IndexCoverage

How much of a store an index can answer for, at one snapshot.

Three counters where CatalogCoverage has two, and the third is the important one. A segment is stale when the index over it is complete and correct and nonetheless unusable at this snapshot: an observation reports only the newest version of each key, but a segment may hold several versions of one key whenever an older snapshot pinned them, and a reader older than the segment's largest sequence is entitled to see a version the index never recorded. The guard is exact — an index over a segment is sound at snapshot S if and only if S.sequence is at or above that segment's largest sequence — and a segment that fails it is scanned instead.

That is the same mechanism as a segment with no sidecar at all: one concept, two causes, both ending in "the caller scans this one". Which is why an index is usable while it is still being built, with no cutover — a half-built index is simply an index with low coverage.