Primary keys are rightly the first tool users turn to when needing to accelerate queries. However, tables are limited to a single primary key, and query access patterns can render this ineffective i.e., for diverse use cases, queries which cannot exploit the primary key efficiently are inevitable. In these cases ClickHouse can be forced to perform a full table scan of each column when applying a WHERE clause condition. Often this will still be sufficiently fast, but in some cases users reach for data skipping indices, hoping to accelerate these queries easily.
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