bigquery reservations are automatic? - google-bigquery

Question

Very basic question. If I purchase flex slots on BigQuery related with a specific project id, without (1) creating a reservation manually, and (2) assigning those slots, are my queries related to this project automatically going to be billed using flex slots?
I assume so - the unclear documentation suggests that a 'default' reservation is created when you purchase slots. Therefore, I imagine BigQuery recognizes the user's intention, unless otherwise specified, is to use the purchased capacity.
It would be a double whammy though if I was charged on-demand pricing while my slots were idle. And, I sense that given I reserved 100 slots, my queries feel slower. But I can't see a way to confirm the jobs used reservations.

Answer 1

Reservations
After you purchase slots, you can assign them to different buckets, called reservations. Reservations let you allocate the slots in ways that make sense for your particular organization.
A reservation named default is automatically created when you purchase
slots.
There is nothing special about the default reservation — it's created as a convenience. You can decide whether you need additional reservations or just use the default reservation.
For example, you might create a reservation named prod for production workloads, and a separate reservation named test for testing. That way, your test jobs won't compete for resources that your production workloads need. Or, you might create reservations for different departments in your organization.
Assignments
To use the slots that you purchase, you will assign projects, folders, or organizations to reservations. Each level in the resource hierarchy inherits the assignment from the level above it, unless you override. In other words, a project inherits the assignment of its parent folder, and a folder inherits the assignment of its organization.
When a job is started from a project that is assigned to a reservation, the job uses that reservation's slots.
If a project is not assigned to a reservation (either directly or by
inheriting from its parent folder or organization), the jobs in that
project use on-demand pricing.
None assignments represent an absence of an assignment. Projects assigned to None use on-demand pricing. The common use case for None assignments is to assign an organization to the reservation and to opt-out some projects or folders from that reservation by assigning them to None. For more information, see Assign a project to None.
Creating assignments
When you create an assignment, you specify the job type for that assignment:
QUERY: Use this reservation for query jobs, including SQL, DDL, DML, and BigQuery ML queries.
PIPELINE: Use this reservation for load, export, and other pipeline jobs.
By default, load and export jobs are free and use a shared pool of slots. BigQuery does not make guarantees about the available capacity of this shared pool. If you are loading large amounts of data, your job may wait as slots become available. In that case, you might want to purchase dedicated slots and assign pipeline jobs to them. We recommend creating an additional dedicated reservation with idle slot sharing disabled.
When load jobs are assigned to a reservation, they lose access to the free pool. Monitor performance to make sure the jobs have enough capacity. Otherwise, performance could actually be worse than using the free pool.
ML_EXTERNAL: Use this reservation for BigQuery ML queries that use services that are external to BigQuery.
Certain BigQuery ML queries use services that are external to BigQuery. To use reserved slots with these external services, create an assignment with job type ML_EXTERNAL.
Screenshots
A full screen guide how to work with Reservations and Assignments is here.