The `user_directory` table has a `room_id` column that seems to be unused.

This issue has been migrated from [#10770](https://github.com/matrix-org/synapse/issues/10770).

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I thought it might be early intention to support per-room user directory entries, but there's also a UNIQUE index on `user_id`, so the table makes this impossible currently.

It's unclear why this is here, but the question now is whether we want to do anything with it?

On librepush.net, `synapse=# select * from user_directory where room_id is not null;` gives me no rows (and I think I did check it was unused in the code the other day, *but I've slept since then!*).

I would suggest we eventually remove it if we're not going to use it, but that's a pain to do with SQLite (if I remember. Though we could still do it since we can assume SQLite homeservers are small).