The unmapped source key as it appears in ExternalRecord.Fields.
Longest observed String(value).length — sizes the future column generously.
Records in which the key appeared with a non-null value.
Bounded sample of observed values, for type inference at promotion time.
Total records scanned for this entity map this run.
Per-key statistics for CUSTOM (unmapped) source keys observed during a sync — aggregated IN MEMORY across every fetched record, REGARDLESS of whether the row was written or the content-hash fast path skipped it. Custom-overflow behaviour: a newly-appearing custom column must NOT affect the row-hash match (so unchanged rows stay skip-cheap), yet its presence + sizing statistics must still surface as a promotion candidate at sync end. This out-of-band aggregation is what makes both true at once — candidates and generous sizing stats exist even when every row was skipped.