archive_userindex table doesn't include ar_comment_id column which is in archive table:
MariaDB [enwiki_p]> describe archive_userindex; +-------------------+---------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------------+---------------------+------+-----+---------+-------+ | ar_id | int(10) unsigned | NO | | 0 | | | ar_namespace | int(11) | NO | | 0 | | | ar_title | varbinary(255) | NO | | | | | ar_text | binary(0) | YES | | NULL | | | ar_comment | binary(0) | YES | | NULL | | <------- ??? | ar_user | int(5) unsigned | NO | | 0 | | | ar_user_text | varbinary(255) | NO | | | | ... +-------------------+---------------------+------+-----+---------+-------+ 20 rows in set (0.00 sec) MariaDB [enwiki_p]> describe archive; +-------------------+---------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------------------+---------------------+------+-----+---------+-------+ | ar_id | int(10) unsigned | NO | | 0 | | | ar_namespace | int(11) | NO | | 0 | | | ar_title | varbinary(255) | NO | | | | | ar_text | binary(0) | YES | | NULL | | | ar_comment | binary(0) | YES | | NULL | | | ar_comment_id | binary(0) | YES | | NULL | | <---------- is here | ar_user | bigint(10) unsigned | YES | | NULL | | | ar_user_text | varbinary(255) | YES | | NULL | | ... +-------------------+---------------------+------+-----+---------+-------+ 21 rows in set (0.00 sec)
This is bad because it means you can't get edit summaries of deleted revisions out of archive_userindex which is helpful. It is also inconsistent with revision and revision_userindex which both have rev_comment_id columns.