Wikimedia Bugzilla is closed!

mysql wmf db1051 root (none)> SELECT s.table_name,s.index_name,rows_read FROM information_schema.statistics s LEFT JOIN information_schema. index_statistics  i ON (i.table_schema=s.table_schema AND i.table_name=s.table_name AND i.index_name=s.index_name) WHERE s.table_schema='enwiki' and s.table_name = 'logging' AND  seq_in_index=1;
+------------+--------------------+-------------+
| table_name | index_name         | rows_read   |
+------------+--------------------+-------------+
| logging    | page_time          |    19794969 |
| logging    | times              | 15178724330 |
| logging    | type_time          |   391424823 |
| logging    | log_user_type_time |    31164428 |
| logging    | PRIMARY            |    11048025 |
| logging    | user_time          |     1442484 |
| logging    | log_page_id_time   |        NULL | <--- never used
+------------+--------------------+-------------+
7 rows in set (0.25 sec)

Same result appeared for various slaves and wikis across all WMF clusters.

log_page_id_time could be dropped.