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Add desc for Default codec

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Anna 2020-10-06 16:16:28 +03:00
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commit 064be8db54
1 changed files with 13 additions and 2 deletions
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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 +138,16 @@ 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.
If the `Default` codec is specified explicitly, the default compression replaced with current default codec which may dependend on different settings (and properties of data) in runtime. In this case, the default compression is set in the CODEC definition:
`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.