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Add BC performance knowledge from OptimAL learnings (#198)

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* 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>

---------

Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
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Jesper Schulz-Wedde 2026-09-29 17:21:28 +02:00 • committed by GitHub
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microsoft/knowledge/performance/temporary-tables-have-no-database-cost.md
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@ -15,8 +15,12 @@ A temporary table stores its rows in Business Central Server memory instead of a
## Best Practice
Do not apply SQL-specific findings such as missing `SetLoadFields`, lock contention, or N+1 database round-trips to a temporary record. Still assess memory volume and repeated scans or lookups. For a pure key-to-value collection, consider an AL `Dictionary`; keep a temporary table when record fields, keys, filtering, or ordered iteration are required.
Do not apply SQL-specific findings such as missing `SetLoadFields`, lock contention, or N+1 database round-trips to a temporary record. Still assess memory volume and repeated scans or lookups. In a matrix, if each cell re-sums the same group, calculate the totals once at the complete group key and reuse them; invalidate or adjust totals if cells change. For a pure key-to-value collection, consider an AL `Dictionary`; keep a temporary table when record fields, keys, filtering, or ordered iteration are required. Measure AL time and peak memory, not just SQL time.
See sample: [`temporary-tables-have-no-database-cost.good.al`](temporary-tables-have-no-database-cost.good.al).
## Anti Pattern
Claiming that every temporary-table access pattern is free because no SQL is involved. A nested scan over a large in-memory buffer can still dominate service-tier CPU, while adding `SetLoadFields` to that buffer addresses a database cost that does not exist.
See sample: [`temporary-tables-have-no-database-cost.bad.al`](temporary-tables-have-no-database-cost.bad.al).