Every database I've ever seen has a method for retrieving the count of the query prior to actually executing it. But I can't figure how to do this simple task in Accumulo.
Just for clarity, I want the Accumulo analog of this Mongo feature.
I checked the Scanner apidocs but I can't find anything. I'm using Java but answers for other languages would be greatly helpful too.
Accumulo is a lower-level application than a traditional RDBMS. It is based on Google's Big Table and not like a relational database. It's more accurately described as a massive parallel sorted map than a database.
It is designed to do different kinds of tasks than a relational database, and its focus is on big data.
To achieve the equivalent of the MongoDB feature you mentioned in Accumulo (to get a count of the size of an arbitrary query's result set), you can write a server-side Iterator which returns counts from each server, which can be summed on the client side to get a total. If you can anticipate your queries, you can also create an index which keeps track of counts during the ingest of your data.
Creating custom Iterators is an advanced activity. Typically, there are important trade-offs (time/space/consistency/convenience) to implementing something as seemingly simple as a count of a result set, so proceed with caution. I would recommend consulting the user mailing list for information and advice.