Why ast must implement PartialOrd/Ord?

I'm working on adding `with` clause support for create index stmt https://www.postgresql.org/docs/current/sql-createindex.html. I plan to use `HashMap` to store `with` content, but failed to compile as `HashMap` doesn't implement PartialOrd/Ord. I'm curious why ast must implement PartialOrd/Ord?

Start by reading the AST definitions and the trait requirements that affect adding CREATE INDEX WITH content. Compare the intended HashMap representation with the existing AST ordering behavior and determine whether the feature needs a different representation or a documented rationale. Done means the design question is resolved and the CREATE INDEX WITH support can be implemented consistently.

Written by the indexing model from the issue text.