Description
What type of bug is this?
Incorrect result
What subsystems and features are affected?
Compression with hypercore tam.
What happened?
I'm unable to get compression stats unless I forcibly recompress chunks.
TimescaleDB version affected
2.18.1, 2.18.2, 2.19.0
PostgreSQL version used
16.6-16.8
What operating system did you use?
timescaledb-ha:pg16.6-ts2.18.1
timescaledb-ha:pg16.7-ts2.18.2
timescaledb-ha:pg16.8-ts2.19.0
What installation method did you use?
Docker
What platform did you run on?
On prem/Self-hosted
How can we reproduce the bug?
DROP TABLE IF EXISTS test_signals;
CREATE TABLE test_signals
(
device_id uuid NOT NULL,
channel text NOT NULL,
signal text NOT NULL,
signal_timestamp TIMESTAMPTZ NOT NULL,
value float NOT NULL,
PRIMARY KEY (device_id, channel, signal, signal_timestamp)
);
SELECT create_hypertable('test_signals', 'signal_timestamp', CHUNK_TIME_INTERVAL => INTERVAL '1 DAY');
ALTER TABLE test_signals SET (
timescaledb.enable_columnstore,
timescaledb.segmentby='device_id',
timescaledb.orderby='channel, signal, signal_timestamp DESC'
);
ALTER TABLE test_signals SET ACCESS METHOD hypercore;
INSERT INTO test_signals(channel, device_id, signal_timestamp, signal, value)
SELECT 'chan', device_ids, signal_timestamps, signals, random()*100
FROM UNNEST(ARRAY[gen_random_uuid(), gen_random_uuid(), gen_random_uuid()]) AS device_ids,
GENERATE_SERIES('2025-02-01', '2025-02-11', '1 second'::INTERVAL) AS signal_timestamps,
UNNEST(ARRAY['signal1', 'signal_longer_name_2', 'signal_much_much_longer_name_3']::TEXT[]) AS signals;
SELECT pg_size_pretty(before_compression_total_bytes) AS before_compress,
pg_size_pretty(after_compression_total_bytes) AS after_compress,
pg_size_pretty(hypertable_size('test_signals')) AS hypertable_size
FROM hypertable_compression_stats('test_signals');initial table size is 1.7GB
before_compress | after_compress | hypertable_size
-----------------+----------------+-----------------
0 bytes | 176 kB | 1736 MB
(1 row)
Let's compress.
SELECT compress_chunk(c) FROM show_chunks('test_signals') c;
SELECT pg_size_pretty(before_compression_total_bytes) AS before_compress,
pg_size_pretty(after_compression_total_bytes) AS after_compress,
pg_size_pretty(hypertable_size('test_signals')) AS hypertable_size
FROM hypertable_compression_stats('test_signals');Stats don't show up at all. table size is shrinking due to the autovacuum daemon presumably.
before_compress | after_compress | hypertable_size
-----------------+----------------+-----------------
0 bytes | 176 kB | 1542 MB
(1 row)
Let's decompress.
SELECT decompress_chunk(c) FROM show_chunks('test_signals') c;
SELECT pg_size_pretty(before_compression_total_bytes) AS before_compress,
pg_size_pretty(after_compression_total_bytes) AS after_compress,
pg_size_pretty(hypertable_size('test_signals')) AS hypertable_size
FROM hypertable_compression_stats('test_signals');Table size neither grows nor shrinks at this point.
before_compress | after_compress | hypertable_size
-----------------+----------------+-----------------
| | 1292 MB
(1 row)
Let's compress again.
SELECT compress_chunk(c) FROM show_chunks('test_signals') c;
SELECT pg_size_pretty(before_compression_total_bytes) AS before_compress,
pg_size_pretty(after_compression_total_bytes) AS after_compress,
pg_size_pretty(hypertable_size('test_signals')) AS hypertable_size
FROM hypertable_compression_stats('test_signals');Finally, expected values from the view (though slightly different from table size)
before_compress | after_compress | hypertable_size
-----------------+----------------+-----------------
1292 MB | 66 MB | 67 MB
(1 row)