Function row_filter 

pub fn row_filter(
    caller: &Caller<'_>,
    names: &NameOverrides,
    schema: &str,
    table: &str,
    verb: Verb,
) -> Result<Option<Value>, Fault>

Which rows a role’s permission lets one operation reach.

Ok(None) where nothing restricts it: an unrestricted caller, a filter of {}, or an insert, which has no rows to choose from.