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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, do not 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 does not guarantee the absence of duplicates.

Note

A detailed guide on ReplacingMergeTree, including best practices and how to optimize performance, is available here.

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 [, is_deleted]])
[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

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

ReplacingMergeTree Parameters​

ver​

ver — column with the version number. Type UInt*, Date, DateTime or DateTime64. 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. If ver is the same for several rows, then it will use "if ver is not specified" rule for them, i.e. the most recent inserted row will remain.

Example:

-- without ver - the last inserted 'wins'
CREATE TABLE myFirstReplacingMT
(
    `key` Int64,
    `someCol` String,
    `eventTime` DateTime
)
ENGINE = ReplacingMergeTree
ORDER BY key;

INSERT INTO myFirstReplacingMT Values (1, 'first', '2020-01-01 01:01:01');
INSERT INTO myFirstReplacingMT Values (1, 'second', '2020-01-01 00:00:00');

SELECT * FROM myFirstReplacingMT FINAL;

┌─key─┬─someCol─┬───────────eventTime─┐
│   1 │ second  │ 2020-01-01 00:00:00 │
└─────┴─────────┴─────────────────────┘


-- with ver - the row with the biggest ver 'wins'
CREATE TABLE mySecondReplacingMT
(
    `key` Int64,
    `someCol` String,
    `eventTime` DateTime
)
ENGINE = ReplacingMergeTree(eventTime)
ORDER BY key;

INSERT INTO mySecondReplacingMT Values (1, 'first', '2020-01-01 01:01:01');
INSERT INTO mySecondReplacingMT Values (1, 'second', '2020-01-01 00:00:00');

SELECT * FROM mySecondReplacingMT FINAL;

┌─key─┬─someCol─┬───────────eventTime─┐
│   1 │ first   │ 2020-01-01 01:01:01 │
└─────┴─────────┴─────────────────────┘

is_deleted​

is_deleted — Name of a column used during a merge to determine whether the data in this row represents the state or is to be deleted; 1 is a "deleted" row, 0 is a "state" row.

Column data type — UInt8.

Note

is_deleted can only be enabled when ver is used.

The row is deleted only when OPTIMIZE ... FINAL CLEANUP. This CLEANUP special keyword is not allowed by default unless allow_experimental_replacing_merge_with_cleanup MergeTree setting is enabled.

No matter the operation on the data, the version must be increased. If two inserted rows have the same version number, the last inserted row is the one kept.

Example:

-- with ver and is_deleted
CREATE OR REPLACE TABLE myThirdReplacingMT
(
    `key` Int64,
    `someCol` String,
    `eventTime` DateTime,
    `is_deleted` UInt8
)
ENGINE = ReplacingMergeTree(eventTime, is_deleted)
ORDER BY key
SETTINGS allow_experimental_replacing_merge_with_cleanup = 1;

INSERT INTO myThirdReplacingMT Values (1, 'first', '2020-01-01 01:01:01', 0);
INSERT INTO myThirdReplacingMT Values (1, 'first', '2020-01-01 01:01:01', 1);

select * from myThirdReplacingMT final;

0 rows in set. Elapsed: 0.003 sec.

-- delete rows with is_deleted
OPTIMIZE TABLE myThirdReplacingMT FINAL CLEANUP;

INSERT INTO myThirdReplacingMT Values (1, 'first', '2020-01-01 00:00:00', 0);

select * from myThirdReplacingMT final;

┌─key─┬─someCol─┬───────────eventTime─┬─is_deleted─┐
│   1 │ first   │ 2020-01-01 00:00:00 │          0 │
└─────┴─────────┴─────────────────────┴────────────┘

Query clauses​

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

Deprecated Method for Creating a Table
Note

Do not use this method in new projects and, if possible, switch 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.

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 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:

CREATE TABLE rmt_example
(
    `number` UInt16
)
ENGINE = ReplacingMergeTree
ORDER BY number

INSERT INTO rmt_example SELECT floor(randUniform(0, 100)) AS number
FROM numbers(1000000000)

0 rows in set. Elapsed: 19.958 sec. Processed 1.00 billion rows, 8.00 GB (50.11 million rows/s., 400.84 MB/s.)

Querying without FINAL produces an incorrect count (exact result will vary depending on merges):

SELECT count()
FROM rmt_example

┌─count()─┐
│     200 │
└─────────┘

1 row in set. Elapsed: 0.002 sec.

Adding final produces a correct result:

SELECT count()
FROM rmt_example
FINAL

┌─count()─┐
│     100 │
└─────────┘

1 row in set. Elapsed: 0.002 sec.

For further details on FINAL, including how to optimize FINAL performance, we recommend reading our detailed guide on ReplacingMergeTree.