ReplacingMergeTree¶

The engine differs from MergeTree in that it removes duplicate entries with the same primary key value (or more accurately, with the same sorting 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.

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])

Original article