MySQL 5.7 Release Notes
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 15, Alternative Storage Engines, and
Section 8.4.7, “Limits on Table Column Count and Row Size”.
See Section 14.11, “InnoDB Row Formats” for information about
storage requirements for InnoDB tables.
Important
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(
column takes M)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
defined as 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 ndb_size.pl 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 21.5.28, “ndb_size.pl — NDBCLUSTER Size Requirement Estimator” for
more information.
| Data Type | Storage Required |
|---|---|
TINYINT |
1 byte |
SMALLINT |
2 bytes |
MEDIUMINT |
3 bytes |
INT,
INTEGER |
4 bytes |
BIGINT |
8 bytes |
FLOAT( |
4 bytes if 0 <= p <= 24, 8 bytes if 25
<= p <= 53 |
FLOAT |
4 bytes |
DOUBLE [PRECISION],
REAL |
8 bytes |
DECIMAL(,
NUMERIC( |
Varies; see following discussion |
BIT( |
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 |
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.
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( |
The compact family of InnoDB row formats optimize storage for
variable-length character sets. See
COMPACT Row Format Storage Characteristics.
Otherwise, M ×
w bytes, <=
255, where
w is the number of bytes
required for the maximum-length character in the character
set. |
BINARY( |
M bytes, 0 <=
255 |
VARCHAR(,
VARBINARY( |
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(' |
1 or 2 bytes, depending on the number of enumeration values (65,535 values maximum) |
SET(' |
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 utf8 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 10.9, “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,
utf8mb3 characters can require up to three
bytes per character, so a VARCHAR
column that uses the utf8mb3 character set
can be declared to be a maximum of 21,844 characters. See
Section 8.4.7, “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 |
|---|---|
BLOB, TEXT |
2000 |
MEDIUMBLOB,
MEDIUMTEXT |
4000 |
LONGBLOB,
LONGTEXT |
13948 |
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. 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 11.3.5, “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
( bytes,
rounded up to 1, 2, 3, 4, or 8 bytes. A
N+7)/8SET can have a maximum of 64
members. See Section 11.3.6, “The SET Type”.
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 11.4.3, “Supported Spatial Data Formats”.
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.