默认情况下Postgresql中字符比较是严格区分大小写的，然而在最新的Postgres 12中支持通过CREATE COLLATION创建非确定性排序规则以支持忽略大小写比较。
创建忽略大小写的排序的Collation如下：
官方说法：CREATE COLLATION case_insensitive (provider = icu, locale = 'und-u-ks-level2', deterministic = false);
然而在Windows版本下尝试官方提供的该方法并不能忽略大小写，据网上的说法，Windows版本附带的ICU版本是一个很旧的版本，也许这就是原因。因而在Windows版本中创建忽略大小写的排序规则应该如下：
CREATE COLLATION case_insensitive ( provider = 'icu', locale = '@colStrength=secondary', deterministic = false );select ('AA' COLLATE "case_insensitive")=('aa' COLLATE "case_insensitive"); --结果为true;
所有标准和预定义的排序规则都是确定性的，所有用户定义的排序规则在默认情况下都是确定性的。虽然不确定性排序提供了更正确的行为，特别是考虑到Unicode的强大功能及其许多特殊情况时，但它们也有一些缺点。最重要的是，它们的使用会导致性能损失。另外，对于非确定性排序，某些操作是不可能的，比如模式匹配操作。因此，它们只能在特别需要的情况下使用。
All standard and predefined collations are deterministic, all user-defined collations are deterministic by default. While nondeterministic collations give a more“correct”behavior, especially when considering the full power of Unicode and its many special cases, they also have some drawbacks. Foremost, their use leads to a performance penalty. Also, certain operations are not possible with nondeterministic collations, such as pattern matching operations. Therefore, they should be used only in cases where they are specifically wanted.
参考https://mlog.club/article/4910683
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