"Hibernate Sequence already Exists" PostgreSQL Warning
Explanation
When starting XWiki with PostgreSQL, you may encounter the following message in the PostgreSQL logs:
Relation "hibernate_sequence" already existsCause
XWiki lets Hibernate generate the identifiers of a few tables, and on PostgreSQL and Oracle Hibernate does that from a sequence named hibernate_sequence. Hibernate would normally create that sequence itself when it updates the schema, but because of HHH-13464 it skips the creation as soon as a sequence of that name exists in any schema, which leaves subwikis without one. XWiki therefore creates the sequence itself, in HibernateStore: it first asks the schema whether the sequence is already there and skips the creation if it is, and only when that query itself fails does it attempt the creation blindly. PostgreSQL then logs this message, which was reported as XWIKI-14055 and closed as won't fix because it is harmless.
Solution
No action is required. XWiki does not report the failed creation as an error in this case, and the wiki starts and operates normally.
