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Generated columns

D1 allows you to define generated columns based on the values of one or more other columns, SQL functions, or even extracted JSON values.

This allows you to normalize your data as you write to it or read it from a table, making it easier to query and reducing the need for complex application logic.

Generated columns can also have indexes defined against them, which can dramatically increase query performance over frequently queried fields.

​​ Types of generated columns

There are two types of generated columns:

  • VIRTUAL (default): the column is generated when read. This has the benefit of not consuming storage, but can increase compute time (and thus reduce query performance), especially for larger queries.
  • STORED: the column is generated when the row is written. The column takes up storage space just as a regular column would, but the column does not need to be generated on every read, which can improve read query performance.

When omitted from a generated column expression, generated columns default to the VIRTUAL type. The STORED type is recommended when the generated column is compute intensive. For example, when parsing large JSON structures.

​​ Define a generated column

Generated columns can be defined during table creation in a CREATE TABLE statement or afterwards via the ALTER TABLE statement.

To create a table that defines a generated column, you use the AS keyword:

CREATE TABLE some_table (
-- other columns omitted
some_generated_column AS <function_that_generates_the_column_data>

As a concrete example, to automatically extract the location value from the following JSON sensor data, you can define a generated column called location (of type TEXT), based on a raw_data column that stores the raw representation of our JSON data.

"measurement": {
"temp_f": "77.4",
"aqi": [21, 42, 58],
"o3": [18, 500],
"wind_mph": "13",
"location": "US-NY"

To define a generated column with the value of $.measurement.location, you can use the json_extract function to extract the value from the raw_data column each time you write to that row:

CREATE TABLE sensor_readings (
raw_data TEXT,
location as (json_extract(raw_data, '$.measurement.location')) STORED

Generated columns can optionally be specified with the column_name GENERATED ALWAYS AS <function> [STORED|VIRTUAL] syntax. The GENERATED ALWAYS syntax is optional and does not change the behavior of the generated column when omitted.

​​ Add a generated column to an existing table

A generated column can also be added to an existing table. If the sensor_readings table did not have the generated location column, you could add it by running an ALTER TABLE statement:

ALTER TABLE sensor_readings
ADD COLUMN location as (json_extract(raw_data, '$.measurement.location'));

This defines a VIRTUAL generated column that runs json_extract on each read query.

Generated column definitions cannot be directly modified. To change how a generated column generates its data, you can use ALTER TABLE table_name REMOVE COLUMN and then ADD COLUMN to re-define the generated column, or ALTER TABLE table_name RENAME COLUMN current_name TO new_name to rename the existing column before calling ADD COLUMN with a new definition.

​​ Examples

Generated columns are not just limited to JSON functions like json_extract: you can use almost any available function to define how a generated column is generated.

For example, you could generate a date column based on the timestamp column from the previous sensor_reading table, automatically converting a Unix timestamp into a YYYY-MM-dd format within your database:

ALTER TABLE your_table
-- date(timestamp, 'unixepoch') converts a Unix timestamp to a YYYY-MM-dd formatted date
ADD COLUMN formatted_date AS (date(timestamp, 'unixepoch'))

Alternatively, you could define an expires_at column that calculates a future date, and filter on that date in your queries:

-- Filter out "expired" results based on your generated column:
-- SELECT * FROM your_table WHERE current_date() > expires_at
ALTER TABLE your_table
-- calculates a date (YYYY-MM-dd) 30 days from the timestamp.
ADD COLUMN expires_at AS (date(timestamp, '+30 days'));

​​ Additional considerations

  • Tables must have at least one non-generated column. You cannot define a table with only generated column(s).
  • Expressions can only reference other columns in the same table and row, and must only use deterministic functions. Functions like random(), sub-queries or aggregation functions cannot be used to define a generated column.
  • Columns added to an existing table via ALTER TABLE ... ADD COLUMN must be VIRTUAL. You cannot add a STORED column to an existing table.