If I have "userID" (primary), "forum topic" (secondary-a), "subtopic" inside "topic" (secondary-b). I need to read/write/search subtopic text.
Can I query Primary (hash+range) + Secondary-A + Secondary-B key in single query to get specific results? Or I query Primary OR Secondary-a OR Secondary-b, so only 1 index choice?
If i understand your question correctly!!! you need to add the Indexes equals to secondary attribute you are creating. Eg: if you have attribute "Secondary-A" and you should create index for it like "Secondary-A-Index". note: your only able to query through the index what you created.