bcquality/microsoft/knowledge/performance/review-overlapping-keys-before-adding-an-index.md

bc-version	domain	keywords	technologies	countries	application-area
all	performance	overlapping-keys redundant-index includedfields sift write-amplification index-portfolio key setrange	al	w1	all

Review overlapping keys as a portfolio

Description

Adding a secondary key to a frequently written table maintains another SQL index for every affected write. Several keys with the same leading fields might be serving distinct filters, sort orders, unique constraints, or SIFT aggregates; they might instead be redundant payload variants. A shared prefix, absent SetCurrentKey calls, or low usage in a short window is not proof that a key is unused.

Best Practice

Before adding or removing a key, inventory keys from the table and installed extensions and identify each key's consumers and purpose: seek, ordering, uniqueness, or aggregation. Check SQLIndex, MaintainSQLIndex, SumIndexFields, and MaintainSIFTIndex, not only the AL key name. For confirmed payload-only variants on BC 19 or later, a single nonclustered key with IncludedFields can be a consolidation candidate (see covering keys); an included field cannot replace a key column used for ordering or a maintained SIFT sum. Measure representative read and write workloads, including periodic reports, integrations, and other companies, before and after a supported extension/schema change. Retain a rollback path for a critical reader that regresses. See sample: review-overlapping-keys-before-adding-an-index.good.al.

Anti Pattern

Keeping another customer/date key for each displayed field without checking whether the trailing fields support distinct operations. Conversely, merging (Customer, Date) with (Customer, Item, Date) merely because they share a prefix can regress item-selective queries; removing a unique key or a SIFT aggregate changes more than write cost. Do not report a key as unused solely because AL never calls SetCurrentKey on it. See sample: review-overlapping-keys-before-adding-an-index.bad.al.

References

• Table keys: benefits, costs, and constraints.
• IncludedFields property.
• Manage index usage (BC 2026 release wave 1 and later).
• SIFT read/write trade-off.