When using an Oracle database, currently using a pre-flyway DSpace version (e.g. DSpace 1.8), and upgrading to DSpace 6, the flyway migration is broken.
The DSpace schema built from the SQL files prior to flyway differs from the DSpace schema built from the SQL files using flyway
An example is the INDEX on metadataschemaregistry(short_id) which is present in old DSpace versions (e.g. DSpace 1.8, https://github.com/DSpace/DSpace/blob/dspace-1_8_x/dspace/etc/oracle/database_schema.sql#L254) but created in https://github.com/DSpace/DSpace/blob/dspace-6_x/dspace-api/src/main/resources/org/dspace/storage/rdbms/sqlmigration/postgres/V6.0_2016.04.04__DS-3086-OAI-Performance-fix.sql causing an error
Another example is the handle migration from https://github.com/DSpace/DSpace/blob/dspace-6_x/dspace-api/src/main/resources/org/dspace/storage/rdbms/sqlmigration/oracle/V6.0_2015.03.07__DS-2701_Hibernate_migration.sql#L359 where line 360 undoes 359 and line 361 undoes the 2 above
We're creating a list of such problems, and have solutions for the problems we encountered so far. I will create a PR once further progressed.