Right now we don't support nested queries that act on different DBs so we need to ensure we don't get any inconsistencies.
See XWIKI-8160 which shows what can happen when you get inconsistencies.
Fail a query if we can't switch the current wiki
Unable to access to different databases via one context in XWikiHibernateBaseStore#execute method
HiddenDocument query filter can put corrupted user documents in the cache (virtual mode)
On HSQLDB each access to the hibernate store generate a switch schema request