Altinity Blog

A Magical Mystery Tour of the LowCardinality Data Type

Mar 27, 2019

Many ClickHouse features like LowCardinality data type seem mysterious to new users.  ClickHouse often deviates from standard SQL and many data types and operations do not even exist in other data warehouses. The key to understanding is that the ClickHouse engineering team values speed more than almost any other property. Mysterious SQL expressions often turn out to be 'secret weapons' to achieve unmatched speed.

In fact, the LowCardinality data type is an example of just such a feature. It has been available since Q4 2018 and was marked as production ready in Feb 2019, but still is not documented, magically appearing in some documentation examples. In this article we will fill the gap  by explaining how LowCardinality works, and when it should be used.

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ClickHouse Networking, Part 2

This post in two parts provides an overview of ClickHouse network configuration with lots of examples. In the first post we describe the overall connectivity design and configuration of listeners and ports. In the second post we describe how to enable encryption, solutions to common problems, and further reading.

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ClickHouse Networking, Part 1

This post in two parts provides an overview of ClickHouse network configuration with lots of examples. In the first post we describe the overall connectivity design and configuration of listeners and ports. In the second post we describe how to enable encryption, solutions to common problems, and further reading.

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Do-It-Yourself Multi-Volume Storage in ClickHouse



Many applications have very different requirements for acceptable latencies / processing speed on different parts of the database. In time-series use cases most of your requests touch only the last day of data (‘hot’ data). Those queries should run very fast. Also a lot of background processing actions happen on the ‘hot’ data--inserts, merges, replications, and so on. Such operations should likewise be processed with the highest possible speed and without significant latencies.

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