When the current database is implicitly used in a query that also uses an
explicit database, it must be routed to the shard which has the current
database.
As cross-shard joins are not supported, the safest, and possibly the most
expected course of action to take, is to route it to the so-called default
shard. The default shard is the shard that contains the database that is
currently set as the active database with a COM_INIT_DB, a text protocol
USE <database> query or it was set at connection time.
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