Unfortunately, every version of glibc thus far tested has bugs whereby
strcoll() ordering does not match strxfrm() ordering as required by
the standard.  This can result in, for example, corrupted indexes.
Disabling abbreviated keys in these cases slows down non-C-collation
string sorting considerably, but there seems to be no practical
alternative.  Users who are confident that their libc implementations
are solid in this regard can re-enable the optimization by compiling
with TRUST_STRXFRM.

Users who have built indexes using PostgreSQL 9.5 or PostgreSQL 9.5.1
should REINDEX if there is a possibility that they may have been
affected by this problem.

Report by Marc-Olaf Jaschke.  Investigation mostly by Tom Lane, with
help from Peter Geoghegan, Noah Misch, Stephen Frost, and me.  Patch
by me, reviewed by Peter Geoghegan and Tom Lane.