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ClickHouse/docs/en/operations/table_engines/replacingmergetree.md
BayoNet b5edce0e32 Updates for Aggregating-,Collapsing-, Replacing- and SummingMergeTree. (#3346) 
* Update of english version of descriprion of the table function `file`.

* New syntax for ReplacingMergeTree.
Some improvements in text.

* Significantly change article about SummingMergeTree.
Article is restructured, text is changed in many places of the document. New syntax for table creation is described.

* Descriptions of AggregateFunction and AggregatingMergeTree are updated. Russian version.

* New syntax for new syntax of CREATE TABLE

* Added english docs on Aggregating, Replacing and SummingMergeTree.

* CollapsingMergeTree docs. English version.

* 1. Update of CollapsingMergeTree. 2. Minor changes in markup

* Update aggregatefunction.md

* Update aggregatefunction.md

* Update aggregatefunction.md

* Update aggregatingmergetree.md

* GraphiteMergeTree docs update.
New syntax for creation of Replicated* tables.
Minor changes in *MergeTree tables creation syntax.

* Markup fix

* Markup and language fixes

* Clarification in the CollapsingMergeTree article
2018-10-19 14:25:22 +03:00

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ReplacingMergeTree

The engine differs from MergeTree in that it removes duplicate entries with the same primary key value.

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]
[SAMPLE BY expr]
[SETTINGS name=value, ...]

For a description of request parameters, see request description.

ReplacingMergeTree Parameters

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

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

    • Last in the selection, if ver not set.
    • 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 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