In a blog post [1], Bruce Momjian notes that expression indexes can help with planning even if they're not used directly. But the examples cited in that post are vague (i.e., they improve stats, but it's not clear how they could change plans), and Bruce's answer to a comment [2] suggests that this is not documented.
Is there any more info on this mechanism? Specifically, if one has unused expression indexes (according to pg_stat_user_indexes), is it safe to drop them? Or could they be providing statistics that materially affect query planning even though the indexes themselves are unused? It looks like a very similar question was asked earlier this year on pgsql-general [3], but got only a vague answer. Any background or tips for where to look in the source regarding this behavior would be greatly appreciated. Thanks, Maciek [1]: https://momjian.us/main/blogs/pgblog/2017.html#February_20_2017 [2]: https://momjian.us/main/comment_item.html?/main/blogs/pgblog.html/February_20_2017#comment-3174376969 [3]: https://www.postgresql.org/message-id/flat/CAHg%3DPn%3DOZu7A3p%2B0Z-CDG4s2CHYe3UFQCTZp4RWGCEn2gmD35A%40mail.gmail.com
