[CASSANDRA-14170] Loss of digits when doing CAST from varint/bigint to decimal

• Type: Bug
• Status: Resolved
• Priority: Low
• Resolution: Fixed
• Fix Version/s: 3.11.3, 4.0-alpha1, 4.0
• Component/s: Legacy/CQL

Environment

Tested with Cassandra 3.11.1 but this issue is present since the implementation of cast functions.

• Severity: Low
• Assignee: Benjamin Lerer
• Reporter: Daniel Fiala

Authors

Benjamin Lerer

Reviewers

Andres de la Peña

• Created: 17/Jan/18 10:22
• Updated: 15/May/20 08:00
• Resolved: 14/Mar/18 09:34

Description

Cast functions from numeric types to decimal type are implemented as conversion to double first and then from double to decimal: https://github.com/apache/cassandra/compare/trunk...blerer:10310-3.0#diff-6aa4a8f76df6c30c5bb4026b8c9251eeR80.

This can cause loss of digits for big values stored in varint or bigint. It is probably unexpected because decimal can store such values precisely.

Examples:

cqlsh> CREATE TABLE cast_bigint_test(k int PRIMARY KEY, bigint_clmn bigint);
cqlsh> INSERT INTO cast_bigint_test(k, decimal_clmn) VALUES(2, 9223372036854775807);
cqlsh> SELECT CAST(bigint_clmn AS decimal) FROM cast_bigint_test;
cast(bigint_clmn as decimal)
------------------------------
9.223372036854776E+18
(1 rows)

cqlsh> CREATE TABLE cast_varint_test (k int PRIMARY KEY, varint_clmn varint);
cqlsh> INSERT INTO cast_varint_test(k, varint_clmn) values(2, 1234567890123456789);
cqlsh> SELECT CAST(varint_clmn AS decimal) FROM cast_varint_test;
cast(varint_clmn as decimal)
------------------------------
1.23456789012345677E+18
(1 rows)