A computed column exposes data generated from other columns by a scalar expression included in the column definition. A stored computed column (set with the STORED SQL keyword) is calculated when a row is inserted or updated, and stores the resulting value of the scalar expression in the primary index similar to a regular column. A virtual computed column (set with the VIRTUAL SQL keyword) is not stored, and the value of the scalar expression is computed during queries as needed.
Why use computed columns?
Computed columns are especially useful when used with partitioning, JSONB columns, or secondary indexes.
Partitioning requires that partitions are defined using columns that are a prefix of the primary key. In the case of geo-partitioning, some applications will want to collapse the number of possible values in this column, to make certain classes of queries more performant. For example, if a users table has a country and state column, then you can make a stored computed column locality with a reduced domain for use in partitioning. For more information, see the partitioning example below.
JSONB columns are used for storing semi-structured
JSONBdata. When the table's primary information is stored inJSONB, it's useful to index a particular field of theJSONBdocument. In particular, computed columns allow for the following use case: a two-column table with aPRIMARY KEYcolumn and apayloadcolumn, whose primary key is computed as some field from thepayloadcolumn. This alleviates the need to manually separate your primary keys from your JSON blobs. For more information, see theJSONBexample below.Secondary indexes can be created on computed columns, which is especially useful when a table is frequently sorted. See the secondary indexes example below.
Considerations
Computed columns:
- Cannot be used to generate other computed columns.
- Cannot be a foreign key reference.
- Behave like any other column, with the exception that they cannot be written to directly.
- Are mutually exclusive with
DEFAULT.
Virtual computed columns:
- Are not stored in the table's primary index.
- Are recomputed as the column data in the expression changes.
- Cannot be used as part of a primary key,
FAMILYdefinition, inCHECKconstraints, orFOREIGN KEYconstraints. - Cannot be stored in indexes.
- Can be index columns.
Creation
To define a stored computed column, use the following syntax:
column_name <type> AS (<expr>) STORED
New in v21.1: To define a virtual computed column, use the following syntax:
column_name <type> AS (<expr>) VIRTUAL
| Parameter | Description |
|---|---|
column_name |
The name/identifier of the computed column. |
<type> |
The data type of the computed column. |
<expr> |
The pure scalar expression used to compute column values. Any functions marked as impure, such as now() or nextval() cannot be used. |
STORED |
(Required for stored computed columns) The computed column is stored alongside other columns. |
VIRTUAL |
(Required for virtual columns) The computed column is virtual, meaning the column data is not stored in the table's primary index. |
For compatibility with PostgresSQL, CockroachDB also supports creating computed columns with the syntax column_name <type> GENERATED ALWAYS AS (<expr>) STORED.
Examples
Create a table with a stored computed column
In this example, let's create a simple table with a computed column:
> CREATE TABLE users (
id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
city STRING,
first_name STRING,
last_name STRING,
full_name STRING AS (CONCAT(first_name, ' ', last_name)) STORED,
address STRING,
credit_card STRING,
dl STRING UNIQUE CHECK (LENGTH(dl) < 8)
);
Then, insert a few rows of data:
> INSERT INTO users (first_name, last_name) VALUES
('Lola', 'McDog'),
('Carl', 'Kimball'),
('Ernie', 'Narayan');
> SELECT * FROM users;
id | city | first_name | last_name | full_name | address | credit_card | dl
+--------------------------------------+------+------------+-----------+---------------+---------+-------------+------+
5740da29-cc0c-47af-921c-b275d21d4c76 | NULL | Ernie | Narayan | Ernie Narayan | NULL | NULL | NULL
e7e0b748-9194-4d71-9343-cd65218848f0 | NULL | Lola | McDog | Lola McDog | NULL | NULL | NULL
f00e4715-8ca7-4d5a-8de5-ef1d5d8092f3 | NULL | Carl | Kimball | Carl Kimball | NULL | NULL | NULL
(3 rows)
The full_name column is computed from the first_name and last_name columns without the need to define a view.
Create a table with a JSONB column and a stored computed column
In this example, create a table with a JSONB column and a stored computed column:
> CREATE TABLE student_profiles (
id STRING PRIMARY KEY AS (profile->>'id') STORED,
profile JSONB
);
Create a compute column after you create a table:
> ALTER TABLE student_profiles ADD COLUMN age INT AS ( (profile->>'age')::INT) STORED;
Then, insert a few rows of data:
> INSERT INTO student_profiles (profile) VALUES
('{"id": "d78236", "name": "Arthur Read", "age": "16", "school": "PVPHS", "credits": 120, "sports": "none"}'),
('{"name": "Buster Bunny", "age": "15", "id": "f98112", "school": "THS", "credits": 67, "clubs": "MUN"}'),
('{"name": "Ernie Narayan", "school" : "Brooklyn Tech", "id": "t63512", "sports": "Track and Field", "clubs": "Chess"}');
> SELECT * FROM student_profiles;
+--------+---------------------------------------------------------------------------------------------------------------------+------+
| id | profile | age |
---------+---------------------------------------------------------------------------------------------------------------------+------+
| d78236 | {"age": "16", "credits": 120, "id": "d78236", "name": "Arthur Read", "school": "PVPHS", "sports": "none"} | 16 |
| f98112 | {"age": "15", "clubs": "MUN", "credits": 67, "id": "f98112", "name": "Buster Bunny", "school": "THS"} | 15 |
| t63512 | {"clubs": "Chess", "id": "t63512", "name": "Ernie Narayan", "school": "Brooklyn Tech", "sports": "Track and Field"} | NULL |
+--------+---------------------------------------------------------------------------------------------------------------------+------|
The primary key id is computed as a field from the profile column. Additionally the age column is computed from the profile column data as well.
Create a virtual computed column using JSONB data
In this example, create a table with a JSONB column and a virtual computed column:
> CREATE TABLE student_profiles (
id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
profile JSONB,
full_name STRING AS (concat_ws(' ',profile->>'firstName', profile->>'lastName')) VIRTUAL
);
Then, insert a few rows of data:
> INSERT INTO student_profiles (profile) VALUES
('{"id": "d78236", "firstName": "Arthur", "lastName": "Read", "birthdate": "2010-01-25", "school": "PVPHS", "credits": 120, "sports": "none"}'),
('{"firstName": "Buster", "lastName": "Bunny", "birthdate": "2011-11-07", "id": "f98112", "school": "THS", "credits": 67, "clubs": "MUN"}'),
('{"firstName": "Ernie", "lastName": "Narayan", "school" : "Brooklyn Tech", "id": "t63512", "sports": "Track and Field", "clubs": "Chess"}');
> SELECT * FROM student_profiles;
id | profile | full_name
---------------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------+----------------
4ebb11a5-8e9a-49dc-905d-fade67027990 | {"clubs": "Chess", "firstName": "Ernie", "id": "t63512", "lastName": "Narayan", "school": "Brooklyn Tech", "sports": "Track and Field"} | Ernie Narayan
6724adfa-610a-4efe-b53d-fd67bd3bd9ba | {"birthdate": "2010-01-25", "credits": 120, "firstName": "Arthur", "id": "d78236", "lastName": "Read", "school": "PVPHS", "sports": "none"} | Arthur Read
75e253d7-a8b6-4be3-a133-bc75611f376e | {"birthdate": "2011-11-07", "clubs": "MUN", "credits": 67, "firstName": "Buster", "id": "f98112", "lastName": "Bunny", "school": "THS"} | Buster Bunny
(3 rows)
Time: 1ms total (execution 1ms / network 0ms)
The virtual column full_name is computed as a field from the profile column's data. The first name and last name are concatenated and separated by a single whitespace character using the concat_ws string function.
Create a table with geo-partitions and a computed column
In this example, create a table with geo-partitioning and a computed column:
> CREATE TABLE user_locations (
locality STRING AS (CASE
WHEN country IN ('ca', 'mx', 'us') THEN 'north_america'
WHEN country IN ('au', 'nz') THEN 'australia'
END) STORED,
id SERIAL,
name STRING,
country STRING,
PRIMARY KEY (locality, id))
PARTITION BY LIST (locality)
(PARTITION north_america VALUES IN ('north_america'),
PARTITION australia VALUES IN ('australia'));
Then, insert a few rows of data:
> INSERT INTO user_locations (name, country) VALUES
('Leonard McCoy', 'us'),
('Uhura', 'nz'),
('Spock', 'ca'),
('James Kirk', 'us'),
('Scotty', 'mx'),
('Hikaru Sulu', 'us'),
('Pavel Chekov', 'au');
> SELECT * FROM user_locations;
+---------------+--------------------+---------------+---------+
| locality | id | name | country |
+---------------+--------------------+---------------+---------+
| australia | 333153890100609025 | Uhura | nz |
| australia | 333153890100772865 | Pavel Chekov | au |
| north_america | 333153890100576257 | Leonard McCoy | us |
| north_america | 333153890100641793 | Spock | ca |
| north_america | 333153890100674561 | James Kirk | us |
| north_america | 333153890100707329 | Scotty | mx |
| north_america | 333153890100740097 | Hikaru Sulu | us |
+---------------+--------------------+---------------+---------+
The locality column is computed from the country column.
The locality values can then be used for geo-partitioning.
Create a table with a secondary index on a computed column
In this example, create a table with a virtual computed column and an index on that column:
> CREATE TABLE gymnastics (
id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
athlete STRING,
vault DECIMAL,
bars DECIMAL,
beam DECIMAL,
floor DECIMAL,
combined_score DECIMAL AS (vault + bars + beam + floor) VIRTUAL,
INDEX total (combined_score DESC)
);
Then, insert a few rows a data:
> INSERT INTO gymnastics (athlete, vault, bars, beam, floor) VALUES
('Simone Biles', 15.933, 14.800, 15.300, 15.800),
('Gabby Douglas', 0, 15.766, 0, 0),
('Laurie Hernandez', 15.100, 0, 15.233, 14.833),
('Madison Kocian', 0, 15.933, 0, 0),
('Aly Raisman', 15.833, 0, 15.000, 15.366);
> SELECT * FROM gymnastics;
+--------------------------------------+------------------+--------+--------+--------+--------+----------------+
| id | athlete | vault | bars | beam | floor | combined_score |
+--------------------------------------+------------------+--------+--------+--------+--------+----------------+
| 3fe11371-6a6a-49de-bbef-a8dd16560fac | Aly Raisman | 15.833 | 0 | 15.000 | 15.366 | 46.199 |
| 56055a70-b4c7-4522-909b-8f3674b705e5 | Madison Kocian | 0 | 15.933 | 0 | 0 | 15.933 |
| 69f73fd1-da34-48bf-aff8-71296ce4c2c7 | Gabby Douglas | 0 | 15.766 | 0 | 0 | 15.766 |
| 8a7b730b-668d-4845-8d25-48bda25114d6 | Laurie Hernandez | 15.100 | 0 | 15.233 | 14.833 | 45.166 |
| b2c5ca80-21c2-4853-9178-b96ce220ea4d | Simone Biles | 15.933 | 14.800 | 15.300 | 15.800 | 61.833 |
+--------------------------------------+------------------+--------+--------+--------+--------+----------------+
Now, run a query using the secondary index:
> SELECT athlete, combined_score FROM gymnastics ORDER BY combined_score DESC;
+------------------+----------------+
| athlete | combined_score |
+------------------+----------------+
| Simone Biles | 61.833 |
| Aly Raisman | 46.199 |
| Laurie Hernandez | 45.166 |
| Madison Kocian | 15.933 |
| Gabby Douglas | 15.766 |
+------------------+----------------+
The athlete with the highest combined score of 61.833 is Simone Biles.
Add a computed column to an existing table
In this example, create a table:
> CREATE TABLE x (
a INT NULL,
b INT NULL AS (a * 2) STORED,
c INT NULL AS (a + 4) STORED,
FAMILY "primary" (a, b, rowid, c)
);
Then, insert a row of data:
> INSERT INTO x VALUES (6);
> SELECT * FROM x;
+---+----+----+
| a | b | c |
+---+----+----+
| 6 | 12 | 10 |
+---+----+----+
(1 row)
Now add another virtual computed column to the table:
> ALTER TABLE x ADD COLUMN d INT AS (a // 2) VIRTUAL;
The d column is added to the table and computed from the a column divided by 2.
> SELECT * FROM x;
+---+----+----+---+
| a | b | c | d |
+---+----+----+---+
| 6 | 12 | 10 | 3 |
+---+----+----+---+
(1 row)
For more information, see ADD COLUMN.
Convert a computed column into a regular column
You can convert a stored, computed column into a regular column by using ALTER TABLE.
In this example, create a simple table with a computed column:
> CREATE TABLE office_dogs (
id INT PRIMARY KEY,
first_name STRING,
last_name STRING,
full_name STRING AS (CONCAT(first_name, ' ', last_name)) STORED
);
Then, insert a few rows of data:
> INSERT INTO office_dogs (id, first_name, last_name) VALUES
(1, 'Petee', 'Hirata'),
(2, 'Carl', 'Kimball'),
(3, 'Ernie', 'Narayan');
> SELECT * FROM office_dogs;
+----+------------+-----------+---------------+
| id | first_name | last_name | full_name |
+----+------------+-----------+---------------+
| 1 | Petee | Hirata | Petee Hirata |
| 2 | Carl | Kimball | Carl Kimball |
| 3 | Ernie | Narayan | Ernie Narayan |
+----+------------+-----------+---------------+
(3 rows)
The full_name column is computed from the first_name and last_name columns without the need to define a view. You can view the column details with the SHOW COLUMNS statement:
> SHOW COLUMNS FROM office_dogs;
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
| column_name | data_type | is_nullable | column_default | generation_expression | indices |
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
| id | INT | false | NULL | | {"primary"} |
| first_name | STRING | true | NULL | | {} |
| last_name | STRING | true | NULL | | {} |
| full_name | STRING | true | NULL | concat(first_name, ' ', last_name) | {} |
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
(4 rows)
Now, convert the computed column (full_name) to a regular column:
> ALTER TABLE office_dogs ALTER COLUMN full_name DROP STORED;
Check that the computed column was converted:
> SHOW COLUMNS FROM office_dogs;
+-------------+-----------+-------------+----------------+-----------------------+-------------+
| column_name | data_type | is_nullable | column_default | generation_expression | indices |
+-------------+-----------+-------------+----------------+-----------------------+-------------+
| id | INT | false | NULL | | {"primary"} |
| first_name | STRING | true | NULL | | {} |
| last_name | STRING | true | NULL | | {} |
| full_name | STRING | true | NULL | | {} |
+-------------+-----------+-------------+----------------+-----------------------+-------------+
(4 rows)
The computed column is now a regular column and can be updated as such:
> INSERT INTO office_dogs (id, first_name, last_name, full_name) VALUES (4, 'Lola', 'McDog', 'This is not computed');
> SELECT * FROM office_dogs;
+----+------------+-----------+----------------------+
| id | first_name | last_name | full_name |
+----+------------+-----------+----------------------+
| 1 | Petee | Hirata | Petee Hirata |
| 2 | Carl | Kimball | Carl Kimball |
| 3 | Ernie | Narayan | Ernie Narayan |
| 4 | Lola | McDog | This is not computed |
+----+------------+-----------+----------------------+
(4 rows)
Yes
No