B,
You haven’t included quite enough information to make a full diagnosis (like 
what command generates the error message) but just guessing, your admin used 
apt-get to update both postgres and postgis packages and then ran pg_upgrade on 
the cluster. Since the old postgis version is gone at this point (it’s been 
removed during the update) when the system catalog tries to make sense of 
postgis objects and references to old library, you get this error.
A hack to work around the issue is to find the location of postgis-2.5.so (the 
new library copy from the upgrade) and then create a symlink from the 
postgis-2.3.so to that file, so that the system will end up finding a library 
to load. The new and old libraries have mostly the same symbols, so this trick 
works fine for the purposes of upgrade.
You may run into yet further problems, but this is a quick way to try and move 
forward. Once you have the database up and running, do not forget to run ALTER 
'EXTENSION postgis UDPATE’ on all your postgis databases. This re-installs all 
the SQL functions, pointing to the new library file. Once that’s done you can 
remove the symlink.
ATB,
P

PS, here’s a link to a blog post about working through this same issue, but on 
a Centos system. So it’s not command-perfect, but provides general explanations 
of problems that an admin might use to navigate the Ubuntu specific issues. 
https://info.crunchydata.com/blog/upgrading-postgis-on-centos-7

> On Aug 13, 2019, at 7:56 AM, Birgit Laggner <birgit.lagg...@thuenen.de> wrote:
> 
> Dear list,
> 
> on our test server, we currently try to test how to upgrade our 
> PostgreSQL/PostGIS database. Sadly, we are stuck somewhere and would be very 
> glad if anybody could help.
> 
> Our test server is running on an Ubuntu 16.04 system in a virtual machine. We 
> are coming from PostgreSQL 9.5.17 with PostGIS 2.3.3 and want to upgrade to 
> PostgreSQL 11.2 and PostGIS 2.5.
> 
> Here the outputs of the version queries:
> SELECT version(); 
> "PostgreSQL 9.5.17 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 
> 5.4.0-6ubuntu1~16.04.10) 5.4.0 20160609, 64-bit"
> 
> SELECT postgis_full_version();
> "POSTGIS="2.3.3 r15473" GEOS="3.5.1-CAPI-1.9.1 r4246" SFCGAL="1.2.2" 
> PROJ="Rel. 4.9.2, 08 September 2015" GDAL="GDAL 1.11.3, released 2015/09/16" 
> LIBXML="2.9.3" LIBJSON="0.11.99" TOPOLOGY RASTER"
> 
> Our admin tried to upgrade PostGIS first and swears he did as told in the 
> PostGIS manual. But apparently, the upgrade was not fully successful, since 
> PostgreSQL still seems to look for PostGIS 2.3.
> 
> His error message while trying to upgrade PostgreSQL is:
> postgres@gis ERROR: could not access file "$libdir/postgis-2.3": No such file 
> or directory.
> 
> Has anybody any idea what we are doing wrong and what we should be doing 
> instead?
> 
> With hopeful regards,
> Birgit
> 
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users@lists.osgeo.org
> https://lists.osgeo.org/mailman/listinfo/postgis-users

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