rowsProjectedFromColumns

Rows whose projected fields were read out of shredded columns rather than out of a document.

The counter that says push-down actually fired. documentsRead == 0 alone cannot: a query returning nothing satisfies it, and so does one projecting only keys. Equal to rowsReturned when every row was served from columns, and zero when no projected path had one — which is not a failure but the ordinary case for a path nobody shredded.