Extension

The extends parameter, supported for cubes and views, allows you to create a child cube (or a view) that reuses all declared members of a parent cube (or a view). This helps build reusable data models.

Cubes declare members such as measures, dimensions, and segments. When a child cube extends the parent cube, lists of measures, dimensions, and segments are merged. For example, if the parent cube defines the a measure and the child cube defines the b measure, the resulting cube will have both measures a and b.

The usual pattern is to extract common measures, dimensions, and joins into the parent cube and then extend from it. This helps prevent code duplication and makes code easier to maintain and refactor.

In the example below, the base_events cube defines the common events measures, dimensions, and a join to the users cube:

cubes:  - name: base_events  sql_table: events    joins:  - name: users  relationship: many_to_one  sql: "{CUBE}.user_id = {users.id}"    measures:  - name: count  type: count    dimensions:  - name: timestamp  sql: time  type: time

The product_purchases and page_views cubes are extended from base_events and define only the specific dimensions: product_name for product purchases and page_path for page views.

cubes:  - name: product_purchases  sql_table: product_purchases  extends: base_events    dimensions:  - name: product_name  sql: product_name  type: string    - name: page_views  sql_table: page_views  extends: base_events    dimensions:  - name: page_path  sql: page_path  type: string

Usage with FILTER_PARAMS

If the parent cube is using FILTER_PARAMS in any sql parameter, then child cubes can accomodate to that in two ways.

First, the sql parameter can be overridden in each child cube:

cubes:  - name: product_purchases  sql: >  SELECT *  FROM events  WHERE {FILTER_PARAMS.product_purchases.timestamp.filter('time')}    # ...

Alternatively, all filters can be put inside the parent cube and referenced in the child cubes using AND. The unused filters will be rendered to 1 = 1 in the SQL query:

cubes:  - name: base_events  sql: >  SELECT *  FROM events  WHERE  {FILTER_PARAMS.base_events.timestamp.filter('time')} AND  {FILTER_PARAMS.product_purchases.timestamp.filter('time')} AND  {FILTER_PARAMS.page_views.timestamp.filter('time')}    # ...