This feature enables Druid to greatly improve the query performance, especially when the query dataSource has a very large number of dimensions but the query only required several dimensions. This feature includes two parts. One is materialized-view-maintenance, and the other is materialized-view-selection.

    In materialized-view-maintenance, dataSources user ingested are called “base-dataSource”. For each base-dataSource, we can submit derivativeDataSource supervisors to create and maintain other dataSources which we called “derived-dataSource”. The dimensions and metrics of derived-dataSources are the subset of base-dataSource’s. The derivativeDataSource supervisor is used to keep the timeline of derived-dataSource consistent with base-dataSource. Each derivativeDataSource supervisor is responsible for one derived-dataSource.

    Supervisor Configuration

    Context

    Materialized-view-selection

    A sample view query spec is shown below:

    1.    {
    2.        "queryType": "view",
    3.        "query": {
    4.            "queryType": "groupBy",
    5.            "dataSource": "wikiticker",
    7.            "dimensions": [
    8.                "user"
    9.            ],
    10.                "type": "default",
    11.                "limit": 1,
    12.                "columns": [
    13.                    {
    14.                        "dimension": "added",
    15.                        "direction": "descending",
    16.                        "dimensionOrder": "numeric"
    17.                    }
    18.                ]
    20.                {
    21.                    "type": "longSum",
    22.                    "name": "added",
    23.                    "fieldName": "added"
    24.                }
    25.            ],
    26.            "intervals": [
    27.                "2015-09-12/2015-09-13"
    28.            ]
    29.        }

    There are 2 parts in a view query: