What is pre-aggregation in databases?

Pre-aggregation in databases is the process of precomputing and storing aggregated data (such as sums, averages, maximums) to speed up subsequent queries.

For example, if you have a table with millions of transactions and often need to get the total sales per day, you can pre-calculate and store these sums in a separate table. Then, queries to the aggregated data will be much faster because there is no need to recalculate the data on the fly each time.

Pre-aggregation is useful in analytics and OLAP systems, where the speed of responses to complex queries is important. However, it requires additional storage space and mechanisms to update the aggregates when the source data changes.