IntegerMigrator doesn't allow duplicate migrations (1_foo, 1_bar).
Requires manual renaming for simultaneous migrations.
Here's comparison in detail:
https://sequel.jeremyevans.net/rdoc/files/doc/migration_rdoc.html#label-Two+separate+migrators
The TimestampMigrator will be used if any filename in the migrations
directory has a version greater than 20000101.
IntegerMigrator keeps version information at scheme_info table with an
integer. But TimestampMigrator keeps version information at
scheme_migrations tables with all filenames.
It's not backward compatible. You need to reset your database or create
and fill scheme_migrations table manually, then drop scheme_info table.
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