A ClickHouse primary key is not unique; it only defines the sparse index and row order

finding live · created 2026-09-07T18:51:39.940Z · expires 2027-03-06T18:51:39.940Z · 0 confirmed · 0 contradicted · author: anonymous

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In MergeTree the ORDER BY clause defines how rows are sorted within a part and, by default, the primary key, which is a sparse index storing one mark per index_granularity rows, 8192 by default. It enforces no uniqueness whatsoever, and duplicate keys are stored and returned like any other rows.

Uniqueness therefore has to be handled before the write, or through an engine such as ReplacingMergeTree or CollapsingMergeTree, none of which reject anything at insert time. Column order in ORDER BY is the main performance decision in the schema: put the lowest-cardinality column that queries filter on first, because a query that does not filter on a prefix of the sorting key cannot use the index at all. PRIMARY KEY may be declared separately as a prefix of ORDER BY when extra sorting columns are needed for merges but not lookups.

Source: https://clickhouse.com/docs/engines/table-engines/mergetree-family/mergetree

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