--limit= mechanism plus new limits for facets

Replaced the --max_returned_rows and --sql_time_limit_ms options to
"datasette serve" with a new --limit option, which supports a larger
list of limits.

Example usage:

	datasette serve --limit max_returned_rows:1000 \
		--limit sql_time_limit_ms:2500 \
		--limit default_facet_size:50 \
		--limit facet_time_limit_ms:1000 \
		--limit facet_suggest_time_limit_ms:500

New docs: https://datasette.readthedocs.io/en/latest/limits.html

Closes #270
Closes #264

docs/getting_started.rst

@@ -101,9 +101,6 @@ datasette serve options
       --cors                       Enable CORS by serving Access-Control-Allow-
                                    Origin: *
       --page_size INTEGER          Page size - default is 100
-      --max_returned_rows INTEGER  Max allowed rows to return at once - default is
-                                   1000. Set to 0 to disable check entirely.
-      --sql_time_limit_ms INTEGER  Max time allowed for SQL queries in ms
       --load-extension PATH        Path to a SQLite extension to load
       --inspect-file TEXT          Path to JSON file created using "datasette
                                    inspect"
@@ -113,4 +110,6 @@ datasette serve options
       --plugins-dir DIRECTORY      Path to directory containing custom plugins
       --static STATIC MOUNT        mountpoint:path-to-directory for serving static
                                    files
+      --limit LIMIT                Set a limit using limitname:integer
+                                   datasette.readthedocs.io/en/latest/limits.html
       --help                       Show this message and exit.

docs/index.rst

@@ -22,6 +22,7 @@ Contents
    facets
    full_text_search
    metadata
+   limits
    custom_templates
    plugins
    changelog

docs/json_api.rst

@@ -132,7 +132,7 @@ Special table arguments
 The Datasette table view takes a number of special querystring arguments:
 
 ``?_size=1000`` or ``?_size=max``
-    Sets a custom page size. This cannot exceed the ``max_returned_rows`` option
+    Sets a custom page size. This cannot exceed the ``max_returned_rows`` limit
     passed to ``datasette serve``. Use ``max`` to get ``max_returned_rows``.
 
 ``?_sort=COLUMN``

docs/limits.rst

@@ -0,0 +1,51 @@
+Limits
+======
+
+To prevent rogue, long-running queries from making a Datasette instance inaccessible to other users, Datasette imposes some limits on the SQL that you can execute.
+
+sql_time_limit_ms
+-----------------
+
+By default, queries have a time limit of one second. If a query takes longer than this to run Datasette will terminate the query and return an error.
+
+If this time limit is too short for you, you can customize it using the ``sql_time_limit_ms`` limit - for example, to increase it to 3.5 seconds::
+
+    datasette mydatabase.db --limit sql_time_limit_ms:3500
+
+You can optionally set a lower time limit for an individual query using the ``_timelimit`` query string argument::
+
+    /my-database/my-table?qSpecies=44&_timelimit=100
+
+This would set the time limit to 100ms for that specific query. This feature is useful if you are working with databases of unknown size and complexity - a query that might make perfect sense for a smaller table could take too long to execute on a table with millions of rows. By setting custom time limits you can execute queries "optimistically" - e.g. give me an exact count of rows matching this query but only if it takes less than 100ms to calculate.
+
+max_returned_rows
+-----------------
+
+Datasette returns a maximum of 1,000 rows of data at a time. If you execute a query that returns more than 1,000 rows, Datasette will return the first 1,000 and include a warning that the result set has been truncated. You can use OFFSET/LIMIT or other methods in your SQL to implement pagination if you need to return more than 1,000 rows.
+
+You can increase or decrease this limit like so::
+
+    datasette mydatabase.db --limit max_returned_rows:2000
+
+default_facet_size
+------------------
+
+The default number of unique rows returned by :ref:`facets` is 30. You can customize it like this::
+
+    datasette mydatabase.db --limit default_facet_size:50
+
+facet_time_limit_ms
+-------------------
+
+This is the time limit Datasette allows for calculating a facet, which defaults to 200ms::
+
+    datasette mydatabase.db --limit facet_time_limit_ms:1000
+
+facet_suggest_time_limit_ms
+---------------------------
+
+When Datasette calculates suggested facets it needs to run a SQL query for every column in your table. The default for this time limit is 50ms to account for the fact that it needs to run once for every column. If the time limit is exceeded the column will not be suggested as a facet.
+
+You can increase this time limit like so::
+
+    datasette mydatabase.db --limit facet_suggest_time_limit_ms:500

docs/sql_queries.rst

@@ -46,39 +46,6 @@ statements can be used to change database settings at runtime. If you need to
 include the string "pragma" in a query you can do so safely using a named
 parameter.
 
-Query limits
-------------
-
-To prevent rogue, long-running queries from making a Datasette instance
-inaccessible to other users, Datasette imposes some limits on the SQL that you
-can execute.
-
-By default, queries have a time limit of one second. If a query takes longer
-than this to run Datasette will terminate the query and return an error.
-
-If this time limit is too short for you, you can customize it using the
-``sql_time_limit_ms`` option - for example, to increase it to 3.5 seconds::
-
-    datasette mydatabase.db --sql_time_limit_ms=3500
-
-You can optionally set a lower time limit for an individual query using the
-``_timelimit`` query string argument::
-
-    /my-database/my-table?qSpecies=44&_timelimit=100
-
-This would set the time limit to 100ms for that specific query. This feature
-is useful if you are working with databases of unknown size and complexity -
-a query that might make perfect sense for a smaller table could take too long
-to execute on a table with millions of rows. By setting custom time limits you
-can execute queries "optimistically" - e.g. give me an exact count of rows
-matching this query but only if it takes less than 100ms to calculate.
-
-Datasette returns a maximum of 1,000 rows of data at a time. If you execute a
-query that returns more than 1,000 rows, Datasette will return the first 1,000
-and include a warning that the result set has been truncated. You can use
-OFFSET/LIMIT or other methods in your SQL to implement pagination if you need to
-return more than 1,000 rows.
-
 Views
 -----