Manual compression

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

warning

Compression alters data on your disk, so always back up before you start.

Compress chunks manually

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

1. 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');

2. This returns a list of chunks. Take a note of the chunk 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

1. At the psql prompt, compress the chunk:

   SELECT compress_chunk( '<chunk_name>');

2. 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

3. 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;