.. _facets:

Facets
======

Datasette facets can be used to add a faceted browse interface to any database table. With facets, tables are displayed along with a summary showing the most common values in specified columns. These values can be selected to further filter the table.

.. image:: facets.png

Facets can be specified in two ways: using querystring parameters, or in ``metadata.json`` configuration for the table.

Facets in querystrings
----------------------

To turn on faceting for specific columns on a Datasette table view, add one or more ``_facet=COLUMN`` parameters to the URL. For example, if you want to turn on facets for the ``city_id`` and ``state`` columns, construct a URL that looks like this::

    /dbname/tablename?_facet=state&_facet=city_id

This works for both the HTML interface and the ``.json`` view. When enabled, facets will cause a ``facet_results`` block to be added to the JSON output, looking something like this::

    {
      "state": {
        "name": "state",
        "results": [
          {
            "value": "CA",
            "label": "CA",
            "count": 10,
            "toggle_url": "http://...?_facet=city_id&_facet=state&state=CA",
            "selected": false
          },
          {
            "value": "MI",
            "label": "MI",
            "count": 4,
            "toggle_url": "http://...?_facet=city_id&_facet=state&state=MI",
            "selected": false
          },
          {
            "value": "MC",
            "label": "MC",
            "count": 1,
            "toggle_url": "http://...?_facet=city_id&_facet=state&state=MC",
            "selected": false
          }
        ],
        "truncated": false
      }
      "city_id": {
        "name": "city_id",
        "results": [
          {
            "value": 1,
            "label": "San Francisco",
            "count": 6,
            "toggle_url": "http://...?_facet=city_id&_facet=state&city_id=1",
            "selected": false
          },
          {
            "value": 2,
            "label": "Los Angeles",
            "count": 4,
            "toggle_url": "http://...?_facet=city_id&_facet=state&city_id=2",
            "selected": false
          },
          {
            "value": 3,
            "label": "Detroit",
            "count": 4,
            "toggle_url": "http://...?_facet=city_id&_facet=state&city_id=3",
            "selected": false
          },
          {
            "value": 4,
            "label": "Memnonia",
            "count": 1,
            "toggle_url": "http://...?_facet=city_id&_facet=state&city_id=4",
            "selected": false
          }
        ],
        "truncated": false
      }
    }

If Datasette detects that a column is a foreign key, the ``"label"`` property will be automatically derived from the detected label column on the referenced table.

Facets in metadata.json
-----------------------

You can turn facets on by default for specific tables by adding them to a ``"facets"`` key in a Datasette :ref:`metadata` file.

Here's an example that turns on faceting by default for the ``qLegalStatus`` column in the ``Street_Tree_List`` table in the ``sf-trees`` database::

    {
      "databases": {
        "sf-trees": {
          "tables": {
            "Street_Tree_List": {
              "facets": ["qLegalStatus"]
            }
          }
        }
      }
    }

Facets defined in this way will always be shown in the interface and returned in the API, regardless of the ``_facet`` arguments passed to the view.

Suggested facets
----------------

Datasette's table UI will suggest facets for the user to apply, based on the following criteria:

For the currently filtered data are there any columns which, if applied as a facet...

* Will return 20 or less unique options
* Will return more than one unique option
* Will return less unique options than the total number of filtered rows
* And the query used to evaluate this criteria can be completed in under 20ms

That last point is particularly important: Datasette runs a query for every column that is displayed on a page, which could get expensive - so to avoid slow load times it sets a time limit of just 20ms for each of those queries. This means suggested facets are unlikely to appear for tables with millions of records in them.