Data retention - Questdb

Background

The nature of time-series data is that the relevance of information diminishes over time. If stale data is no longer required, users can delete old data from QuestDB to either save disk space or adhere to a data retention policy. This is achieved in QuestDB by removing data partitions from a table.

QuestDB offers two approaches for data retention:

Automatic: Use Time To Live (TTL) to automatically drop partitions when data ages beyond a specified threshold. This is the simplest approach for most use cases.
Manual: Use DROP PARTITION commands as described on this page for explicit control over which partitions to remove and when.

This page provides a high-level overview of partitioning with examples to drop data by date. For more details on partitioning, see the partitioning page.

Manual partition management

This section covers the manual approach to removing stale data by dropping partitions. A table must have a designated timestamp assigned and a partitioning strategy specified during a CREATE TABLE operation to achieve this.

:::note

Users cannot alter the partitioning strategy after a table is created.

:::

Tables can be partitioned by one of the following:

YEAR
MONTH
WEEK
DAY
HOUR

questdb-sql

CREATE TABLE my_table(ts TIMESTAMP, symb SYMBOL, price DOUBLE) timestamp(ts)
PARTITION BY DAY;

Dropping partitions

:::caution

Use DROP PARTITION with care, as QuestDB cannot recover data from dropped partitions.

:::

To drop partitions, users can use the ALTER TABLE DROP PARTITION syntax. Partitions may be dropped by:

DROP PARTITION LIST - specifying a comma-separated list of partitions to drop

questdb-sql

--Delete a partition
ALTER TABLE my_table DROP PARTITION LIST '2021-01-01';

--Delete a list of two partitions
ALTER TABLE my_table DROP PARTITION LIST '2021-01-01', '2021-01-02';

WHERE timestamp = - exact date matching by timestamp

questdb-sql

ALTER TABLE my_table DROP PARTITION
WHERE timestamp = to_timestamp('2021-01-01', 'yyyy-MM-dd');

WHERE timestamp < - using comparison operators (< / >) to delete by time range relative to a timestamp. Note that the now() function may be used to automate dropping of partitions relative to the current time, i.e.:
questdb-sql
```
--Drop partitions older than 30 days
ALTER TABLE my_table DROP PARTITION
WHERE timestamp < dateadd('d', -30, now());
```

Usage notes:

The most chronologically recent partition cannot be deleted
Arbitrary partitions may be dropped, which means they may not be the oldest chronologically. Depending on the types of queries users are performing on a dataset, it may not be desirable to have gaps caused by dropped partitions.
Unlike TTL, DROP PARTITION commands must be triggered manually or via external scheduling (e.g., cron jobs). For fully automated retention, consider using TTL instead.

Example

The following example demonstrates how to create a table with partitioning and to drop partitions based on time. This example produces 5 days' worth of data with one incrementing LONG value inserted per hour.

questdb-sql

CREATE TABLE my_table (timestamp TIMESTAMP, x LONG) timestamp(timestamp)
PARTITION BY DAY;

INSERT INTO my_table
SELECT timestamp_sequence(
    to_timestamp('2021-01-01T00:00:00', 'yyyy-MM-ddTHH:mm:ss'),100000L * 36000), x
FROM long_sequence(120);

For reference, the following functions are used to generate the example data:

timestamp_sequence with 1 hour stepping
long_sequence which creates a x:long column

The result of partitioning is visible when listing as directories on disk:

bash

my_table
├── 2021-01-01
├── 2021-01-02
├── 2021-01-03
├── 2021-01-04
└── 2021-01-05

Partitions can be dropped using the following query:

--Delete days before 2021-01-03
ALTER TABLE my_table DROP PARTITION
WHERE timestamp < to_timestamp('2021-01-03', 'yyyy-MM-dd');