Merge pull request #2770 from CSCfi/DBAAS-484

Add warning about reindexing on certain pgsql upgrades

Author: lvarin

docs/cloud/dbaas/operations.md

@@ -33,6 +33,14 @@ The commands to use:
 
 5. Verify with your preferred tool that your database is working as expected.
 
+!!! warning "Certain PostgreSQL upgrades will cause databases to be reindexed"
+    The libraries used by PostgreSQL internally for collation (sorting, comparing, and ordering data) might change between datastore versions.
+    When this happens, a full reindex of all databases is required to prevent issues with data consistency.
+    This reindexing can take a considerable amount of time, especially with large databases containing complex indexes.
+    Currently upgrading from 17.5 or earlier to 17.6 or newer triggers the reindexing. Upgrading between
+    minor versions of PostgreSQL 14 also triggers the reindexing, as does upgrading from major version 14 to 17.
+    Please plan your database upgrades accordingly.
+
 ## Major database upgrades
 
 Major version upgrades are no different from the user's point of view, but there's a bit more happening in the background, which creates more possible points of failure.