register_output_renderer() should support streaming data

I’d like to implement this by first extending the register_output_renderer() hook to support streaming huge responses, then switching CSV to use the plugin hook in addition to TSV using it.

_Originally posted by @simonw in https://github.com/simonw/datasette/issues/1096#issuecomment-732542285_

[simonw]

Idea: instead of returning a dictionary, register_output_renderer could return an object. The object could have the following properties:

• .extension - the extension to use
• .can_render(...) - says if it can render this
• .can_stream(...) - says if streaming is supported
• async .stream_rows(rows_iterator, send) - method that loops through all rows and uses send to send them to the response in the correct format

I can then deprecate the existing dict return type for 1.0.

[eyeseast]

Ha! That was your idea (and a good one).

But it’s probably worth measuring to see what overhead it adds. It did require both passing in the database and making the whole thing async.

Just timing the queries themselves:

1. Using AsGeoJSON(geometry) as geometry takes 10.235 ms
2. Leaving as binary takes 8.63 ms

Looking at the network panel:

1. Takes about 200 ms for the fetch request
2. Takes about 300 ms

I’m not sure how best to time the GeoJSON generation, but it would be interesting to check. Maybe I’ll write a plugin to add query times to response headers.

The other thing to consider with async streaming is that it might be well-suited for a slower response. When I have to get the whole result and send a response in a fixed amount of time, I need the most efficient query possible. If I can hang onto a connection and get things one chunk at a time, maybe it’s ok if there’s some overhead.