thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-11-25 00:52:02 +00:00
Code Issues Packages Projects Releases Wiki Activity

DOCS-85: Update docs for SELECT ... FROM ... FINAL (#7723)

Browse Source 
* Doc links fix.

* More links fix.

* CLICKHOUSEDOCS-85: Update of FROM clause description.

* CLICKHOUSEDOCS-85: Clarification

* Update docs/en/query_language/select.md

Co-Authored-By: Ivan Blinkov <github@blinkov.ru>

* Update docs/en/query_language/select.md

Co-Authored-By: Ivan Blinkov <github@blinkov.ru>

* CLICKHOUSEDOCS-85: Update by comment.
This commit is contained in:
BayoNet 2019-12-06 14:58:31 +03:00 committed by GitHub
parent 5a45c7561d
commit aeaac4d573
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 14 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
docs/en/query_language/select.md
View File

@ -112,8 +112,20 @@ In contrast to standard SQL, a synonym does not need to be specified after a sub
To execute a query, all the columns listed in the query are extracted from the appropriate table. Any columns not needed for the external query are thrown out of the subqueries.
If a query does not list any columns (for example, `SELECT count() FROM t`), some column is extracted from the table anyway (the smallest one is preferred), in order to calculate the number of rows.
The `FINAL` modifier can be used in the `SELECT` select query for engines from the [MergeTree](../operations/table_engines/mergetree.md) family. When you specify `FINAL`, data is selected fully "merged". Keep in mind that using `FINAL` leads to reading columns related to the primary key, in addition to the columns specified in the query. Additionally, the query will be executed in a single thread, and data will be merged during query execution. This means that when using `FINAL`, the query is processed slowly. In the most cases, avoid using `FINAL`.
The `FINAL` modifier can be applied for all engines of MergeTree family that do data transformations in background merges (except GraphiteMergeTree).
#### FINAL Modifier {#select-from-final}
Appliable when selecting data from tables of the [MergeTree](../operations/table_engines/mergetree.md)-engine family, except `GraphiteMergeTree`. When `FINAL` is specified, ClickHouse fully merges data before returning the result and thus performs all data transformations that are supposed to happen during merges for given table engine.
Also supported for:
- [Replicated](../operations/table_engines/replication.md) versions of `MergeTree` engines.
- [View](../operations/table_engines/view.md), [Buffer](../operations/table_engines/buffer.md), [Distributed](../operations/table_engines/distributed.md), [MaterializedView](../operations/table_engines/materializedview.md) engines that operate over other engines, if they created over `MergeTree`-engine tables.
The queries that use `FINAL` are executed slower than similar queries that don't, because:
- Query is executed in a single thread, and data is merged during query execution.
- Queries with `FINAL` read primary key columns additionally to the columns specified in the query.
In the most cases, avoid using `FINAL`.
### SAMPLE Clause {#select-sample-clause}