Seehttps://lists.debian.org/debian-glibc/2019/03/msg00030.html,https://bugs.debian.org/926627, andhttps://salsa.debian.org/ddp-team/release-notes/commit/ad75c4b for some context.

When upgrading from stretch to buster, the glibc locale data is upgraded.
Specifically, this changes how PostgreSQL sorts data in text indexes.
To avoid corruption, such indexes need to beREINDEXed
immediately after upgrading thelocales or
locales-all packages, before putting back the database
into production.

Suggested command:sudo -u postgres reindexdb --all

I'm not entirely sure how we're going to handle this, but it does mean it won't be reasonable for us to simply upgrade the images from Stretch to Buster once Buster is released. 😕 😞