Comment 2 for bug 1545129

The cause of the unexpected behavior is that pt-osc uses INSERT LOW_PRIORITY IGNORE to copy chunks, which only raises a warning instead of an error when there is a UNIQUE constraint violation (Percona and Oracle 5.6.35) so pt-osc silently ignores those rows.

example
# pt_online_schema_change:10906 41234 INSERT LOW_PRIORITY IGNORE INTO `xy`.`_table_new` () SELECT ) WHERE ((`user_id` >= ?)) AND ((`user_id` <= ?)) LOCK IN SHARE MODE /*pt-online-schema-change 41234 copy nibble*/ lower boundary: 18757143 upper boundary: 18773685
# pt_online_schema_change:10925 41234 SHOW WARNINGS