There are many properties stored in VARCHAR(255) fields, which sometimes isn't enough. This already causes http://jira.xwiki.org/jira/browse/XWIKI-883 . Changing to variable length field doesn't have any major side effects, as I know.
As pro arguments:
1. The size of the database will not increase
2. There are already some fields stored as mediumtext and longblob, so it's not something new in the database
3. The fewer limits there are, the better
4. Issues like XWIKI-883 will be fixed
... and maybe more
Is there something I'm missing that prevents this?
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