8 characters use the same number of bytes and can require up to three bytes per character. For a breakdown of the storage used for different categories of utf8 characters, see Section 9.1.8, “Unicode Support”.

VARCHAR, VARBINARY, and the BLOB and TEXT types are variable-length types. For each, the storage requirements depend on these factors:

For example, a VARCHAR(255) column can hold a string with a maximum length of 255 characters. Assuming that the column uses the latin1 character set (one byte per character), the actual storage required is the length of the string (L), plus one byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is five bytes. If the same column is instead declared to use the ucs2 double-byte character set, the storage requirement is 10 bytes: The length of 'abcd' is eight bytes and the column requires two bytes to store lengths because the maximum length is greater than 255 (up to 510 bytes).

The NDBCLUSTER storage engine in MySQL 5.1 supports variable-width columns. This means that a VARCHAR column in a MySQL Cluster table requires the same amount of storage as it would using any other storage engine, with the exception that such values are 4-byte aligned. Thus, the string 'abcd' stored in a VARCHAR(50) column using the latin1 character set requires 8 bytes (rather than 6 bytes for the same column value in a MyISAM table). This represents a change in behavior from earlier versions of NDBCLUSTER, where a VARCHAR(50) column would require 52 bytes storage per record regardless of the length of the string being stored.

TEXT and BLOB columns are implemented differently in the NDB Cluster storage engine, wherein each row in a TEXT column is made up of two separate parts. One of these is of fixed size (256 bytes), and is actually stored in the original table. The other consists of any data in excess of 256 bytes, which is stored in a hidden table. The rows in this second table are always 2,000 bytes long. This means that the size of a TEXT column is 256 if size <= 256 (where size represents the size of the row); otherwise, the size is 256 + size + (2000 – (size – 256) % 2000).

The size of an ENUM object is determined by the number of different enumeration values. One byte is used for enumerations with up to 255 possible values. Two bytes are used for enumerations having between 256 and 65,535 possible values. See Section 10.4.4, “The ENUM Type”.

The size of a SET object is determined by the number of different set members. If the set size is N, the object occupies (N+7)/8 bytes, rounded up to 1, 2, 3, 4, or 8 bytes. A SET can have a maximum of 64 members. See Section 10.4.5, “The SET Type”.