When a PostgreSQL database is created, the administrator can choose the encoding, which should be used to store the data. Basically, the configuration decides which characters exist and in which order they are displayed. Here is an example—de_AT@UTF-8. In this case, we will use Unicode characters, which will be displayed in an Austrian sort order (Austrians speak some sort of German). So, de_AT will define the order in which the data will be sorted.
To achieve this kind of sorting, PostgreSQL relies heavily on the operating system. The trouble is that if the sort order of characters changes in the operating system for some reason (maybe because of a bug or because of some other reason), PostgreSQL will have troubles with its indexes. A normal b-tree index is basically a sorted list, and if the sort order changes, naturally, there is a problem.
The introduction of the ICU library is supposed to fix this problem. ICU offers stronger promises than the operating system and is, therefore, more suitable for long-term storage of data. With the introduction of PostgreSQL 10.0, ICU encodings can be enabled.