Specifying Tests During Deployment When Deploying Unit Tests

Question

When we deploy change sets that contain Apex classes such as controllers, we'll always deploy using default unit testing thus running all test classes. In my mind, this is best practise to ensure the change set and emulation of it running touches everything, and everything is validated.

However, when the change set only contains unit tests, is it bad practice to only run those unit tests to speed up their deployment given they're ran privately?

I haven't encountered nor read of any problems this could cause, but I'm curious.

Tl;dr:

Is it bad practice when deploying only unit tests to only run those unit tests during deployment?

Answer 1

If you're deploying to production, you'll have to run all tests anyways. If you're deploying to another sandbox, feel free to run just those tests.

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See these notes:

Default Test Execution in Production

When no test level is specified in the deployment options, the default test execution behavior depends on the contents of your deployment package. When deploying to production, all tests, except those that originate from managed packages, are executed if your deployment package contains Apex classes or triggers. If your package doesn’t contain Apex components, no tests are run by default.