Document pre-aggregation measure identity, role matching, filter placement and limitations - #2380
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…tations Measure matching now compares expression AND Phase-1 aggregation, so the "matched by expression hash" description was inaccurate and the guarantee it provides was unstated. Adds a section on registering several aggregations of one column, documents where filters are applied relative to the roll-up, and records five limitations that were previously only discoverable by experiment: no dimension joins onto an aggregate, one uncovered metric affecting a whole grain group, registrations pinned to a node revision, freshness not gating routing, and the raw-values requirement for COUNT(DISTINCT) columns. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Grain matching canonicalizes dimension references, so a bare and a role-qualified spelling of the same dimension match each other and a registration made with one spelling still serves queries written with the other. A bare name is rejected only when the dimension is reachable by several roles, where it names none of them. Output aliases still follow the caller's spelling. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
The "What this means when you model" bullet still said measures are matched by hash, which reads as expression-only. Measure identity is the expression plus its aggregation, as the matching section above now states. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Written before the role-free-link preference landed in #2375: a bare name and a role-qualified spelling only match when exactly one named role reaches the dimension. When a role-free link also exists, the bare name means that link and the two stay distinct. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Summary
Documents pre-aggregation behavior that shipped in #2369, #2370 and #2375 but wasn't written down, plus the limitations that are easiest to get wrong in practice.
Test Plan
make checkpassesmake testshows 100% unit test coverageDeployment Plan
None -- docs only.