Let’s imagine that we have an electronic payment system with the database table operations. For example, we want to calculate the size of average operation. It is easy, here's a query:

SELECT avg(amount) FROM transfer;
65.125965782378
generated in 3850 seconds

Now let’s imagine that the index should be the latest one, the entries are made every second in the table, and after a month there are millions of them. To aggregate the same data each time is very expensive. Traditional databases do not offer such optimizations. What to do?
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xially 26 march 2012, 15:00