How to sample rows if not all rows are scanned. Meaningful only when
used in conjunction with either rows_limit or rows_limit_percent. If
not specified, rows are scanned in the order BigQuery reads them.
Scan groups of rows in the order BigQuery
provides (default). Multiple groups of rows may
be scanned in parallel, so results may not
appear in the same order the rows are read.
Randomly pick groups of rows to scan.