When tehre is a table having a key like:
PRIMARY KEY (`id`,`start`,`end`,`user_id`(13),`interval`),
DBI driver is taking (13) as part of the field name.
Fixed the way pt-archiver composes the key using DBI cols field
instead of the colnames field since the cols field has the correct
field names for the key.
In the case we are trying to migrate a table with no PK nor an unique
key, pt-archiver was failing to select the correct rows for deletion.
This fix implemented here is to add ALL columns in the WHERE clause of
the DELETE command. This way, we are deleting only the exact same row we
just migrated but using columns instead of an index.
