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26 lines
1.7 KiB
Markdown
26 lines
1.7 KiB
Markdown
---
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bc-version: [all]
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domain: performance
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keywords: [temporary-table, in-memory, findset, findfirst, get, no-db-cost]
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technologies: [al]
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countries: [w1]
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application-area: [all]
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---
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# Temporary tables avoid SQL I/O, not in-memory work
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## Description
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A temporary table stores its rows in Business Central Server memory instead of a physical SQL table. Its reads and writes therefore do not incur SQL round-trips, locking, or SIFT maintenance. They still allocate memory and execute record filtering, key lookup, sorting, insertion, and iteration in the service tier; those costs grow with the temporary dataset and access pattern.
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## Best Practice
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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.
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See sample: [`temporary-tables-have-no-database-cost.good.al`](temporary-tables-have-no-database-cost.good.al).
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## Anti Pattern
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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.
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See sample: [`temporary-tables-have-no-database-cost.bad.al`](temporary-tables-have-no-database-cost.bad.al).
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