5.3. Generated Columns
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## 5.3. Generated Columns #

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). PostgreSQL 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 and partitioning:

  - If a parent column is a generated column, its child column must also be a generated column; however, the child column can have a different generation expression. The generation expression that is actually applied during insert or update of a row is the one associated with the table that the row is physically in. (This is unlike the behavior for column defaults: for those, the default value associated with the table named in the query applies.)

  - If a parent column is not a generated column, its child column must not be generated either.

  - For inherited tables, if you write a child column definition without any `GENERATED` clause in `CREATE TABLE ... INHERITS`, then its `GENERATED` clause will automatically be copied from the parent. `ALTER TABLE ... INHERIT` will insist that parent and child columns already match as to generation status, but it will not require their generation expressions to match.

  - Similarly for partitioned tables, if you write a child column definition without any `GENERATED` clause in `CREATE TABLE ... PARTITION OF`, then its `GENERATED` clause will automatically be copied from the parent. `ALTER TABLE ... ATTACH PARTITION` will insist that parent and child columns already match as to generation status, but it will not require their generation expressions to match.

  - In case of multiple inheritance, if one parent column is a generated column, then all parent columns must be generated columns. If they do not all have the same generation expression, then the desired expression for the child must be specified explicitly.

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.

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