Thomas Kellerer <spam_eater@gmx.net> writes:
> Interesting enough, the same error message appears when I run "create extension xml2;" on Postgres 10.
Sure, that no doubt is exactly the command getting issued during the
pg_upgrade attempt.
> It somehow looks as if there is a DLL dependency between XML2 and whatever uses pgxml.dll (the new xmltable perhaps?)
pgxml.dll *is* the dll name for contrib/xml2.
I'm suspicious that what you're seeing is some sort of binary-level
incompatibility between BigSQL's build of contrib/xml2 and whatever
version of libxml2 you have installed. Hard to guess more than that.
If you're not actually using contrib/xml2 in the 9.6 installation,
probably dropping that extension would let you do the upgrade.
regards, tom lane