Enum for Join Restrictions policy.

JOIN_CONDITION_UNSPECIFIED A join is neither required nor restricted on any column. Default value.
JOIN_ANY A join is required on at least one of the specified columns.
JOIN_ALL A join is required on all specified columns.
JOIN_NOT_REQUIRED A join is not required, but if present it is only permitted on 'joinAllowedColumns'
JOIN_BLOCKED Joins are blocked for all queries.