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pg_upgrade checks whether all the shared libraries used in the old cluster are also available in the new one by issuing LOAD for each library name. Previously, it cared not what order it did the LOADs in. Ideally it should not have to care, but currently the transform modules in contrib fail unless both the language and datatype modules they depend on are loaded first. A backend-side solution for that looks possible but probably not back-patchable, so as a stopgap measure, let's do the LOAD tests in order by library name length. That should fix the problem for reasonably-named transform modules, eg "hstore_plpython" will be loaded after both "hstore" and "plpython". (Yeah, it's a hack.) In a larger sense, having a predictable order of these probes is a good thing, since it will make upgrades predictably work or not work in the face of inter-library dependencies. Also, this patch replaces O(N^2) de-duplication logic with O(N log N) logic, which could matter in installations with very many databases. So I don't foresee reverting this even after we have a proper fix for the library-dependency problem. In passing, improve a couple of SQL queries used here. Per complaint from Andrew Dunstan that pg_upgrade'ing the transform contrib modules failed. Back-patch to 9.5 where transform modules were introduced. Discussion: <f7ac29f3-515c-2a44-21c5-ec925053265f@dunslane.net>
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