To avoid the index data being too large.
This was first seen in guix commit 1bb4fd64b7bbe5a17eda6f0ca8105283c038f7c8:
psql-query-error (fatal-error PGRES_FATAL_ERROR ERROR: index row size 2808
exceeds maximum 2712 for index "package_descriptions_locale_description_key"
HINT: Values larger than 1/3 of a buffer page cannot be indexed.
Christopher Baines