Manually compress chunks

In most cases, an automated compression policy is sufficient to automatically compress your chunks. However, if you want more control over compression, you can also manually compress specific chunks.

Before you start, you need a list of chunks to compress. In this example, you use a hypertable called example, and compress chunks older than three days.

Selecting chunks to compress

At the psql prompt, select all chunks in the table example that are older than three days:
```
SELECT show_chunks('example', older_than => INTERVAL '3 days');
```

This returns a list of chunks. Take note of the chunks' names:


||show_chunks|
|---|---|
|1|_timescaledb_internal_hyper_1_2_chunk|
|2|_timescaledb_internal_hyper_1_3_chunk|

When you are happy with the list of chunks, you can use the chunk names to manually compress each one.

Compressing chunks manually

At the psql prompt, compress the chunk:
```
SELECT compress_chunk( '<chunk_name>');
```

Check the results of the compression with this command:


SELECT *
FROM chunk_compression_stats('example');

The results show the chunks for the given hypertable, their compression status, and some other statistics:


|chunk_schema|chunk_name|compression_status|before_compression_table_bytes|before_compression_index_bytes|before_compression_toast_bytes|before_compression_total_bytes|after_compression_table_bytes|after_compression_index_bytes|after_compression_toast_bytes|after_compression_total_bytes|node_name|
|---|---|---|---|---|---|---|---|---|---|---|---|
|_timescaledb_internal|_hyper_1_1_chunk|Compressed|8192 bytes|16 kB|8192 bytes|32 kB|8192 bytes|16 kB|8192 bytes|32 kB||
|_timescaledb_internal|_hyper_1_20_chunk|Uncompressed||||||||||

Repeat for all chunks you want to compress.

Manually compress chunks in a single command

Alternatively, you can select the chunks and compress them in a single command by using the output of the show_chunks command to compress each one. For example, use this command to compress chunks between one and three weeks old if they are not already compressed:


SELECT compress_chunk(i, if_not_compressed => true)
    FROM show_chunks(
        'example',
        now()::timestamp - INTERVAL '1 week',
        now()::timestamp - INTERVAL '3 weeks'
    ) i;

Roll up uncompressed chunks when compressing

In Timescale 2.9 and later, you can roll up multiple uncompressed chunks into a previously compressed chunk as part of your compression procedure. This allows you to have much smaller uncompressed chunk intervals, which reduces the disk space used for uncompressed data. For example, if you have multiple smaller uncompressed chunks in your data, you can roll them up into a single compressed chunk.

To roll up your uncompressed chunks into a compressed chunk, alter the compression settings to set the compress chunk time interval and run compression operations to roll up the chunks while compressing.

Note

The default setting of compress_orderby is 'time DESC' (the descending or DESC command is used to sort the data returned in ascending order), which causes chunks to be re-compressed many times during the rollup, possibly leading to a steep performance penalty. Set timescaledb.compress_orderby = 'time ASC' to avoid this penalty.


ALTER TABLE example SET (timescaledb.compress_chunk_time_interval = '<time_interval>',
                            timescaledb.compress_orderby = 'time ASC');
SELECT compress_chunk(c, if_not_compressed => true)
    FROM show_chunks(
        'example',
        now()::timestamp - INTERVAL '1 week'
    ) c;

The time interval you choose must be a multiple of the uncompressed chunk interval. For example, if your uncompressed chunk interval is one week, your <time_interval> of the compressed chunk could be two weeks or six weeks, but not one month.