As other said, you need a spatial indexing structure, the good news is that nowadays, a lot of database vendors (SQL & noSQL) support spatial indexing , whether in memory or on disk.
You could probably use one of these out of the box implementations, and perform spatial queries on them to get the list of players to be notified when you move an object. The main drawback being that what you really want is not "querying" but rather be "notified" when you move objects around. Something that Elasticsearch does through its percolation feature... But I really don't know if the kind of load and latencies you expect are compatible with this tool. I didn't experiment the thing yet, sadly.
At the end, I think the best solution for you depends on these factors:
- You require really low latencies and a lot of queries, but persistence and dataset size is not the core concern: Build your own grid memory based system.
- Your main concern is the dataset size and you don't need to put everything in the process memory: an existing spatial database could possibly do the trick, with a little experimentation.
Edited by JMDeruty, 02 May 2014 - 09:24 AM.