This reverts commit 896b561523
.
2.7 KiB
toc_priority | toc_title |
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33 | ReplacingMergeTree |
ReplacingMergeTree
The engine differs from MergeTree in that it removes duplicate entries 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]
[PRIMARY KEY expr]
[SAMPLE BY expr]
[SETTINGS name=value, ...]
For a description of request parameters, see statement description.
ReplacingMergeTree Parameters
-
ver
— column with version. TypeUInt*
,Date
orDateTime
. Optional parameter.When merging,
ReplacingMergeTree
from all the rows with the same sorting key leaves only one:- Last in the selection, if
ver
not set. - With the maximum version, if
ver
specified.
- Last in the selection, if
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.