?_nocount=1 for opting out of table counts

[simonw]

Running a trace against a CSV streaming export with the new _trace=1 feature from #1351 shows that the following code is executing a select count(*) from table for every page of results returned:

datasette/datasette/views/table.py

Lines 700 to 705 in d1d06ac

	if count_sql and filtered_table_rows_count is None:
	try:
	count_rows = list(await db.execute(count_sql, from_sql_params))
	filtered_table_rows_count = count_rows[0][0]
	except QueryInterrupted:
	pass

This is inefficient - a new ?_nocount=1 option would let us disable this count in the same way as #1349:

datasette/datasette/views/base.py

Lines 264 to 276 in d1d06ac

	async def as_csv(self, request, database, hash, **kwargs):
	stream = request.args.get("_stream")
	# Do not calculate facets:
	if not request.args.get("_nofacets"):
	if not request.query_string:
	new_query_string = "_nofacets=1"
	else:
	new_query_string = request.query_string + "&_nofacets=1"
	new_scope = dict(
	request.scope, query_string=new_query_string.encode("latin-1")
	)
	request.scope = new_scope
	if stream:

[simonw]

I'm going to rename ?_nofacets=1 to ?_nofacet=1 to keep it consistent with the new ?_nocount=1 option (and because I don't like ?_nocounts=1).

[simonw]

Documented here: https://docs.datasette.io/en/latest/json_api.html#special-table-arguments