[CASSANDRA-18647] CASTing a float to decimal adds wrong digits

• Type: Bug
• Status: Resolved
• Priority: Normal
• Resolution: Fixed
• Fix Version/s: 3.11.16, 4.0.11, 4.1.3, 5.0-alpha1, 5.0
• Component/s: CQL/Semantics
• Bug Category: Correctness
• Severity: Normal
• Complexity: Normal
• Discovered By: User Report
• Platform: All
• Since Version: 3.11.0

Source Control Link

https://github.com/apache/cassandra/commit/7ec5126870f1bee853e63bfd98a73769c63abf3b

Test and Documentation Plan

CI

• Assignee: Stefan Miklosovic
• Reporter: Nadav Har'El

Authors

Stefan Miklosovic

Reviewers

Andres de la Peña

• Created: 04/Jul/23 21:27
• Updated: 27/Mar/24 17:39
• Resolved: 12/Jul/23 10:30

Description

If I create a table with a float (32-bit) column, and cast it to the decimal type, the casting wrongly passes through the double (64-bit) type and picks up extra, wrong, digits. For example, if we have a column e of type "float", and run

INSERT INTO tbl (p, e) VALUES (1, 5.2)

SELECT CAST(e AS decimal) FROM tbl WHERE p=1

The result is the "decimal" value 5.199999809265137, with all those extra wrong digits. It would have been better to get back the decimal value 5.2, with only two significant digits.

It appears that this happens because Cassandra's implementation first converts the 32-bit float into a 64-bit double, and only then converts that - with all the silly extra digits it picked up in the first conversion - into a "decimal" value.

Contrast this with CAST(e AS text) which works correctly - it returns the string "5.2" - only the actual digits of the 32-bit floating point value are converted to the string, without inventing additional digits in the process.

Issue Links

• links to: GitHub Pull Request #2468