Add BC performance knowledge from OptimAL learnings (#198)

* Add BC performance knowledge from OptimAL learnings

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

* Address review feedback on performance guidance

Clarify predicate-supporting keys versus covering queries, demonstrate proven cache reuse, and evaluate the updated partial-load and bulk-update rules.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

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

@@ -0,0 +1,29 @@
+---
+bc-version: [all]
+domain: performance
+keywords: [overlapping-keys, redundant-index, includedfields, sift, write-amplification, index-portfolio, key, setrange]
+technologies: [al]
+countries: [w1]
+application-area: [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 [read-pattern key design](design-covering-keys-from-read-pattern.md)); an included field cannot replace a key column used for ordering or a maintained SIFT sum. Including the explicit payload does not prove that the resulting index covers every automatically selected field. 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`](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`](review-overlapping-keys-before-adding-an-index.bad.al).
+
+## References
+
+- [Table keys: benefits, costs, and constraints](https://learn.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/devenv-table-keys).
+- [IncludedFields property](https://learn.microsoft.com/en-us/dynamics365/business-central/dev-itpro/developer/properties/devenv-includedfields-property).
+- [Manage index usage (BC 2026 release wave 1 and later)](https://learn.microsoft.com/en-us/dynamics365/business-central/manage-indexes).
+- [SIFT read/write trade-off](choose-maintainsiftindex-by-read-write-ratio.md).