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Merge pull request #15679 from adevyatova/annadevyatova-DOCSUP-2861-codec

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DOCSUP-2861: Add ability to specify Default codec
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alexey-milovidov 2020-10-10 20:37:39 +03:00 committed by GitHub
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docs/en/sql-reference/statements/create/table.md
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@ -121,7 +121,9 @@ Defines storage time for values. Can be specified only for MergeTree-family tabl
## Column Compression Codecs {#codecs}
By default, ClickHouse applies the `lz4` compression method. For `MergeTree`-engine family you can change the default compression method in the [compression](../../../operations/server-configuration-parameters/settings.md#server-settings-compression) section of a server configuration. You can also define the compression method for each individual column in the `CREATE TABLE` query.
By default, ClickHouse applies the `lz4` compression method. For `MergeTree`-engine family you can change the default compression method in the [compression](../../../operations/server-configuration-parameters/settings.md#server-settings-compression) section of a server configuration.
You can also define the compression method for each individual column in the `CREATE TABLE` query.
``` sql
CREATE TABLE codec_example
@ -136,7 +138,18 @@ ENGINE = <Engine>
...
```
If a codec is specified, the default codec doesn’t apply. Codecs can be combined in a pipeline, for example, `CODEC(Delta, ZSTD)`. To select the best codec combination for you project, pass benchmarks similar to described in the Altinity [New Encodings to Improve ClickHouse Efficiency](https://www.altinity.com/blog/2019/7/new-encodings-to-improve-clickhouse) article. One thing to note is that codec can't be applied for ALIAS column type.
The `Default` codec can be specified to reference default compression which may dependend on different settings (and properties of data) in runtime.
Example: `value UInt64 CODEC(Default)` - the same as lack of codec specification.
Also you can remove current CODEC from the column and use default compression from config.xml:
``` sql
ALTER TABLE codec_example MODIFY COLUMN float_value CODEC(Default);
```
Codecs can be combined in a pipeline, for example, `CODEC(Delta, Default)`.
To select the best codec combination for you project, pass benchmarks similar to described in the Altinity [New Encodings to Improve ClickHouse Efficiency](https://www.altinity.com/blog/2019/7/new-encodings-to-improve-clickhouse) article. One thing to note is that codec can't be applied for ALIAS column type.
!!! warning "Warning"
You can’t decompress ClickHouse database files with external utilities like `lz4`. Instead, use the special [clickhouse-compressor](https://github.com/ClickHouse/ClickHouse/tree/master/programs/compressor) utility.