value: 0 Unspecified partition key. If used, it means using non-partitioned table.
READ_TIME
value: 1 The time when the snapshot is taken. If specified as partition key, the
result table(s) is partitoned by the additional timestamp column,
readTime. If [read_time] in ExportAssetsRequest is specified, the
readTime column's value will be the same as it. Otherwise, its value will
be the current time that is used to take the snapshot.
REQUEST_TIME
value: 2 The time when the request is received and started to be processed. If
specified as partition key, the result table(s) is partitoned by the
requestTime column, an additional timestamp column representing when the
request was received.