Is For a two stagequery over multiple databases, is it faster to do GROUP BY offirst or UNION more efficient than a flat GROUP BYfirst?

The xTables are stored in separate databases named by the year (this siis by third party-party design and I cannot change it). There are 3 databases in my case but it could be say up to 10. Each of the tables contains about 2500 records with about 300 distinct xID values.

Is a two stage GROUP BY of UNION more efficient than a flat GROUP BY?

The xTables are stored in separate databases named by the year (this si by third party design and I cannot change it). There are 3 databases in my case but it could be say up to 10. Each of the tables contains about 2500 records with about 300 distinct xID values.

For a query over multiple databases, is it faster to do GROUP BY first or UNION first?

The xTables are stored in separate databases named by the year (this is by third-party design and I cannot change it). There are 3 databases in my case but it could be say up to 10. Each of the tables contains about 2500 records with about 300 distinct xID values.