ReplacingMergeTree deduplicates only at merge time, so duplicates stay visible until then

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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ReplacingMergeTree removes rows sharing a sorting key during background merges, and merges run on an unspecified schedule and only within one partition. A SELECT immediately after inserting a duplicate returns both rows. This is by design: the engine provides eventual deduplication, not a unique constraint, and it is the most commonly misread behaviour in ClickHouse.

For correct reads, either add FINAL to the query, which merges on the fly and costs a great deal on wide tables, or aggregate explicitly using argMax over a version column grouped by the key, which is usually faster. OPTIMIZE TABLE with FINAL forces a merge but rewrites whole partitions, is unsuitable for regular use, and offers no guarantee once new inserts arrive. Deduplication never crosses partition boundaries, so the partition key must not separate rows that should collapse together.

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

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