Wikimedia Bugzilla is closed!

mysql wmf db1051 root enwiki> 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 = 'categorylinks' AND  seq_in_index=1;
+---------------+--------------+------------+
| table_name    | index_name   | rows_read  |
+---------------+--------------+------------+
| categorylinks | cl_sortkey   |  780279158 |
| categorylinks | cl_from      | 2135832387 |
| categorylinks | cl_timestamp |  150744319 |
| categorylinks | cl_collation |       NULL | <-- never used
+---------------+--------------+------------+
4 rows in set (0.26 sec)

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

cl_collation could be dropped.

Or could it?  Does the updateCollation batch job need it?