Indexes are created where you told them to. So, if you include an arbitrary field, it's probably not going to be indexed to start with. And yes, query is for indexed fields.
I guess, we can have another type of behavior where all new fields are indexed. It doesn't sound like a "clean way" to me, however some developers could desire that
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How are you going to manage querying ad-hoc fields? AFAIK every field within your ZODB-query needs to be indexed in ZODB first?
Indexes are created where you told them to. So, if you include an arbitrary field, it's probably not going to be indexed to start with. And yes, query is for indexed fields.
I guess, we can have another type of behavior where all new fields are indexed. It doesn't sound like a "clean way" to me, however some developers could desire that