CER-0301
This inefficiency occurs when BigQuery slot reservations are sized for peak or anticipated demand but are not adjusted as workloads evolve. When actual query concurrency or complexity is lower than expected, a portion of the reserved slots remains idle. Because slot reservations are billed independently of usage, underutilized capacity results in sustained waste even while on-demand query costs elsewhere may continue.
This commonly happens when reservations are created during migrations, one-time analytical initiatives, or early scaling phases and are not revisited once usage stabilizes.
BigQuery slot reservations are billed as fixed commitments. Charges accrue for the full reserved capacity regardless of how much of that capacity is actually used.
• Review whether reserved slot capacity is consistently underutilized over representative time periods
• Identify reservations sized for historical or forecasted peaks that no longer occur
• Assess whether workloads relying on reservations have shifted to other projects or usage patterns
• Reduce or right-size slot reservations to better align with current workload demand
• Consolidate reservations across teams or projects where appropriate
• Reevaluate reservation sizing periodically as query patterns and business needs change
https://docs.cloud.google.com/bigquery/docs/reservations-intro
