2e1f6bc56d
* replace exit with assert in test_single_page * improve save_raw_single_page docs option * More grammar fixes * "Built from" link in new tab * fix mistype * Example of include in docs * add anchor to meeting form * Draft of translation helper * WIP on translation helper * Replace some fa docs content with machine translation * add normalize-en-markdown.sh * normalize some en markdown * normalize some en markdown * admonition support * normalize * normalize * normalize * support wide tables * normalize * normalize * normalize * normalize * normalize * normalize * normalize * normalize * normalize * normalize * normalize * normalize * normalize * lightly edited machine translation of introdpection.md * lightly edited machhine translation of lazy.md * WIP on translation utils * Normalize ru docs * Normalize other languages * some fixes * WIP on normalize/translate tools * add requirements.txt * [experimental] add es docs language as machine translated draft * remove duplicate script * Back to wider tab-stop (narrow renders not so well)
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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]
[PRIMARY KEY expr]
[SAMPLE BY expr]
[SETTINGS name=value, ...]
For a description of request parameters, see request description.
ReplacingMergeTree Parameters
-
ver— column with version. TypeUInt*,DateorDateTime. Optional parameter.When merging,
ReplacingMergeTreefrom all the rows with the same primary key leaves only one:- Last in the selection, if
vernot set. - With the maximum version, if
verspecified.
- 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.