Citus Blog

HyperLogLog is an awesome approximation algorithm that addresses the distinct count problem. I am a big fan of HyperLogLog (HLL), so much so that I already wrote about the internals and how HLL solves the distributed distinct count problem. But there’s more to talk about, including HLL with rollup tables.

Rollup Tables and Postgres

Rollup tables are commonly used in Postgres when you don’t need to perform detailed analysis, but you still need to answer basic aggregation queries on older data.

With rollup tables, you can pre-aggregate your older data for the queries you still need to answer. Then you no longer need to store all of the older data, rather, you can delete the older data or roll it off to slower storage—saving space and computing power.

Let’s walk through a rollup table example in Postgres without using HLL.

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