A BigQuery slot is a unit of computational capacity required to execute SQL queries. BigQuery automatically calculates how many slots are required by each query, depending on query size and complexity.
See the quota policy for queries for the per-project slot quota.
Most users find the default slot capacity more than sufficient. Access to more slots does not guarantee faster per-query performance. However, a larger pool of slots might improve performance of very large or very complex queries, as well as performance of highly concurrent workloads. To check how many slots your account uses, see Monitoring BigQuery using Stackdriver.
BigQuery automatically manages your slots quota based on customer history, usage, and spending. For customers who require consistent monthly expenditures on analytics spending, BigQuery offers flat-rate pricing.
When you enroll in a flat-rate pricing plan, you purchase a dedicated number of slots to use for query processing. You can specify the allocation of slots by location for all projects attached to that billing account. Contact your sales representative if you are interested in flat-rate pricing.