Do we need to sort `_tidb_rowid` when dumping TiDB data?

[lichunzhu]

Question

TiDB-Lightning doesn't need data to keep in order of _tidb_rowid, but we still sort data in the order of _tidb_rowid. This may increase TiDB's memory usage because we'll need a global sort in TiDB.

[kennytm]

we don't really need ORDER BY _tidb_rowid but we must support WHERE xxx <= _tidb_rowid AND _tidb_rowid < yyy.