Add documentation for aggregate functions (#701)

docs/references/api.rst

@@ -17,6 +17,7 @@ PostgREST exposes three database objects of a schema as resources: tables, views
    api/resource_embedding.rst
    api/resource_representation.rst
    api/media_type_handlers.rst
+   api/aggregate_functions.rst
    api/openapi.rst
    api/preferences.rst
    api/*

docs/references/api/aggregate_functions.rst

@@ -0,0 +1,342 @@
+.. _aggregate_functions:
+
+Aggregate Functions
+###################
+
+Aggregate functions allow you to summarize data by performing calculations across groups of rows. For instance, if you have an ``orders`` table that has an ``amount`` column, you could use an aggregate function to get the sum of the ``amount`` column, either for all rows, or for each group of rows that share specific values, for instance all rows that share the same ``order_date``.
+
+.. note::
+ Aggregate functions are *disabled* by default in PostgREST, as without appropriate safeguards, aggregate functions can create performance problems. See :ref:`db-aggregates-enabled` for further details.
+
+PostgREST supports the following aggregate functions: ``avg()``, ``count()``, ``max()``, ``min()``, and ``sum()``. Please refer to the `section on aggregate functions in the PostgreSQL documentation <https://www.postgresql.org/docs/current/functions-aggregate.html>`_ for a detailed explanation of these functions.
+
+To use an aggregate function, you append the function to a value in the ``select`` parameter, like so:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=amount.sum() HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=amount.sum()"
+
+With the above query, PostgREST will return a single row with a single column named ``sum`` that contains the sum of all the values in the ``amount`` column:
+
+.. code-block:: json
+
+  [
+    {
+      "sum": 1234.56
+    }
+  ]
+
+You can use multiple aggregate functions by just adding more columns with aggregate functions to the ``select`` parameter.
+
+To group by other columns, you simply add those columns to the ``select`` parameter. For instance:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=amount.sum(),amount.avg(),order_date HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=amount.sum(),amount.avg(),order_date"
+
+This will return a row for each unique value in the ``order_date`` column, with the sum and average of the ``amount`` column for all rows that share the same ``order_date``:
+
+.. code-block:: json
+
+  [
+    {
+      "sum": 1234.56,
+      "avg": 123.45,
+      "order_date": "2023-01-01"
+    },
+    {
+      "sum": 2345.67,
+      "avg": 234.56,
+      "order_date": "2023-01-02"
+    }
+  ]
+
+.. note::
+  Aggregate functions work alongside other PostgREST features, like :ref:`h_filter`, :ref:`json_columns`, and :ref:`ordering`. Please note at this time aggregate functions are not compatible with :ref:`domain_reps`. Additionally, PostgreSQL's ``HAVING`` clause and ordering by aggregated columns are not yet supported.
+
+The Case of ``count()``
+===========================
+
+.. note::
+  Before the addition of aggregate functions, it was possible to count by adding ``count`` (without parentheses) to the ``select`` parameter. While this is still supported, it may be deprecated in the future, and thus use of this legacy feature is **not recommended.** Please use ``count()`` (with parentheses) instead.
+
+
+``count()`` is treated specially, as it can be used without an associated column. Take for example the following query:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=count(),order_date HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=count(),order_date"
+
+This would return a row for each unique value in the ``order_date`` column, with the count of all rows that share the same ``order_date``:
+
+.. code-block:: json
+
+  [
+    {
+      "count": 4,
+      "order_date": "2023-01-01"
+    },
+    {
+      "count": 2,
+      "order_date": "2023-01-02"
+    }
+  ]
+
+When ``count()`` is used with an associated column, its behaviour is slightly different: It will return the count of all values that are not ``NULL``. This is due to how PostgreSQL itself implements the ``count()`` function.
+
+Renaming and Casting
+====================
+
+Renaming Aggregates
+-------------------
+
+Just like with other columns, you can rename aggregated columns too. See :ref:`renaming_columns` for details.
+
+Renaming columns is especially helpful in the context of aggregate functions, as by default a column with an aggregate function applied will take on the name of the applied aggregate function. You may want to provide a more semantically meaningful name or prevent collisions when using multiple aggregate functions of the same type.
+
+Casting Aggregates
+------------------
+
+When applying an aggregate function to a column, you are able to cast both the value of the input to the aggregate function *and* the value of the output from the aggregate function. In both cases, the syntax works as described in :ref:`casting_columns`, with the only difference being the placement of the cast.
+
+Casting the Value of the Input
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For instance, imagine that the ``orders`` table has a JSON column, ``order_details``, and this column contains a JSON object that has a key, ``tax_amount``. Let's say you want to get the sum of the tax amount for every order. You can use the ``->`` or ``->>`` operators to extract the value with this key (see :ref:`json_columns`), but these operators will return values of the types JSON and ``text`` respectively, and neither of these types can be used with ``sum()``.
+
+Therefore, you will need to first cast the input value to a type that is compatible with ``sum()`` (e.g. ``numeric``). Casting the input value is done in exactly the same way as casting any other value:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=order_details->tax_amount::numeric.sum() HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=order_details->tax_amount::numeric.sum()"
+
+With this, you will receive the sum of the casted ``tax_amount`` value:
+
+.. code-block:: json
+
+  [
+    {
+      "sum": 1234.56
+    }
+  ]
+
+Casting the Value of the Output
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Now let's return to an example involving the ``amount`` column of the ``orders`` table. Imagine that we want to get the rounded average of the ``amount`` column. One way to do this is to use the ``avg()`` aggregate function and then to cast the output value of the function to ``int``. To cast the value of the output of the function, we simply place the cast *after* the aggregate function:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=amount.avg()::int HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=amount.avg()::int"
+
+You will then receive the rounded average as the result:
+
+.. code-block:: json
+
+  [
+    {
+      "avg": 201
+    }
+  ]
+
+Of course, you can use both input and output casts at the same time, if you so desire.
+
+
+Using Aggregate Functions with Resource Embedding
+=================================================
+
+Aggregate functions can be used in conjunction with :ref:`resource_embedding`. You can use embedded resources as grouping columns, use aggregate functions within the context of an embedded resource, or use columns from a spreaded resource as grouping columns or as inputs to aggregate functions.
+
+Using Embedded Resources as Grouping Columns
+--------------------------------------------
+
+Using an embedded resource as a grouping column allows you to use data from an association to group the results of an aggregation.
+
+For example, imagine that the ``orders`` table from the examples above is related to a ``customers`` table. If you want to get the sum of the ``amount`` column grouped by the ``name`` column from the ``customers`` table, you can include the customer name, using the standard :ref:`resource_embedding` syntax, and perform a sum on the ``amount`` column.
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=amount.sum(),customers(name) HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=amount.sum(),customers(name)"
+
+You will then get the summed amount, along with the embedded customer resource:
+
+.. code-block:: json
+
+  [
+    {
+      "sum": 100,
+      "customers": {
+        "name": "Customer A"
+      }
+    },
+    {
+      "sum": 200,
+      "customers": {
+        "name": "Customer B"
+      }
+    }
+  ]
+
+.. note::
+ The previous example uses a has-one association to demonstrate this functionality, but you may also use has-many associations as grouping columns, although there are few obvious use cases for this.
+
+Using Aggregate Functions Within the Context of an Embedded Resource
+--------------------------------------------------------------------
+
+When embedding a resource, you can apply aggregate functions to columns from the associated resource to perform aggregations within the context of an embedded resource.
+
+Continuing with the example relationship between ``orders`` and ``customers`` from the previous section, imagine that you want to fetch the ``name``, ``city``, and ``state`` for each customer, along with the sum of amount of the customer's orders, grouped by the order date. This can be done in the following way:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /customers?select=name,city,state,orders(amount.sum(),order_date) HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/customers?select=name,city,state,orders(amount.sum(),order_date)"
+
+.. code-block:: json
+
+  [
+    {
+      "name": "Customer A",
+      "city": "New York",
+      "state": "NY",
+      "orders": [
+        {
+          "sum": 215.22,
+          "order_date": "2023-09-01"
+        },
+        {
+          "sum": 905.73,
+          "order_date": "2023-09-02"
+        }
+      ]
+    },
+    {
+      "name": "Customer B",
+      "city": "Los Angeles",
+      "state": "CA",
+      "orders": [
+        {
+          "sum": 329.71,
+          "order_date": "2023-09-01"
+        },
+        {
+          "sum": 425.87,
+          "order_date": "2023-09-03"
+        }
+      ]
+    }
+  ]
+
+In this example, the ``amount`` column is summed and grouped by the ``order_date`` *within* the context of the embedded resource. That is, the ``name``, ``city``, and ``state`` from the ``customers`` table have no bearing on the aggregation performed in the context of the ``orders`` association; instead, each aggregation can be seen as being performed independently on just the orders belonging to a particular customer, using only the data from the embedded resource for both grouping and aggregation.
+
+Using Columns from a Spreaded Resource
+--------------------------------------
+
+When you :ref:`spread an embedded resource <spread_embed>`, the columns from the spreaded resource are treated as if they were columns of the top-level resource, both when using them as grouping columns and when applying aggregate functions to them.
+
+Grouping with Columns from a Spreaded Resource
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For instance, assume you want to sum the ``amount`` column from the ``orders`` table, using the ``city`` and ``state`` columns from the ``customers`` table as grouping columns. To achieve this, you may select these two columns from the ``customers`` table and spread them; they will then be used as grouping columns:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=amount.sum(),...customers(city,state) HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=amount.sum(),...customers(city,state)
+
+The result will be the same as if ``city`` and ``state`` were columns from the ``orders`` table:
+
+.. code-block:: json
+
+  [
+    {
+      "sum": 2000.29,
+      "city": "New York",
+      "state": "NY"
+    },
+    {
+      "sum": 9241.21,
+      "city": "Los Angeles",
+      "state": "CA"
+    }
+  ]
+
+Aggregate Functions with Columns from a Spreaded Resource
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Now imagine that the ``customers`` table has a ``joined_date`` column that represents the date that the customer joined. You want to get both the most recent and the oldest ``joined_date`` for customers that placed an order on every distinct order date. This can be expressed as follows:
+
+.. tabs::
+
+  .. code-tab:: http
+
+    GET /orders?select=order_date,...customers(joined_date.max(),joined_date.min()) HTTP/1.1
+
+  .. code-tab:: bash Curl
+
+    curl "http://localhost:3000/orders?select=order_date,...customers(joined_date.max(),joined_date.min())
+
+As columns from a spreaded resource are treated as if they were columns from the top-level resource, the ``max()`` and ``min()`` are applied *within* the context of the top-level, rather than within the context of the embedded resource, as in the previous section.
+
+The result will be the same as if the aggregations were applied to columns from the top-level:
+
+.. code-block:: json
+
+  [
+    {
+      "order_date": "2023-11-01",
+      "max": "2023-10-15",
+      "min": "2013-10-01"
+    },
+    {
+      "order_date": "2023-11-02",
+      "max": "2023-10-30",
+      "min": "2016-02-11"
+    }
+  ]

docs/references/api/tables_views.rst

@@ -273,6 +273,8 @@ When certain columns are wide (such as those holding binary data), it is more ef
 
 The default is ``*``, meaning all columns. This value will become more important below in :ref:`resource_embedding`.
 
+.. _renaming_columns:
+
 Renaming Columns
 ~~~~~~~~~~~~~~~~
 

docs/references/configuration.rst

@@ -178,6 +178,28 @@ app.settings.*
 
   Arbitrary settings that can be used to pass in secret keys directly as strings, or via OS environment variables. For instance: :code:`app.settings.jwt_secret = "$(MYAPP_JWT_SECRET)"` will take :code:`MYAPP_JWT_SECRET` from the environment and make it available to postgresql functions as :code:`current_setting('app.settings.jwt_secret')`.
 
+.. _db-aggregates-enabled:
+
+db-aggregates-enabled
+---------------------
+
+  =============== =======================
+  **Type**        Boolean
+  **Default**     False
+  **Reloadable**  Y
+  **Environment** PGRST_DB_AGGREGATES_ENABLED
+  **In-Database** pgrst.db_aggregates_enabled
+  =============== =======================
+
+
+  When this is set to :code:`true`, the use of :ref:`aggregate_functions` is allowed.
+
+  It is recommended that this be set to ``false`` unless proper safeguards are in place to prevent potential performance problems from arising. For example, it is possible that a user may request the ``max()`` of an unindexed column in a table with millions of rows. At best, this would result in a slow query, and at worst, it could be abused to prevent other users from accessing your API (i.e. a form of denial-of-service attack.)
+
+  Proper safeguards could include:
+    - Use of a statement timeout. See :ref:`impersonated_settings`.
+    - Use of the `pg_plan_filter extension <https://github.com/pgexperts/pg_plan_filter>`_ to block excessively expensive queries.
+
 .. _db-anon-role:
 
 db-anon-role

postgrest.dict

@@ -14,6 +14,7 @@ buildpack
 BOM
 Bytea
 Cardano
+casted
 cd
 centric
 coercible
@@ -143,6 +144,8 @@ SHA
 signup
 SIGUSR
 sl
+spreaded
+Spreaded
 SQL
 sql
 sr