A generated column is a special column that is always computed from other columns. Thus, it is for columns what a view is for tables. There are two kinds of generated columns: stored and virtual. A stored generated column is computed when it is written (inserted or updated) and occupies storage as if it were a normal column. A virtual generated column occupies no storage and is computed when it is read. Thus, a virtual generated column is similar to a view and a stored generated column is similar to a materialized view (except that it is always updated automatically). LightDB currently implements only stored generated columns.
To create a generated column, use the GENERATED ALWAYS
AS clause in CREATE TABLE, for example:
CREATE TABLE people (
...,
height_cm numeric,
height_in numeric GENERATED ALWAYS AS (height_cm / 2.54) STORED
);
The keyword STORED must be specified to choose the
stored kind of generated column. See CREATE TABLE for
more details.
A generated column cannot be written to directly. In
INSERT or UPDATE commands, a value
cannot be specified for a generated column, but the keyword
DEFAULT may be specified.
Consider the differences between a column with a default and a generated
column. The column default is evaluated once when the row is first
inserted if no other value was provided; a generated column is updated
whenever the row changes and cannot be overridden. A column default may
not refer to other columns of the table; a generation expression would
normally do so. A column default can use volatile functions, for example
random() or functions referring to the current time;
this is not allowed for generated columns.
Several restrictions apply to the definition of generated columns and tables involving generated columns:
The generation expression can only use immutable functions and cannot use subqueries or reference anything other than the current row in any way.
A generation expression cannot reference another generated column.
A generation expression cannot reference a system column, except
tableoid.
A generated column cannot have a column default or an identity definition.
A generated column cannot be part of a partition key.
Foreign tables can have generated columns. See CREATE FOREIGN TABLE for details.
For inheritance:
If a parent column is a generated column, a child column must also be
a generated column using the same expression. In the definition of
the child column, leave off the GENERATED clause,
as it will be copied from the parent.
In case of multiple inheritance, if one parent column is a generated column, then all parent columns must be generated columns and with the same expression.
If a parent column is not a generated column, a child column may be defined to be a generated column or not.
Additional considerations apply to the use of generated columns.
Generated columns maintain access privileges separately from their underlying base columns. So, it is possible to arrange it so that a particular role can read from a generated column but not from the underlying base columns.
Generated columns are, conceptually, updated after
BEFORE triggers have run. Therefore, changes made to
base columns in a BEFORE trigger will be reflected in
generated columns. But conversely, it is not allowed to access
generated columns in BEFORE triggers.