How to query the ActivityFieldHistory big object

Hello,

We recently turned on field history tracking for Tasks and we're trying to use the captured data. To do so, I'm querying the related big object, ActivityFieldHistory. However, I'm not able for the life of me to figure out which fields in the table make up the composite key and in which order. I keep getting the error "filters may not have any gaps within the composite key". But there isn't any Salesforce documentation on what fields compose the composite key on this object! Any help is appreciated!

For reference, I'm trying to query based on the ChangedDate field.