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28 lines
1.5 KiB
Markdown
28 lines
1.5 KiB
Markdown
---
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bc-version: [all]
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domain: performance
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keywords: [case, branch, frequency, control-flow, hot-path]
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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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# Order case branches by frequency
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> Contributions welcome — open a PR to refine or extend this article.
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## Description
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The AL `case` statement evaluates branches in the order they appear. When the distribution of the discriminator is heavily skewed — one or two values handle the vast majority of records, and the rest handle edge cases — the average cost of the statement is dominated by how many branches precede the common one. For evenly distributed discriminators the order does not matter; for skewed distributions it changes the hot-path cost of every call site.
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## Best Practice
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Where the runtime frequency of values is known or measurable, list the common branches first. An `else` arm that handles unexpected values belongs last. When the common branch is also the simplest to evaluate, the placement compounds: the hot path is both short and cheap, and the uncommon branches are never touched on typical records.
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See sample: `order-case-branches-by-frequency.good.al`.
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## Anti Pattern
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Ordering branches alphabetically, by enum declaration order, or by "logical grouping" when the runtime distribution is heavily skewed. Every common record pays the cost of evaluating every uncommon branch first; on a posting routine processing thousands of rows the overhead is measurable.
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See sample: `order-case-branches-by-frequency.bad.al`.
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