A slot is one thread on one machine, and Dremel estimates the execution of every query in slots.
The estimate is a guess: how many machines, into the number of threads those machines have, the query is going to use. A query that pulls 50 thousand data points on threads that each pull 1000 a second needs 50 slots.
The client does not ask for slots. The load balancer makes the estimate:
- A pool. The system has 3000 slots.
- A query of 2000 slots. It goes ahead.
- Another query of 2000 slots. Only a thousand are sent, and the query waits. It is slow, or it fails and the client retries, and the system is not overloaded.
A node holds several slots. A quad core processor has 4 threads and so 4 slots, an octa core has 8 and a single core has 1.
Slots are also what the customer pays for. AWS states how many IO operations a database does per second and charges only for that, and Google charges only for slots.