MySQL 9.3 Reference Manual Including MySQL NDB Cluster 9.3
The row format of a table determines how its rows are physically stored, which in turn can affect the performance of queries and DML operations. As more rows fit into a single disk page, queries and index lookups can work faster, less cache memory is required in the buffer pool, and less I/O is required to write out updated values.
The data in each table is divided into pages. The pages that make up each table are arranged in a tree data structure called a B-tree index. Table data and secondary indexes both use this type of structure. The B-tree index that represents an entire table is known as the clustered index, which is organized according to the primary key columns. The nodes of a clustered index data structure contain the values of all columns in the row. The nodes of a secondary index structure contain the values of index columns and primary key columns.
Variable-length columns are an exception to the rule that column values are stored in B-tree index nodes. Variable-length columns that are too long to fit on a B-tree page are stored on separately allocated disk pages called overflow pages. Such columns are referred to as off-page columns. The values of off-page columns are stored in singly-linked lists of overflow pages, with each such column having its own list of one or more overflow pages. Depending on column length, all or a prefix of variable-length column values are stored in the B-tree to avoid wasting storage and having to read a separate page.
The InnoDB storage engine supports four row
formats: REDUNDANT, COMPACT,
DYNAMIC, and COMPRESSED.
Table 17.12 InnoDB Row Format Overview
| Row Format | Compact Storage Characteristics | Enhanced Variable-Length Column Storage | Large Index Key Prefix Support | Compression Support | Supported Tablespace Types |
|---|---|---|---|---|---|
REDUNDANT |
No | No | No | No | system, file-per-table, general |
COMPACT |
Yes | No | No | No | system, file-per-table, general |
DYNAMIC |
Yes | Yes | Yes | No | system, file-per-table, general |
COMPRESSED |
Yes | Yes | Yes | Yes | file-per-table, general |
The topics that follow describe row format storage characteristics and how to define and determine the row format of a table.
The REDUNDANT format provides compatibility
with older versions of MySQL.
Tables that use the REDUNDANT row format store
the first 768 bytes of variable-length column values
(VARCHAR,
VARBINARY, and
BLOB and
TEXT types) in the index record
within the B-tree node, with the remainder stored on overflow
pages. Fixed-length columns greater than or equal to 768 bytes are
encoded as variable-length columns, 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.
If the value of a column is 768 bytes or less, an overflow page is
not used, and some savings in I/O may result, since the value is
stored entirely in the B-tree node. This works well for relatively
short BLOB column values, but may cause B-tree
nodes to fill with data rather than key values, reducing their
efficiency. Tables with many BLOB columns could
cause B-tree nodes to become too full, and contain too few rows,
making the entire index less efficient than if rows were shorter
or column values were stored off-page.
The REDUNDANT row format has the following
storage characteristics:
Each index record contains a 6-byte header. The header is used to link together consecutive records, and for row-level locking.
Records in the clustered index contain fields for all user-defined columns. In addition, there is a 6-byte transaction ID field and a 7-byte roll pointer field.
If no primary key is defined for a table, each clustered index record also contains a 6-byte row ID field.
Each secondary index record contains all the primary key columns defined for the clustered index key that are not in the secondary index.
A record contains a pointer to each field of the record. If the total length of the fields in a record is less than 128 bytes, the pointer is one byte; otherwise, two bytes. The array of pointers is called the record directory. The area where the pointers point is the data part of the record.
Internally, fixed-length character columns such as
CHAR(10) in stored in
fixed-length format. Trailing spaces are not truncated from
VARCHAR columns.
Fixed-length columns greater than or equal to 768 bytes are
encoded as variable-length columns, 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.
An SQL NULL value reserves one or two bytes
in the record directory. An SQL NULL value
reserves zero bytes in the data part of the record if stored
in a variable-length column. For a fixed-length column, the
fixed length of the column is reserved in the data part of the
record. Reserving fixed space for NULL
values permits columns to be updated in place from
NULL to non-NULL values
without causing index page fragmentation.
The COMPACT row format reduces row storage
space by about 20% compared to the REDUNDANT
row format, at the cost of increasing CPU use for some operations.
If your workload is a typical one that is limited by cache hit
rates and disk speed, COMPACT format is likely
to be faster. If the workload is limited by CPU speed, compact
format might be slower.
Tables that use the COMPACT row format store
the first 768 bytes of variable-length column values
(VARCHAR,
VARBINARY, and
BLOB and
TEXT types) in the index record
within the B-tree node, with
the remainder stored on overflow pages. Fixed-length columns
greater than or equal to 768 bytes are encoded as variable-length
columns, 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.
If the value of a column is 768 bytes or less, an overflow page is
not used, and some savings in I/O may result, since the value is
stored entirely in the B-tree node. This works well for relatively
short BLOB column values, but may cause B-tree
nodes to fill with data rather than key values, reducing their
efficiency. Tables with many BLOB columns could
cause B-tree nodes to become too full, and contain too few rows,
making the entire index less efficient than if rows were shorter
or column values were stored off-page.
The COMPACT row format has the following
storage characteristics:
Each index record contains a 5-byte header that may be preceded by a variable-length header. The header is used to link together consecutive records, and for row-level locking.
The variable-length part of the record header contains a bit
vector for indicating NULL columns. If the
number of columns in the index that can be
NULL is N, the
bit vector occupies
CEILING(
bytes. (For example, if there are anywhere from 9 to 16
columns that can be N/8)NULL, the bit vector
uses two bytes.) Columns that are NULL do
not occupy space other than the bit in this vector. The
variable-length part of the header also contains the lengths
of variable-length columns. Each length takes one or two
bytes, depending on the maximum length of the column. If all
columns in the index are NOT NULL and have
a fixed length, the record header has no variable-length part.
For each non-NULL variable-length field,
the record header contains the length of the column in one or
two bytes. Two bytes are only needed if part of the column is
stored externally in overflow pages or the maximum length
exceeds 255 bytes and the actual length exceeds 127 bytes. For
an externally stored column, the 2-byte length indicates the
length of the internally stored part plus the 20-byte pointer
to the externally stored part. The internal part is 768 bytes,
so the length is 768+20. The 20-byte pointer stores the true
length of the column.
The record header is followed by the data contents of
non-NULL columns.
Records in the clustered index contain fields for all user-defined columns. In addition, there is a 6-byte transaction ID field and a 7-byte roll pointer field.
If no primary key is defined for a table, each clustered index record also contains a 6-byte row ID field.
Each secondary index record contains all the primary key columns defined for the clustered index key that are not in the secondary index. If any of the primary key columns are variable length, the record header for each secondary index has a variable-length part to record their lengths, even if the secondary index is defined on fixed-length columns.
Internally, for nonvariable-length character sets,
fixed-length character columns such as
CHAR(10) are stored in a
fixed-length format.
Trailing spaces are not truncated from
VARCHAR columns.
Internally, for variable-length character sets such as
utf8mb3 and utf8mb4,
InnoDB attempts to store
CHAR(
in N)N bytes by trimming trailing
spaces. If the byte length of a
CHAR(
column value exceeds N)N bytes,
trailing spaces are trimmed to a maximum of the column value
byte length. The maximum length of a
CHAR(
column is the maximum character byte length ×
N)N.
A minimum of N bytes is reserved
for
CHAR(.
Reserving the minimum space N)N in
many cases enables column updates to be done in place without
causing index page fragmentation. By comparison,
CHAR(
columns occupy the maximum character byte length ×
N)N when using the
REDUNDANT row format.
Fixed-length columns greater than or equal to 768 bytes are
encoded 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 DYNAMIC row format offers the same storage
characteristics as the COMPACT row format but
adds enhanced storage capabilities for long variable-length
columns and supports large index key prefixes.
When a table is created with
ROW_FORMAT=DYNAMIC, InnoDB
can store long variable-length column values (for
VARCHAR,
VARBINARY, and
BLOB and
TEXT types) fully off-page, with
the clustered index record containing only a 20-byte pointer to
the overflow page. Fixed-length fields greater than or equal to
768 bytes are encoded as variable-length fields. 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.
Whether columns are stored off-page depends on the page size and
the total size of the row. When a row is too long, the longest
columns are chosen for off-page storage until the clustered index
record fits on the B-tree page.
TEXT and
BLOB columns that are less than or
equal to 40 bytes are stored in line.
The DYNAMIC row format maintains the efficiency
of storing the entire row in the index node if it fits (as do the
COMPACT and REDUNDANT
formats), but the DYNAMIC row format avoids the
problem of filling B-tree nodes with a large number of data bytes
of long columns. The DYNAMIC row format is
based on the idea that if a portion of a long data value is stored
off-page, it is usually most efficient to store the entire value
off-page. With DYNAMIC format, shorter columns
are likely to remain in the B-tree node, minimizing the number of
overflow pages required for a given row.
The DYNAMIC row format supports index key
prefixes up to 3072 bytes.
Tables that use the DYNAMIC row format can be
stored in the system tablespace, file-per-table tablespaces, and
general tablespaces. To store DYNAMIC tables in
the system tablespace, either disable
innodb_file_per_table and use a
regular CREATE TABLE or ALTER
TABLE statement, or use the TABLESPACE [=]
innodb_system table option with CREATE
TABLE or ALTER TABLE. The
innodb_file_per_table variable is
not applicable to general tablespaces, nor is it applicable when
using the TABLESPACE [=] innodb_system table
option to store DYNAMIC tables in the system
tablespace.
The DYNAMIC row format is a variation of the
COMPACT row format. For storage
characteristics, see
COMPACT Row Format Storage Characteristics.
The COMPRESSED row format offers the same
storage characteristics and capabilities as the
DYNAMIC row format but adds support for table
and index data compression.
The COMPRESSED row format uses similar internal
details for off-page storage as the DYNAMIC row
format, with additional storage and performance considerations
from the table and index data being compressed and using smaller
page sizes. With the COMPRESSED row format, the
KEY_BLOCK_SIZE option controls how much column
data is stored in the clustered index, and how much is placed on
overflow pages. For more information about the
COMPRESSED row format, see
Section 17.9, “InnoDB Table and Page Compression”.
The COMPRESSED row format supports index key
prefixes up to 3072 bytes.
Tables that use the COMPRESSED row format can
be created in file-per-table tablespaces or general tablespaces.
The system tablespace does not support the
COMPRESSED row format. To store a
COMPRESSED table in a file-per-table
tablespace, the
innodb_file_per_table variable
must be enabled. The
innodb_file_per_table variable is
not applicable to general tablespaces. General tablespaces support
all row formats with the caveat that compressed and uncompressed
tables cannot coexist in the same general tablespace due to
different physical page sizes. For more information, see
Section 17.6.3.3, “General Tablespaces”.
The COMPRESSED row format is a variation of the
COMPACT row format. For storage
characteristics, see
COMPACT Row Format Storage Characteristics.
The default row format for InnoDB tables is
defined by
innodb_default_row_format
variable, which has a default value of DYNAMIC.
The default row format is used when the
ROW_FORMAT table option is not defined
explicitly or when ROW_FORMAT=DEFAULT is
specified.
The row format of a table can be defined explicitly using the
ROW_FORMAT table option in a
CREATE TABLE or
ALTER TABLE statement. For example:
CREATE TABLE t1 (c1 INT) ROW_FORMAT=DYNAMIC;
An explicitly defined ROW_FORMAT setting
overrides the default row format. Specifying
ROW_FORMAT=DEFAULT is equivalent to using the
implicit default.
The innodb_default_row_format
variable can be set dynamically:
mysql> SET GLOBAL innodb_default_row_format=DYNAMIC;
Valid innodb_default_row_format
options include DYNAMIC,
COMPACT, and REDUNDANT. The
COMPRESSED row format, which is not supported
for use in the system tablespace, cannot be defined as the
default. It can only be specified explicitly in a
CREATE TABLE or
ALTER TABLE statement. Attempting
to set the
innodb_default_row_format
variable to COMPRESSED returns an error:
mysql> SET GLOBAL innodb_default_row_format=COMPRESSED;
ERROR 1231 (42000): Variable 'innodb_default_row_format'
can't be set to the value of 'COMPRESSED'
Newly created tables use the row format defined by the
innodb_default_row_format
variable when a ROW_FORMAT option is not
specified explicitly, or when
ROW_FORMAT=DEFAULT is used. For example, the
following CREATE TABLE statements
use the row format defined by the
innodb_default_row_format
variable.
CREATE TABLE t1 (c1 INT);
CREATE TABLE t2 (c1 INT) ROW_FORMAT=DEFAULT;
When a ROW_FORMAT option is not specified
explicitly, or when ROW_FORMAT=DEFAULT is used,
an operation that rebuilds a table silently changes the row format
of the table to the format defined by the
innodb_default_row_format
variable.
Table-rebuilding operations include ALTER
TABLE operations that use
ALGORITHM=COPY or
ALGORITHM=INPLACE where table rebuilding is
required. See Section 17.12.1, “Online DDL Operations” for
more information. OPTIMIZE TABLE is
also a table-rebuilding operation.
The following example demonstrates a table-rebuilding operation that silently changes the row format of a table created without an explicitly defined row format.
mysql>SELECT @@innodb_default_row_format;+-----------------------------+ | @@innodb_default_row_format | +-----------------------------+ | dynamic | +-----------------------------+ mysql>CREATE TABLE t1 (c1 INT);mysql>SELECT * FROM INFORMATION_SCHEMA.INNODB_TABLES WHERE NAME LIKE 'test/t1' \G*************************** 1. row *************************** TABLE_ID: 54 NAME: test/t1 FLAG: 33 N_COLS: 4 SPACE: 35 ROW_FORMAT: Dynamic ZIP_PAGE_SIZE: 0 SPACE_TYPE: Single mysql>SET GLOBAL innodb_default_row_format=COMPACT;mysql>ALTER TABLE t1 ADD COLUMN (c2 INT);mysql>SELECT * FROM INFORMATION_SCHEMA.INNODB_TABLES WHERE NAME LIKE 'test/t1' \G*************************** 1. row *************************** TABLE_ID: 55 NAME: test/t1 FLAG: 1 N_COLS: 5 SPACE: 36 ROW_FORMAT: Compact ZIP_PAGE_SIZE: 0 SPACE_TYPE: Single
Consider the following potential issues before changing the row
format of existing tables from REDUNDANT or
COMPACT to DYNAMIC.
The REDUNDANT and
COMPACT row formats support a maximum index
key prefix length of 767 bytes whereas
DYNAMIC and COMPRESSED
row formats support an index key prefix length of 3072 bytes.
In a replication environment, if the
innodb_default_row_format
variable is set to DYNAMIC on the source,
and set to COMPACT on the replica, the
following DDL statement, which does not explicitly define a
row format, succeeds on the source but fails on the replica:
CREATE TABLE t1 (c1 INT PRIMARY KEY, c2 VARCHAR(5000), KEY i1(c2(3070)));
For related information, see Section 17.21, “InnoDB Limits”.
Importing a table that does not explicitly define a row format
results in a schema mismatch error if the
innodb_default_row_format
setting on the source server differs from the setting on the
destination server. For more information, see
Section 17.6.1.3, “Importing InnoDB Tables”.
To determine the row format of a table, use
SHOW TABLE STATUS:
mysql> SHOW TABLE STATUS IN test1\G
*************************** 1. row ***************************
Name: t1
Engine: InnoDB
Version: 10
Row_format: Dynamic
Rows: 0
Avg_row_length: 0
Data_length: 16384
Max_data_length: 0
Index_length: 16384
Data_free: 0
Auto_increment: 1
Create_time: 2016-09-14 16:29:38
Update_time: NULL
Check_time: NULL
Collation: utf8mb4_0900_ai_ci
Checksum: NULL
Create_options:
Comment:
Alternatively, query the Information Schema
INNODB_TABLES table:
mysql> SELECT NAME, ROW_FORMAT FROM INFORMATION_SCHEMA.INNODB_TABLES WHERE NAME='test1/t1';
+----------+------------+
| NAME | ROW_FORMAT |
+----------+------------+
| test1/t1 | Dynamic |
+----------+------------+