When i tried the Api_DoQuery in a table with more than 50 fields i am getting the entire fields.
https://[target_domain]/db/?a=API_DoQuery&qid=[reportId]&appToken=[appToken]
target_domain : aaa.com
tableId : Go to the corresponding table that you want to fetch , on The URI the value after db
reportId: Go to any reports that have all the fields in your case , you will see a qid=some value in URI copy that value
appToken: In Application Go to home -> settings -> app properties -> under Advanced Settings -> Security options -> require application token check it -> go to manage application token and create a new token
Please remove the square brackets
After making the URI proper with values just paste that URI in a tab. You will get the response as XML and you can see all the fields
I Tried it. It's working fine for me
I hope this helps for you.