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 65,534 bytes. Each BLOB and TEXT column accounts for only five to nine bytes toward this size.

Important: For tables using the NDBCluster storage engine, there is the factor of 4-byte alignment to be taken into account when calculating storage requirements. This means that all NDB data storage is done in multiples of 4 bytes. Thus, a column value that — in a table using a storage engine other than NDB — would take 15 bytes for storage, requires 16 bytes in an NDB table. This requirement applies in addition to any other considerations that are discussed in this section. For example, in NDBCluster tables, the TINYINT, SMALLINT, MEDIUMINT, and INTEGER (INT) column types each require 4 bytes storage per record.

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.

When calculating Cluster memory requirements, you may find useful the ndb_size.pl utility which is available on MySQLForge. 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.

Storage Requirements for Numeric Types

The storage requirements for DECIMAL (and NUMERIC) are version-specific:

As of MySQL 5.0.3, values for DECIMAL 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:

Before MySQL 5.0.3, DECIMAL columns are represented as strings and storage requirements are: M+2 bytes if D > 0, M+1 bytes if D = 0 (D+2, if M < D)

Storage Requirements for Date and Time Types

Storage Requirements for String Types

For the CHAR, VARCHAR, and TEXT types, the values L and M in the preceding table should be interpreted as number of characters, and lengths for these types in column specifications indicate the number of characters. For example, to store a TINYTEXT value requires L characters plus one byte.

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(10) column can hold a string with a maximum length of 10. 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 was instead declared as VARCHAR(500), the string 'abcd' requires 4 + 2 = 6 bytes. Two bytes rather than one are required for the prefix because the length of the column is greater than 255 characters.

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. 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. For a breakdown of the storage used for different categories of utf8 characters, see Section 10.7, “Unicode Support”.

Note: In MySQL 5.0.3 and later, the effective maximum length for a VARCHAR or VARBINARY column is 65,532.

As of MySQL 5.0.3, the NDBCLUSTER engine supports only fixed-width columns. This means that a VARCHAR column from a table in a MySQL Cluster will behave as follows:

Note that the number of bytes required per character varies according to the character set used. For example, if a VARCHAR(100) column in a Cluster table uses the utf8 character set, then each character requires 3 bytes storage. This means that each record in such a column takes up 100 × 3 + 1 = 301 bytes for storage, regardless of the length of the string actually stored in any given record. For a VARCHAR(1000) column in a table using the NDBCLUSTER storage engine with the utf8 character set, each record will use 1000 × 3 + 2 = 3002 bytes storage; that is, the column is 1,000 characters wide, each character requires 3 bytes storage, and each record has a 2-byte overhead because 1,000 > 256.

The BLOB and TEXT types require 1, 2, 3, or 4 bytes to record the length of the column value, depending on the maximum possible length of the type. See Section 11.4.3, “The BLOB and TEXT Types”.

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 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 11.4.4, “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 11.4.5, “The SET Type”.