You could have your static objects on a grid. That way you can just iterate through all the grid cells that are within 30m, and query the grid about which objects it contains. To implement it efficiently you could:

- Only worry about updating a player's static object list when he moves from one grid cell to the next.

- When a player moves from one grid cell to the next, only worry about the grid cells that are included now and not included before (so assuming the player isn't moving very fast, it should only be a single line of cells at a time (or a square of cells if we're talking 3D)

- If you have large sparse worlds, then instead of having some crazy 100000x100000 grid which is mostly empty cells, you could have a 1000x1000 grid in memory, and objects can put themselves in an appropriate grid slot by calculating iGridSlot % 1000, so an object at grid slot (64363, 25783) would be in grid cell (363, 783) in your structure.

Google spatial hashing.

Look into spatial datastructures, such as grid partioning, quadtrees, spatial hashing, etc... that let you answer the "what is the closest object to the player" question efficiently. Also consider that maybe the server could give more information to the client so that it can figure this out for itself, instead of the server doing absolutely everything.

Simplest is a grid (voxel in 3D) representation. It's easy to compute, easy to query, very fast. It take comparatively more memory. Spatial hashing is kind of the same principle, with better memory footprint.

There are other spatial partitioning structures. Physics engines use them all the time, on 1,000's of interacting and static objects (check out NextCarGame / BeamNG).

The other common strategy is using portals / PVS (Potentially Visible Set). You build a list of areas that are potentially of interest from another area.

the good news is that nowadays, a lot of database vendors (SQL & noSQL) support spatial indexing smile.png, whether in memory or on disk

You do not want to base a real-time spatial query on a SQL database implementation. You don't want to base *any* real-time information on direct queries to persistent databases.

This is why we have game servers, as separate from application servers or database servers. The game servers keep a game-specific, "reified," up-to-date in-RAM copy of the game objects and can enforce the rules and update the world state very cheaply. Things that really matter go back to the persistent store in an asynchronous manner. If the game server crashes, you may lose some amount of progress, but that's hopefully rare, and the orders of magnitude of efficiency gains are worth it. The kind of real-time gaming experience you can deliver from an in-RAM custom server compared to what you can deliver AT ALL from a persistent server (much less can deliver with economics that make sense) make it "not a choice."

How often do the objects change location or otherwise mutate? Most games pre-install a "level" that contains the object locations, and update this through "patching." If that's too static, then can you send all the objects within a kilometer when a user connects, and just update this when the user moves more than half a kilometer?