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Deduplicating at query time can be achieved by grouping the data on the fields, which uniquely identify a row, and using the argMax function with a date field to identify the last value for other fields. ReplacingMergeTree allows rows with the same sorting key (ORDER BY key) to be deduplicated on merges. Note this is "best effort" only: sometimes parts will not be merged with the merge process scheduled at non-deterministic intervals. It, therefore, does not guarantee the absence of duplicates. Users can also utilize the FINAL modifier to force this deduplication at SELECT time. While FINAL does add overhead, it has received significant performance improvements in recent versions including multi-threaded processing, and is suitable for many production workloads.
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Алевтина Запольская (редактор отдела «Бывший СССР»)
No argument re-reads the current buffer from the filesystem.，推荐阅读超级权重获取更多信息