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Dale Mcdiarmid 2024-10-25 17:52:30 +01:00
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docs/en/engines/table-engines/mergetree-family/replacingmergetree.md
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@ -165,7 +165,7 @@ All of the parameters excepting `ver` have the same meaning as in `MergeTree`.
## Query time de-duplication & FINAL
At merge time, the ReplacingMergeTree identifies duplicate rows, using the values of the `ORDER BY` columns (used to create the table) as a unique identifier, and either retains only the highest version or removes all duplicates if the latest version indicates a delete. This, however, offers eventual correctness only - it does not guarantee rows will be deduplicated, and you should not rely on it. Queries can, therefore, produce incorrect answers due to update and delete rows being considered in queries.
At merge time, the ReplacingMergeTree identifies duplicate rows, using the values of the `ORDER BY` columns (used to create the table) as a unique identifier, and retains only the highest version. This, however, offers eventual correctness only - it does not guarantee rows will be deduplicated, and you should not rely on it. Queries can, therefore, produce incorrect answers due to update and delete rows being considered in queries.
To obtain correct answers, users will need to complement background merges with query time deduplication and deletion removal. This can be achieved using the `FINAL` operator. For example, consider the following example: