Upgrade to 5.2.x fails due to MySQL collation mismatch on columns 'FOLLOW_CONNECTIONS.FOLLOWER' and 'user_mapping.user_key'

NB: the database should be backed up before making any changes.

1. Follow the instructions in this document. This is necessary to create all the foreign keys that you should have in your database, to ensure referential integrity.
2. Then attempt the upgrade to 5.2 again. NB: Do not attempt to perform the upgrade at the same time. Follow the instructions above to fix the storage engine, and then upgrade as normal once that is complete.

You may also want to check the Known Issues for MySQL to see if there are any other settings you may be missing that could prevent issues like this in future.

NB: the database should be backed up before making any changes.

1. Shut down Confluence
2. If any of the following SQL statements returns any rows, migration to utf8 should be performed before proceeding:

   use confluence;
   SHOW variables WHERE Variable_name = "character_set_database" AND Value != "utf8";
   

   SELECT table_name, column_name, character_set_name FROM information_schema.`COLUMNS` C
   WHERE table_schema = "confluence"
     AND character_set_name != "utf8";
   

   SELECT T.table_name, CCSA.character_set_name FROM information_schema.`TABLES` T,
          information_schema.`COLLATION_CHARACTER_SET_APPLICABILITY` CCSA
   WHERE CCSA.collation_name = T.table_collation
     AND T.table_schema = "confluence"
     AND CCSA.character_set_name != "utf8";
   

3. Check database collation:

   use confluence;
   show variables like "collation_database";
   

4. If database collation is not "utf8_bin", fix it with:

   alter database confluence character set utf8 collate utf8_bin;
   

5. Find tables using non utf8_bin collation:

   SELECT DISTINCT T.table_name FROM information_schema.`TABLES` T, information_schema.`COLUMNS` C 
   WHERE T.table_schema = "confluence"
     AND T.table_name = C.table_name
     AND (T.table_collation != "utf8_bin" OR C.collation_name != "utf8_bin");
   

6. Fix each table with a command like the following:

   ALTER TABLE FOLLOW_CONNECTIONS CONVERT TO CHARACTER SET DEFAULT; 
   

   OR

   SELECT CONCAT('ALTER TABLE `', t.`TABLE_SCHEMA`, '`.`', t.`TABLE_NAME`, '` CONVERT TO CHARACTER SET DEFAULT') as stmt 
   FROM `information_schema`.`TABLES` t
   WHERE 1
   AND t.`TABLE_SCHEMA` = 'confluence'
   ORDER BY 1;
   

   Ensure to change the t.table_schema to reflect your Confluence database name.

1. Restart Confluence. The remaining upgrade tasks should run at this point.

You may also want to check the Known Issues for MySQL to see if there are any other settings you may be missing that could prevent issues like this in future.