Dustin Smith created SPARK-60843:
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Summary: Truncate decimals exactly when casting to INT, SMALLINT
and TINYINT
Key: SPARK-60843
URL: https://issues.apache.org/jira/browse/SPARK-60843
Project: Spark
Issue Type: Bug
Components: SQL
Affects Versions: 5.0.0
Reporter: Dustin Smith
{{Decimal.roundToInt}}, {{roundToShort}} and {{roundToByte}} check the range of
a BigDecimal-backed value by converting it to a {{Double}}. A value that
truncates to the bound but sits just below the next integer rounds up past the
bound as a double, so the check fails even though the truncated result fits.
Example, with {{spark.sql.ansi.enabled=true}}:
{code:sql}
SELECT CAST(CAST('2147483647.99999999' AS DECIMAL(20,8)) AS INT);
-- fails with CAST_OVERFLOW, should return 2147483647
SELECT try_cast(CAST('2147483647.99999999' AS DECIMAL(20,8)) AS INT);
-- returns NULL, should return 2147483647
{code}
The same happens at {{Int.MinValue}} and at the {{SMALLINT}} and {{TINYINT}}
bounds. When the same value is held as an unscaled long (precision <= 18), the
check is already exact and the cast succeeds, as does {{roundToLong}}, which
truncates through {{BigInteger}}. So the result of the cast depends on the
internal representation of the value.
Proposed change: truncate BigDecimal-backed values exactly via {{BigInteger}},
as {{roundToLong}} does, and range-check the truncated integer. Values that
used to fail with {{CAST_OVERFLOW}} (or return NULL from {{try_cast}}) now
return the truncated value; values that are actually out of range still
overflow. Add a line to the SQL migration guide.
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