Improvements + docs for db.execute() and Results class

* Including new results.first() and results.single_value() methods. Closes #685

docs/internals.rst

@@ -94,13 +94,74 @@ Database class
 
 Instances of the ``Database`` class can be used to execute queries against attached SQLite databases, and to run introspection against their schemas.
 
-SQLite only allows one database connection to write at a time. Datasette handles this for you by maintaining a queue of writes to be executed against a given database. Plugins can submit write operations to this queue and they will be executed in the order in which they are received.
+.. _database_execute:
+
+await db.execute(sql, ...)
+--------------------------
+
+Executes a SQL query against the database and returns the resulting rows (see :ref:`database_results`).
+
+``sql`` - string (required)
+    The SQL query to execute. This can include ``?`` or ``:named`` parameters.
+
+``params`` - list or dict
+    A list or dictionary of values to use for the parameters. List for ``?``, dictionary for ``:named``.
+
+``truncate`` - boolean
+    Should the rows returned by the query be truncated at the maximum page size? Defaults to ``True``, set this to ``False`` to disable truncation.
+
+``custom_time_limit`` - integer ms
+    A custom time limit for this query. This can be set to a lower value than the Datasette configured default. If a query takes longer than this it will be terminated early and raise a ``dataette.database.QueryInterrupted`` exception.
+
+``page_size`` - integer
+    Set a custom page size for truncation, over-riding the configured Datasette default.
+
+``log_sql_errors`` - boolean
+    Should any SQL errors be logged to the console in addition to being raised as an error? Defaults to ``True``.
+
+.. _database_results:
+
+Results
+-------
+
+The ``db.execute()`` method returns a single ``Results`` object. This can be used to access the rows returned by the query.
+
+Iterating over a ``Results`` object will yield SQLite `Row objects <https://docs.python.org/3/library/sqlite3.html#row-objects>`__. Each of these can be treated as a tuple or can be accessed using ``row["column"]`` syntax:
+
+.. code-block:: python
+
+    info = []
+    results = await db.execute("select name from sqlite_master")
+    for row in results:
+        info.append(row["name"])
+
+The ``Results`` object also has the following properties and methods:
+
+``.truncated`` - boolean
+    Indicates if this query was truncated - if it returned more results than the specified ``page_size``. If this is true then the results object will only provide access to the first ``page_size`` rows in the query result. You can disable truncation by passing ``truncate=False`` to the ``db.query()`` method.
+
+``.columns`` - list of strings
+    A list of column names returned by the query.
+
+``.rows`` - list of sqlite3.Row
+    This property provides direct access to the list of rows returned by the database. You can access specific rows by index using ``results.rows[0]``.
+
+``.first()`` - row or None
+    Returns the first row in the results, or ``None`` if no rows were returned.
+
+``.single_value()``
+    Returns the value of the first column of the first row of results - but only if the query returned a single row with a single column. Raises a ``datasette.database.MultipleValues`` exception otherwise.
+
+``.__len__()``
+    Calling ``len(results)`` returns the (truncated) number of returned results.
 
 .. _database_execute_write:
 
 await db.execute_write(sql, params=None, block=False)
 -----------------------------------------------------
 
+SQLite only allows one database connection to write at a time. Datasette handles this for you by maintaining a queue of writes to be executed against a given database. Plugins can submit write operations to this queue and they will be executed in the order in which they are received.
+
 This method can be used to queue up a non-SELECT SQL query to be executed against a single write connection to the database.
 
 You can pass additional SQL parameters as a tuple or dictionary.