What is t.index used for and do the details of the functional index matter?

In schema.rb there indices are shown. During an upgrade a functional index has been reduced to an index on a column. Since I am not sure how they are used I wonder if this can cause problems later on.

I assume this could make problems in a re-initialized database for example when testing. Is there another use case? This Upgrade was from Rails 4 to Rails 6. Did something change here?

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Just to clarify. It was changed in the schema.rb, so it wouldn’t match the actual DB schema anymore. Not sure why. In this case it seems to mostly be a performance issue.

To be clear when I say functional index this is what I mean:

(also looking at Jack’s profile and the link in his post I think that’s a spammer)