thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-11-28 10:31:57 +00:00
Code Issues Packages Projects Releases Wiki Activity

Update replacingmergetree.md

Browse Source
This commit is contained in:
Denny Crane 2020-11-30 23:06:36 -04:00 committed by GitHub
parent cf96f990c3
commit 534cec15b2
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 5 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
docs/en/engines/table-engines/mergetree-family/replacingmergetree.md
View File

@ -5,7 +5,7 @@ toc_title: ReplacingMergeTree
# ReplacingMergeTree {#replacingmergetree} # ReplacingMergeTree {#replacingmergetree}
The engine differs from [MergeTree](../../../engines/table-engines/mergetree-family/mergetree.md#table_engines-mergetree) in that it removes duplicate entries with the same [sorting key](../../../engines/table-engines/mergetree-family/mergetree.md) value. The engine differs from [MergeTree](../../../engines/table-engines/mergetree-family/mergetree.md#table_engines-mergetree) in that it removes duplicate entries with the same [sorting key](../../../engines/table-engines/mergetree-family/mergetree.md) 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, don’t count on using it, because the `OPTIMIZE` query will read and write a large amount of data. 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.
@ -29,13 +29,16 @@ CREATE TABLE [IF NOT EXISTS] [db.]table_name [ON CLUSTER cluster]
For a description of request parameters, see [statement description](../../../sql-reference/statements/create/table.md). For a description of request parameters, see [statement description](../../../sql-reference/statements/create/table.md).
!!! note "Attention"
Uniqueness of rows is determined by the `ORDER BY` table section, not `PRIMARY KEY`.
**ReplacingMergeTree Parameters** **ReplacingMergeTree Parameters**
- `ver` — column with version. Type `UInt*`, `Date` or `DateTime`. Optional parameter. - `ver` — column with version. Type `UInt*`, `Date` or `DateTime`. Optional parameter.
When merging, `ReplacingMergeTree` from all the rows with the same sorting key leaves only one: When merging, `ReplacingMergeTree` from all the rows with the same sorting key leaves only one:
- Last in the selection, if `ver` not set. - 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. - With the maximum version, if `ver` specified.
**Query clauses** **Query clauses**