SQL Lab: keep the selected schema when switching to a database that has it

[shaoyinz]

Is your feature request related to a problem?
We have one Superset database connection per environment (dev, stage, prod) for the same Postgres application, and every environment has the same schemas. In SQL Lab, when I switch databases in "Select Database and Schema", the schema I had selected is cleared. I have to pick it again every time, even though the new database has a schema with exactly that name.

Describe the solution you'd like
When the database changes, keep the selected schema if the new database has a schema with the same name, and clear it only if it doesn't. DatabaseSelector already does this check when the schema list loads (the useSchemas onSuccess handler), but changeDatabase() clears the schema before that check runs.

Describe alternatives you've considered

• One SQL Lab tab per environment, each with its own database and schema.
• Schema-qualifying every table name in queries.

Both work, but switching environments to compare results is a routine task, and the reset adds a step every time.

Additional context
The same DatabaseSelector is used when creating datasets, where keeping the schema would help too.

Are you willing to contribute?
Yes. I can open a PR with tests and a before/after recording.