How to install a separated Civi database with Drupal 8?
I was able to separate my Drupal 8 database from my CiviCRM database fairly easily, here are my notes I took on how to do it. Note that these steps aren't particularly specific to Drupal 8, and can be used to convert any single-database install into a two-database install.


Create a new database (e.g. "example_civi") on the same server.
Do NOT create a separate MySQL user - just use the same MySQL user
Drupal uses.
Grant permissions to the user.  Here's the permissions you need:


TABLES, CREATE TEMPORARY TABLES, TRIGGER, CREATE ROUTINE, ALTER ROUTINE,
CREATE VIEW, SHOW VIEW, REFERENCES ON ``.* TO ''@'localhost';



Load a dump that just has CiviCRM tables into the new database.  Steps to dump tables by prefix are available. Since you sound like you're potentially upgrading an existing site, you can just copy the existing database instead.
Drop the CiviCRM tables from the Drupal database.  There's a good
explanation of how to remove tables by prefix.


On the last step, note that foreign key constraints will cause some tables not to delete.  You'll either need to run those steps multiple times or disable FK constraints before dropping the tables.

UPDATE:  See also how to enable Views integration on D8 with separated databases.