Performance knowledge: keep low-selectivity fields out of key columns

[Miljan Milosavljević (miljance)]

Fixes #224

Mistake prevented

design-covering-keys-from-read-pattern.md says which predicates a key should support, but no article says which predicates should stay out of a key. Agents applying the guidance turn every filtered field into a key column, including Boolean, small Option/Enum and status fields (Processed, Blocked, Status), sometimes as the leading column. Such a column:

• widens every index row;
• adds write cost, most of all when the process the key serves rewrites the field (filter Processed = false, then ModifyAll(Processed, true));
• as a leading column, gives no seek to any read that does not filter it with equality.

Change

• New article microsoft/knowledge/performance/keep-low-selectivity-fields-out-of-key-columns.md, with a .bad.al/.good.al pair. The samples use the same reader code, and only the key differs.
• The article lists the legitimate exceptions so reviewers do not report false positives:
  • SIFT keys, which must contain every filtered field (cross-referenced to flowfield-source-key-needs-sumindexfields.md);
  • read-heavy covering through IncludedFields, when the process does not update the field;
  • skewed distributions filtered on the rare value;
  • unique keys.
• al-performance-review.md gains one worklist sentence that loads the article when an added or changed key, or its IncludedFields, contains a Boolean, Option, Enum or status field. The existing rule that a changed key alone is not a finding still applies.

Evidence

• The article links Microsoft Learn pages on table keys and their costs, on keys and performance, on SIFT, and the SQL Server index design guide on column order and selectivity.
• The claim that SQL Server cannot skip-scan past an unfiltered leading column is general SQL Server behaviour. The article states it in those terms, not as a BC platform guarantee.

Versions and domain

• bc-version: [all]. IncludedFields requires runtime 8.0 (BC 19).
• The performance domain is Microsoft-owned, and this is general key behaviour rather than an organization policy, so the article belongs in the Microsoft layer.

Checks

• validate_frontmatter.py: 0 errors. Its 2 warnings come from pre-existing files this PR does not touch.
• Test-KnowledgeIndex.ps1, Test-ReviewFixtures.ps1 and Test-ReviewContract.ps1 all pass.
• No model-based evaluation was run.

🤖 Generated with Claude Code