# Use ZSTD compression levels
<a name="compression-support-zstd-levels"></a>

The [Zstandard real-time data compression algorithm](http://facebook.github.io/zstd/) is a fast compression algorithm that provides high compression ratios. The Zstandard (ZSTD) library is open source software and uses a BSD license. Athena supports reading and writing ZSTD compressed ORC, Parquet, and text file data.

You can use ZSTD compression levels to adjust the compression ratio and speed according to your requirements. The ZSTD library supports compression levels from 1 to 22. Athena uses ZSTD compression level 3 by default.

Compression levels provide granular trade-offs between compression speed and the amount of compression achieved. Lower compression levels provide faster speed but larger file sizes. For example, you can use level 1 if speed is most important and level 22 if size is most important. Level 3 is suitable for many use cases and is the default. Use levels greater than 19 with caution as they require more memory. The ZSTD library also offers negative compression levels that extend the range of compression speed and ratios. For more information, see the [Zstandard Compression RFC](https://datatracker.ietf.org/doc/html/rfc8478).

The abundance of compression levels offers substantial opportunities for fine tuning. However, make sure that you measure your data and consider the tradeoffs when deciding on a compression level. We recommend using the default level of 3 or a level in the range from 6 to 9 for a reasonable tradeoff between compression speed and compressed data size. Reserve levels 20 and greater for cases where size is most important and compression speed is not a concern.

## Considerations and limitations
<a name="compression-support-zstd-levels-considerations-and-limitations"></a>

When using ZSTD compression level in Athena, consider the following points.
+ The ZSTD `compression_level` property is supported only in Athena engine version 3.
+ The ZSTD `compression_level` property is supported for the `ALTER TABLE`, `CREATE TABLE`, `CREATE TABLE AS` (CTAS), and `UNLOAD` statements.
+ The `compression_level` property is optional.
+ The `compression_level` property is supported only for ZSTD compression.
+ Possible compression levels are 1 through 22.
+ The default compression level is 3.

For information about Apache Hive ZSTD compression support in Athena, see [Use Hive table compression](compression-support-hive.md). For information about Apache Iceberg ZSTD compression support in Athena, see [Use Iceberg table compression](compression-support-iceberg.md).

## Specify ZSTD compression levels
<a name="compression-support-zstd-levels-specifying"></a>

To specify the ZSTD compression level for the `ALTER TABLE`, `CREATE TABLE`, `CREATE TABLE AS`, and `UNLOAD` statements, use the `compression_level` property. To specify ZSTD compression itself, you must use the individual compression property that the syntax for the statement uses.

### ALTER TABLE SET TBLPROPERTIES
<a name="compression-support-zstd-levels-alter-table"></a>

In the [ALTER TABLE SET TBLPROPERTIES](alter-table-set-tblproperties.md) statement `SET TBLPROPERTIES` clause, specify ZSTD compression using `'write.compression' = ' ZSTD'` or `'parquet.compression' = 'ZSTD'`. Then use the `compression_level` property to specify a value from 1 to 22 (for example, '`compression_level' = '5'`). If you do not specify a compression level property, the compression level defaults to 3.

#### Example
<a name="compression-support-zstd-levels-alter-table-example"></a>

The following example modifies the table `existing_table` to use Parquet file format with ZSTD compression and ZSTD compression level 4. Note that in the `TBLPROPERTIES` clause the compression level value must be entered as a string rather an integer and therefore must be enclosed in either single or double quotes.

```
ALTER TABLE existing_table 
SET TBLPROPERTIES ('parquet.compression' = 'ZSTD', 'compression_level' = '4')
```

### CREATE TABLE
<a name="compression-support-zstd-levels-create-table"></a>

In the [CREATE TABLE](create-table.md) statement `TBLPROPERTIES` clause, specify '`write.compression' = 'ZSTD'` or `'parquet.compression' = 'ZSTD'`, and then use `compression_level = {{compression_level}}` and specify a value from 1 to 22 as a string. If the `compression_level` property is not specified, the default compression level is 3.

#### Example
<a name="compression-support-zstd-levels-create-table-example"></a>

The following example creates a table in Parquet file format using ZSTD compression and ZSTD compression level 4. 

```
CREATE EXTERNAL TABLE new_table ( 
  `col0` string COMMENT '', 
  `col1` string COMMENT '' 
) 
STORED AS PARQUET 
LOCATION 's3://amzn-s3-demo-bucket/' 
TBLPROPERTIES ('write.compression' = 'ZSTD', 'compression_level' = '4')
```

### CREATE TABLE AS (CTAS)
<a name="compression-support-zstd-levels-ctas"></a>

In the [CREATE TABLE AS](create-table-as.md) statement `WITH` clause, specify `write_compression = 'ZSTD'`, or `parquet_compression = 'ZSTD'`, and then use `compression_level = {{compression_level}}` and specify a value from 1 to 22 as an integer. If the `compression_level` property is not specified, the default compression level is 3.

#### Example
<a name="compression-support-zstd-levels-ctas-example"></a>

The following CTAS example specifies Parquet as the file format using ZSTD compression with compression level 4. Note that, in the `WITH` clause, the value for compression level must be specified as an integer, not as a string.

```
CREATE TABLE new_table  
WITH ( format = 'PARQUET', write_compression = 'ZSTD', compression_level = 4)  
AS SELECT * FROM old_table
```

### UNLOAD
<a name="compression-support-zstd-levels-unload"></a>

In the [UNLOAD](unload.md) statement `WITH` clause, specify `compression = 'ZSTD'`, and then use `compression_level = {{compression_level}}` and specify a value from 1 to 22 as an integer. If the `compression_level` property is not specified, the default compression level is 3.

#### Example
<a name="compression-support-zstd-levels-unload-example"></a>

The following example unloads the query results to the specified location using the Parquet file format, ZSTD compression, and ZSTD compression level 4.

```
UNLOAD (SELECT * FROM old_table) 
TO 's3://amzn-s3-demo-bucket/' 
WITH (format = 'PARQUET', compression = 'ZSTD', compression_level = 4)
```