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MySQL 8.4 Reference Manual  /  Data Types  /  Data Type Storage Requirements

13.7 Data Type Storage Requirements

The storage requirements for table data on disk depend on several factors. Different storage engines represent data types and store raw data differently. Table data might be compressed, either for a column or an entire row, complicating the calculation of storage requirements for a table or column.

Despite differences in storage layout on disk, the internal MySQL APIs that communicate and exchange information about table rows use a consistent data structure that applies across all storage engines.

This section includes guidelines and information for the storage requirements for each data type supported by MySQL, including the internal format and size for storage engines that use a fixed-size representation for data types. Information is listed by category or storage engine.

The internal representation of a table has a maximum row size of 65,535 bytes, even if the storage engine is capable of supporting larger rows. This figure excludes BLOB or TEXT columns, which contribute only 9 to 12 bytes toward this size. For BLOB and TEXT data, the information is stored internally in a different area of memory than the row buffer. Different storage engines handle the allocation and storage of this data in different ways, according to the method they use for handling the corresponding types. For more information, see Chapter 18, Alternative Storage Engines, and Section 10.4.7, “Limits on Table Column Count and Row Size”.

InnoDB Table Storage Requirements

See Section 17.10, “InnoDB Row Formats” for information about storage requirements for InnoDB tables.

NDB Table Storage Requirements


NDB tables use 4-byte alignment; all NDB data storage is done in multiples of 4 bytes. Thus, a column value that would typically take 15 bytes requires 16 bytes in an NDB table. For example, in NDB tables, the TINYINT, SMALLINT, MEDIUMINT, and INTEGER (INT) column types each require 4 bytes storage per record due to the alignment factor.

Each BIT(M) column takes M bits of storage space. Although an individual BIT column is not 4-byte aligned, NDB reserves 4 bytes (32 bits) per row for the first 1-32 bits needed for BIT columns, then another 4 bytes for bits 33-64, and so on.

While a NULL itself does not require any storage space, NDB reserves 4 bytes per row if the table definition contains any columns allowing NULL, up to 32 NULL columns. (If an NDB Cluster table is defined with more than 32 NULL columns up to 64 NULL columns, then 8 bytes per row are reserved.)

Every table using the NDB storage engine requires a primary key; if you do not define a primary key, a hidden primary key is created by NDB. This hidden primary key consumes 31-35 bytes per table record.

You can use the Perl script to estimate NDB storage requirements. It connects to a current MySQL (not NDB Cluster) database and creates a report on how much space that database would require if it used the NDB storage engine. See Section 25.5.29, “ — NDBCLUSTER Size Requirement Estimator” for more information.

Numeric Type Storage Requirements

Data Type Storage Required
TINYINT 1 byte
SMALLINT 2 bytes
INT, INTEGER 4 bytes
BIGINT 8 bytes
FLOAT(p) 4 bytes if 0 <= p <= 24, 8 bytes if 25 <= p <= 53
FLOAT 4 bytes
DECIMAL(M,D), NUMERIC(M,D) Varies; see following discussion
BIT(M) approximately (M+7)/8 bytes

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.

Leftover Digits Number of Bytes
0 0
1 1
2 1
3 2
4 2
5 3
6 3
7 4
8 4

Date and Time Type Storage Requirements

For TIME, DATETIME, and TIMESTAMP columns, the storage required for tables created before MySQL 5.6.4 differs from tables created from 5.6.4 on. This is due to a change in 5.6.4 that permits these types to have a fractional part, which requires from 0 to 3 bytes.

Data Type Storage Required Before MySQL 5.6.4 Storage Required as of MySQL 5.6.4
YEAR 1 byte 1 byte
DATE 3 bytes 3 bytes
TIME 3 bytes 3 bytes + fractional seconds storage
DATETIME 8 bytes 5 bytes + fractional seconds storage
TIMESTAMP 4 bytes 4 bytes + fractional seconds storage

As of MySQL 5.6.4, storage for YEAR and DATE remains unchanged. However, TIME, DATETIME, and TIMESTAMP are represented differently. DATETIME is packed more efficiently, requiring 5 rather than 8 bytes for the nonfractional part, and all three parts have a fractional part that requires from 0 to 3 bytes, depending on the fractional seconds precision of stored values.

Fractional Seconds Precision Storage Required
0 0 bytes
1, 2 1 byte
3, 4 2 bytes
5, 6 3 bytes

For example, TIME(0), TIME(2), TIME(4), and TIME(6) use 3, 4, 5, and 6 bytes, respectively. TIME and TIME(0) are equivalent and require the same storage.

For details about internal representation of temporal values, see MySQL Internals: Important Algorithms and Structures.

String Type Storage Requirements

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

Data Type Storage Required
CHAR(M) The compact family of InnoDB row formats optimize storage for variable-length character sets. See COMPACT Row Format Storage Characteristics. Otherwise, M × w bytes, <= M <= 255, where w is the number of bytes required for the maximum-length character in the character set.
BINARY(M) M bytes, 0 <= M <= 255
VARCHAR(M), VARBINARY(M) L + 1 bytes if column values require 0 − 255 bytes, L + 2 bytes if values may require more than 255 bytes
TINYBLOB, TINYTEXT L + 1 bytes, where L < 28
BLOB, TEXT L + 2 bytes, where L < 216
MEDIUMBLOB, MEDIUMTEXT L + 3 bytes, where L < 224
LONGBLOB, LONGTEXT L + 4 bytes, where L < 232
ENUM('value1','value2',...) 1 or 2 bytes, depending on the number of enumeration values (65,535 values maximum)
SET('value1','value2',...) 1, 2, 3, 4, or 8 bytes, depending on the number of set members (64 members maximum)

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 multibyte characters. In particular, when using a UTF-8 Unicode character set, you must keep in mind that not all characters use the same number of bytes. utf8mb3 and utf8mb4 character sets can require up to three and four bytes per character, respectively. For a breakdown of the storage used for different categories of utf8mb3 or utf8mb4 characters, see Section 12.9, “Unicode Support”.

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

  • The actual length of the column value

  • The column's maximum possible length

  • The character set used for the column, because some character sets contain multibyte characters

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 effective maximum number of bytes that can be stored in a VARCHAR or VARBINARY column is subject to the maximum row size of 65,535 bytes, which is shared among all columns. For a VARCHAR column that stores multibyte characters, the effective maximum number of characters is less. For example, utf8mb4 characters can require up to four bytes per character, so a VARCHAR column that uses the utf8mb4 character set can be declared to be a maximum of 16,383 characters. See Section 10.4.7, “Limits on Table Column Count and Row Size”.

InnoDB encodes fixed-length fields greater than or equal to 768 bytes in length as variable-length fields, which can be stored off-page. For example, a CHAR(255) column can exceed 768 bytes if the maximum byte length of the character set is greater than 3, as it is with utf8mb4.

The NDB storage engine supports variable-width columns. This means that a VARCHAR column in an NDB Cluster table requires the same amount of storage as would 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 5 bytes for the same column value in a MyISAM table).

TEXT, BLOB, and JSON columns are implemented differently in the NDB storage engine, wherein each row in the column is made up of two separate parts. One of these is of fixed size (256 bytes for TEXT and BLOB, 4000 bytes for JSON), 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 blob parts table. The size of the rows in this second table are determined by the exact type of the column, as shown in the following table:

Type Blob Part Size
JSON 8100

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).

No blob parts are stored separately by NDB for TINYBLOB or TINYTEXT column values.

You can increase the size of an NDB blob column's blob part to the maximum of 13948 using NDB_COLUMN in a column comment when creating or altering the parent table. NDB also supports setting the inline size for a TEXT, BLOB, or JSON column, using NDB_TABLE in a column comment. See NDB_COLUMN Options, for more information.

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 13.3.5, “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 13.3.6, “The SET Type”.

Spatial Type Storage Requirements

MySQL stores geometry values using 4 bytes to indicate the SRID followed by the WKB representation of the value. The LENGTH() function returns the space in bytes required for value storage.

For descriptions of WKB and internal storage formats for spatial values, see Section 13.4.3, “Supported Spatial Data Formats”.

JSON Storage Requirements

In general, the storage requirement for a JSON column is approximately the same as for a LONGBLOB or LONGTEXT column; that is, the space consumed by a JSON document is roughly the same as it would be for the document's string representation stored in a column of one of these types. However, there is an overhead imposed by the binary encoding, including metadata and dictionaries needed for lookup, of the individual values stored in the JSON document. For example, a string stored in a JSON document requires 4 to 10 bytes additional storage, depending on the length of the string and the size of the object or array in which it is stored.

In addition, MySQL imposes a limit on the size of any JSON document stored in a JSON column such that it cannot be any larger than the value of max_allowed_packet.