The Glibc library version 2.28 has been released on a majority of Linux systems. This new version introduced changes that affect the sort order of database queries. 

If your system contains Glibc 2.28, you must issue the REINDEX command on every PostgreSQL database residing on the updated host server. This procedure must be applied to the following:

• all Linux hosts
• hosts whose operating system uses the Linux kernel and have PostgreSQL installed on them

To learn more about the reindex of PostgreSQL databases and see examples, reference the following pages:

• Postgres 9.1
• Postgres 9.4
• Postgres 9.5

For more information about the Glibc library, see The GNU C Library (glibc).

For more information about this specific issue, see the "Beware of your next glibc upgrade" article.