<type> [GENERATED ALWAYS] AS ( <expression> ) [VIRTUAL | PERSISTENT | STORED] [UNIQUE] [UNIQUE KEY] [COMMENT <text>]
MariaDB's generated columns syntax is designed to be similar to the syntax for Microsoft SQL Server's computed columns and Oracle Database's virtual columns. In MariaDB 10.2 and later, the syntax is also compatible with the syntax for MySQL's generated columns.
A generated column is a column in a table that cannot explicitly be set to a specific value in a DML query. Instead, its value is automatically generated based on an expression. This expression might generate the value based on the values of other columns in the table, or it might generate the value by calling built-in functions or user-defined functions (UDFs).
There are two types of generated columns:
PERSISTENT (a.k.a. STORED): This type's value is actually stored in the table. VIRTUAL: This type's value is not stored at all. Instead, the value is generated dynamically when the table is queried. This type is the default. Generated columns are also sometimes called computed columns or virtual columns.
ERROR 1910 (HY000): TokuDB storage engine does not support computed columns
PERSISTENT generated column. VIRTUAL and PERSISTENT generated column. ERROR 1903 (HY000): Primary key cannot be defined upon a computed column
PERSISTENT generated columns as part of a foreign key is supported. PERSISTENT generated columns as part of a foreign key is also supported. ON UPDATE CASCADE, ON UPDATE SET NULL, or ON DELETE SET NULL clauses is not supported. If you try to use an unsupported clause, then you will see an error similar to the following: ERROR 1905 (HY000): Cannot define foreign key with ON UPDATE SET NULL clause on a computed column
VIRTUAL and PERSISTENT generated columns is supported. VIRTUAL and PERSISTENT generated columns differ in how their data is stored. PERSISTENT generated columns are generated whenever a DML queries inserts or updates the row with the special DEFAULT value. This generates the columns value, and it is stored in the table like the other "real" columns. This value can be read by other DML queries just like the other "real" columns. VIRTUAL generated columns are not stored in the table. Instead, the value is generated dynamically whenever the column is queried. If other columns in a row are queried, but the VIRTUAL generated column is not one of the queried columns, then the column's value is not generated. VIRTUAL or PERSISTENT generated columns cannot be explicitly set to any other values than NULL or DEFAULT. If a generated column is explicitly set to any other value, then the outcome depends on whether strict mode is enabled in sql_mode. If it is not enabled, then a warning will be raised and the default generated value will be used instead. If it is enabled, then an error will be raised instead. MODIFY and CHANGE clauses for PERSISTENT generated columns. MODIFY clause for VIRTUAL generated columns if ALGORITHM is not set to COPY. See MDEV-15476 for more information. CHANGE clause for VIRTUAL generated columns if ALGORITHM is not set to COPY. See MDEV-17035 for more information. COPY if the table has a VIRTUAL generated column that is indexed. See MDEV-14046 for more information. VIRTUAL generated column with the ADD clause if the same statement is also adding other columns if ALGORITHM is not set to COPY. See MDEV-17468 for more information. VIRTUAL generated column. Extra column is set to either VIRTUAL or PERSISTENT. For example: DESCRIBE table1; +-------+-------------+------+-----+---------+------------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+------------+ | a | int(11) | NO | | NULL | | | b | varchar(32) | YES | | NULL | | | c | int(11) | YES | | NULL | VIRTUAL | | d | varchar(5) | YES | | NULL | PERSISTENT | +-------+-------------+------+-----+---------+------------+
NEW and OLD rows in triggers. ERROR 1901 (HY000): Function or expression 'dayname()' cannot be used in the GENERATED ALWAYS AS clause of `v`
VIRTUAL generated columns. PERSISTENT or indexed VIRTUAL generated columns. VIRTUAL generated columns. CREATE TABLE t1 (a int as (1), b int as (a));
CREATE TABLE t1 (a int as (1));
When a generated column is PERSISTENT or indexed, the value of the expression needs to be consistent regardless of the SQL Mode flags in the current session. If it is not, then the table will be seen as corrupted when the value that should actually be returned by the computed expression and the value that was previously stored and/or indexed using a different sql_mode setting disagree.
There are currently two affected classes of inconsistencies: character padding and unsigned subtraction:
VARCHAR or TEXT generated column the length of the value returned can vary depending on the PAD_CHAR_TO_FULL_LENGTH sql_mode flag. To make the value consistent, create the generated column using an RTRIM() or RPAD() function. Alternately, create the generated column as a CHAR column so that its data is always fully padded. SIGNED generated column is based on the subtraction of an UNSIGNED value, the resulting value can vary depending on how large the value is and the NO_UNSIGNED_SUBTRACTION sql_mode flag. To make the value consistent, use CAST() to ensure that each UNSIGNED operand is SIGNED before the subtraction. Beginning in MariaDB 10.5, there is a fatal error generated when trying to create a generated column whose value can change depending on the SQL Mode when its data is PERSISTENT or indexed.
For an existing generated column that has a potentially inconsistent value, a warning about a bad expression is generated the first time it is used (if warnings are enabled).
Beginning in MariaDB 10.4.8, MariaDB 10.3.18, and MariaDB 10.2.27 a potentially inconsistent generated column outputs a warning when created or first used (without restricting their creation).
Here is an example of two tables that would be rejected in MariaDB 10.5 and warned about in the other listed versions:
CREATE TABLE bad_pad ( txt CHAR(5), -- CHAR -> VARCHAR or CHAR -> TEXT can't be persistent or indexed: vtxt VARCHAR(5) AS (txt) PERSISTENT ); CREATE TABLE bad_sub ( num1 BIGINT UNSIGNED, num2 BIGINT UNSIGNED, -- The resulting value can vary for some large values vnum BIGINT AS (num1 - num2) VIRTUAL, KEY(vnum) );
The warnings for the above tables look like this:
Warning (Code 1901): Function or expression '`txt`' cannot be used in the GENERATED ALWAYS AS clause of `vtxt` Warning (Code 1105): Expression depends on the @@sql_mode value PAD_CHAR_TO_FULL_LENGTH Warning (Code 1901): Function or expression '`num1` - `num2`' cannot be used in the GENERATED ALWAYS AS clause of `vnum` Warning (Code 1105): Expression depends on the @@sql_mode value NO_UNSIGNED_SUBTRACTION
To work around the issue, force the padding or type to make the generated column's expression return a consistent value. For example:
CREATE TABLE good_pad ( txt CHAR(5), -- Using RTRIM() or RPAD() makes the value consistent: vtxt VARCHAR(5) AS (RTRIM(txt)) PERSISTENT, -- When not persistent or indexed, it is OK for the value to vary by mode: vtxt2 VARCHAR(5) AS (txt) VIRTUAL, -- CHAR -> CHAR is always OK: txt2 CHAR(5) AS (txt) PERSISTENT ); CREATE TABLE good_sub ( num1 BIGINT UNSIGNED, num2 BIGINT UNSIGNED, -- The indexed value will always be consistent in this expression: vnum BIGINT AS (CAST(num1 AS SIGNED) - CAST(num2 AS SIGNED)) VIRTUAL, KEY(vnum) );
STORED keyword is supported as an alias for the PERSISTENT keyword. Generated columns are subject to various constraints in other DBMSs that are not present in MariaDB's implementation. Generated columns may also be called computed columns or virtual columns in different implementations. The various details for a specific implementation can be found in the documentation for each specific DBMS.
MariaDB's generated columns implementation does not enforce the following restrictions that are present in Microsoft SQL Server's computed columns implementation:
QUOTED_IDENTIFIER to be set in sql_mode. In MariaDB, if data is inserted without ANSI_QUOTES set in sql_mode, then it will be processed and stored differently in a generated column that contains quoted identifiers. Microsoft SQL Server enforces the above restrictions by doing one of the following things:
In MariaDB, as long as the sql_mode, language, and other settings that were in effect during the CREATE TABLE remain unchanged, the generated column expression will always be evaluated the same. If any of these things change, then please be aware that the generated column expression might not be evaluated the same way as it previously was.
If you try to update a virtual column, you will get an error if the default strict mode is enabled in sql_mode, or a warning otherwise.
Generated columns was originally developed by Andrey Zhakov. It was then modified by Sanja Byelkin and Igor Babaev at Monty Program for inclusion in MariaDB. Monty did the work on MariaDB 10.2 to lift a some of the old limitations.
Here is an example table that uses both VIRTUAL and PERSISTENT virtual columns:
USE TEST;
CREATE TABLE table1 (
a INT NOT NULL,
b VARCHAR(32),
c INT AS (a mod 10) VIRTUAL,
d VARCHAR(5) AS (left(b,5)) PERSISTENT);
If you describe the table, you can easily see which columns are virtual by looking in the "Extra" column:
DESCRIBE table1; +-------+-------------+------+-----+---------+------------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+------------+ | a | int(11) | NO | | NULL | | | b | varchar(32) | YES | | NULL | | | c | int(11) | YES | | NULL | VIRTUAL | | d | varchar(5) | YES | | NULL | PERSISTENT | +-------+-------------+------+-----+---------+------------+
To find out what function(s) generate the value of the virtual column you can use SHOW CREATE TABLE:
SHOW CREATE TABLE table1; | table1 | CREATE TABLE `table1` ( `a` int(11) NOT NULL, `b` varchar(32) DEFAULT NULL, `c` int(11) AS (a mod 10) VIRTUAL, `d` varchar(5) AS (left(b,5)) PERSISTENT ) ENGINE=MyISAM DEFAULT CHARSET=latin1 |
If you try to insert non-default values into a virtual column, you will receive a warning and what you tried to insert will be ignored and the derived value inserted instead:
WARNINGS; Show warnings enabled. INSERT INTO table1 VALUES (1, 'some text',default,default); Query OK, 1 row affected (0.00 sec) INSERT INTO table1 VALUES (2, 'more text',5,default); Query OK, 1 row affected, 1 warning (0.00 sec) Warning (Code 1645): The value specified for computed column 'c' in table 'table1' has been ignored. INSERT INTO table1 VALUES (123, 'even more text',default,'something'); Query OK, 1 row affected, 2 warnings (0.00 sec) Warning (Code 1645): The value specified for computed column 'd' in table 'table1' has been ignored. Warning (Code 1265): Data truncated for column 'd' at row 1 SELECT * FROM table1; +-----+----------------+------+-------+ | a | b | c | d | +-----+----------------+------+-------+ | 1 | some text | 1 | some | | 2 | more text | 2 | more | | 123 | even more text | 3 | even | +-----+----------------+------+-------+ 3 rows in set (0.00 sec)
If the ZEROFILL clause is specified, it should be placed directly after the type definition, before the AS (<expression>):
CREATE TABLE table2 (a INT, b INT ZEROFILL AS (a*2) VIRTUAL); INSERT INTO table2 (a) VALUES (1); SELECT * FROM table2; +------+------------+ | a | b | +------+------------+ | 1 | 0000000002 | +------+------------+ 1 row in set (0.00 sec)
You can also use virtual columns to implement a "poor man's partial index". See example at the end of Unique Index.
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https://mariadb.com/kb/en/virtual-columns/