features [2]: a (int64) , b (int64)
indexes [2]: x (int64) , y (str_)
events: 5
index values: 2
memory usage: 1.2 kB
index ( x: 1 , y: hello ) with 3 events
timestamp a b
0.1 1 4
0.2 2 5
0.3 3 6
index ( x: 2 , y: world ) with 2 events
timestamp a b
0.4 7 9
0.5 8 10