title: what exactly is a map dimension in a multi-dimensional map?
body: <p>Like many these days, I am an old relational-model user approaching Cassandra for the first time.  I have been trying to understand Cassandra's data model, and when I read about it I frequently encounter statements that encourage me to think about it as 4 and 5 dimensional maps.</p>
  
  <p>Now I'm familiar with an ordinary key/value Map, but I have never thought of how many dimensions it has, and that gives me no basis to plunge headlong into trying to visualize 4 and 5 dimensions.</p>
  
  <p>Is there a more gentle introduction to dimensionality in maps?   How many dimensions are there in an ordinary hashtable?  One?  Two?  Zero?</p>
  
  <p>If an ordinary hashtable has, say, just one dimension, then what would a two-dimensional map be?  If two, then what would a 3-dimensional map be?</p>