Release 6.0 introduces breaking changes in table transaction files. An automated conversion process has been included in the release which will migrate table transaction files to use the new format. The following sections describe the automated upgrade process with notes for manually downgrading tables for compatibility with older versions.

Upgrading QuestDB

When QuestDB v6.0 starts up, and tables from older QuestDB versions are detected, a migration to the new transaction file format will run automatically. The migration scans for the existence of tables within the QuestDB storage directory and upgrades transaction (_txn) files for each table. All other table data is untouched by the upgrade.

If the migration fails for a table, an error message will be printed in the QuestDB logs on startup. QuestDB will not terminate, but tables which have not been successfully upgraded cannot be used for querying or writing.

Starting QuestDB again will trigger another attempt to migrate tables using an older transaction file format.

Reverting transaction files

During the upgrade process, _txn files are backed up and renamed using the format _txn.v417. Users who wish to revert the table migration can downgrade tables by following these steps:

1. delete the folder /path/to/questdb/db/_upgrade.d
2. for each table, rename _txn.v417 to _txn

Table downgrade example

This section illustrates how to revert transaction files to a format used by QuestDB versions earlier than 6.0. Given storage directories for two table example_table and sensors:

├── conf
├── db
│   ├── _tab_index.d
│   ├── _upgrade.d
│   ├── example_table
│   │   ├── 2021
│   │   │   ├── tempF.d
│   │   │   ├── ...
│   │   │   └── visMiles.d
│   │   ├── _meta
│   │   ├── _txn
│   │   └── _txn.v417
│   └── sensors
│       ├── 2021
│       │   ├── device_id.d
│       │   ├── ...
│       │   └── temperature.d
│       ├── _meta
│       ├── _txn
│       └── _txn.v417
└── public

The tables may be downgraded in the following manner:

rm db/_upgrade.d
mv db/example_table/_txn.v417 db/example_table/_txn
mv db/sensors/_txn.v417 db/sensors/_txn

After these steps have been completed, QuestDB v5.x may be started and the table data will be loaded as usual.

Breaking SQL changes

Release 6.0.1 contains breaking changes relating to SQL syntax to simplify working with TIMESTAMP types and for improved compatibility with ANSI SQL expectations.

:::info

For more information on these changes, see the 6.0.1 software version release notes on GitHub.

:::

To illustrate how timestamps are handled, a table my_table containing 48 records with timestamps every hour beginning at 00:00:00 on 2020-01-01 will be used in the following examples:

Timestamp string equality

The following example SQL uses a WHERE clause to evaluate if records match using string equality.

SELECT * FROM my_table
WHERE timestamp = '2020-01-01'

The result will be 1 record with exact match of 2020-01-01T00:00:00.000000Z. In other words, the string 2020-01-01 does not represent an interval, but a single TIMESTAMP data point of 2020-01-01T00:00:00.000000Z

Before software version 6.0.1, this would result in 24 records of all hours during date ‘2020-01-01’

In order to use the old semantics, the query must use the IN keyword instead of =:

SELECT * FROM my_table
WHERE timestamp IN '2020-01-01'

Timestamp string comparison

Timestamps may also be compared using > greater-than and < less-than operators. The following example SQL uses a > greater-than operator to evaluate if records occur later than a timestamp provided as a string:

SELECT * FROM my_table
WHERE timestamp > '2020-01-01'

The results are 47 records which have timestamps strictly greater than 2020-01-01T00:00:00.000000Z. The string 2020-01-01 does not represent an interval, but a single TIMESTAMP data point of 2020-01-01T00:00:00.000000Z:

Before software version 6.0.1, this would result in 24 records, one for each hour during the date 2020-01-02:

In order to use the old semantics, the query must use >= instead of >, and <= instead of <:

SELECT * FROM my_table
WHERE timestamp >= '2020-01-02'

Timestamp IN list

The IN keyword is used to check equality with a list of 2 elements:

SELECT * FROM my_table
WHERE timestamp IN ('2020-01-01T00:00:00.000000Z', '2020-01-02T00:00:00.000000Z')

The result is two records matching exactly 2020-01-01T00:00:00.000000Z and 2020-01-02T00:00:00.000000Z

Before software version 6.0.1, this would result in 25 records, one for each hour during the date 2020-01-01 and the 00:00:00 data point on 2020-01-02:

In order to use the old semantics, the BETWEEN keyword should be used:

SELECT * FROM my_table
WHERE timestamp BETWEEN '2020-01-01T00:00:00.000000Z' AND '2020-01-02T00:00:00.000000Z'