In most cases, an automated compression policy is sufficient. However, if you want more control over compression, you can also manually compress specific chunks.
Compression alters data on your disk, so always back up before you start.
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.
At the psql prompt, select all chunks in the table
examplethat are older than three days:SELECT show_chunks('example', older_than => INTERVAL '3 days');
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.
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.
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;
In TimescaleDB 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, use this command, specifying the interval to use:
SELECT compress_chunk_time_interval('<time_interval>',if_not_compressed => true)FROM show_chunks('example',now()::timestamp - INTERVAL '1 week',) ;
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.
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