This question appeared after the following consideration: consider the immense world map of a MMOG like Travian or Tiberium wars. Even if they are 2D maps, they normally have a lot of different identities like player’s towns, terrain features and others.
How are those maps saved on a database? Is there an entry for every tile, with a foreign key to the identity which is at that position? Or is the other way around: each identity as two position attributes (x,y) which define its position?
In a more broad sense, my question is the following: what is the most efficient way to store a positional lattice (square tiles, hexagonal tiles, etc) on a mysql-like database? What is the bottleneck (if any) of making such a storage when querying the db for a position, or list of positions?
This is entirely dependent upon the scenario. For example, if the map is sparse, it might make sense to store the position of entities with coordinates in a single row. If the topology is irregular (i.e., there are arbitrarily many paths out of any given tile), then you might store it as a graph (i.e., separate “node” and “edge” tables, in a one-to-many configuration). You might decide to use continuous (e.g., real) coordinates for items in the database, and then simply partition those into logical “tiles” in the application.
Similarly, the bottlenecks are going to be application-dependent. Do you regularly need to ask “what is presently around me?”, or are you merely asking “Can I get from here to there?”. If you had particular use-cases in mind, perhaps you could add more detail and we could attempt to provide more detailed answers.
And as always in anything talking about performance, random speculation in the hypothetical case is never a substitute for prototyping and measuring.