ClickHouse/docs/en/engines/table-engines/mergetree-family/replacingmergetree.md

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33	ReplacingMergeTree

ReplacingMergeTree

The engine differs from MergeTree in that it removes duplicate entries with the same sorting key value (ORDER BY table section, not PRIMARY KEY).

Data deduplication occurs only during a merge. Merging occurs in the background at an unknown time, so you can’t plan for it. Some of the data may remain unprocessed. Although you can run an unscheduled merge using the OPTIMIZE query, don’t count on using it, because the OPTIMIZE query will read and write a large amount of data.

Thus, ReplacingMergeTree is suitable for clearing out duplicate data in the background in order to save space, but it doesn’t guarantee the absence of duplicates.

Creating a Table

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
) ENGINE = ReplacingMergeTree([ver])
[PARTITION BY expr]
[ORDER BY expr]
[PRIMARY KEY expr]
[SAMPLE BY expr]
[SETTINGS name=value, ...]

For a description of request parameters, see statement description.

!!! note "Attention" Uniqueness of rows is determined by the ORDER BY table section, not PRIMARY KEY.

ReplacingMergeTree Parameters

• ver — column with version. Type UInt*, Date or DateTime. Optional parameter.

  When merging, ReplacingMergeTree from all the rows with the same sorting key leaves only one:

  • The last in the selection, if ver not set. A selection is a set of rows in a set of parts participating in the merge. The most recently created part (the last insert) will be the last one in the selection. Thus, after deduplication, the very last row from the most recent insert will remain for each unique sorting key.
  • With the maximum version, if ver specified.

Query clauses

When creating a ReplacingMergeTree table the same clauses are required, as when creating a MergeTree table.

Deprecated Method for Creating a Table

!!! attention "Attention" Do not use this method in new projects and, if possible, switch the old projects to the method described above.

CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
(
    name1 [type1] [DEFAULT|MATERIALIZED|ALIAS expr1],
    name2 [type2] [DEFAULT|MATERIALIZED|ALIAS expr2],
    ...
) ENGINE [=] ReplacingMergeTree(date-column [, sampling_expression], (primary, key), index_granularity, [ver])

All of the parameters excepting ver have the same meaning as in MergeTree.

• ver - column with the version. Optional parameter. For a description, see the text above.

Original article