Runtime support for ATTACHing multiple databases

[simonw]

The implementation in #1232 is ready to land. It's the simplest-thing-that-could-possibly-work: you can run datasette one.db two.db three.db --crossdb and then use the /_memory page to run joins across tables from multiple databases.

It only works on the first 10 databases that were passed to the command-line. This means that if you have a Datasette instance with hundreds of attached databases (see Datasette Library) this won't be particularly useful for you.

So... a better, future version of this feature would be one that lets you join across databases on command - maybe by hitting /_memory?attach=db1&attach=db2 to get a special connection.

Originally posted by @simonw in #283 (comment)

[simonw]

I'm not going to work on this for a while, but if anyone has needs or ideas around that they can add them to this issue.