gengliangwang opened a new pull request, #37160:
URL: https://github.com/apache/spark/pull/37160
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### What changes were proposed in this pull request?
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Currently, casting decimal as string type will result in Strings with
exponential notations if the adjusted exponent is less than -6. This is
consistent with BigDecimal.toString
https://docs.oracle.com/javase/8/docs/api/java/math/BigDecimal.html#toString
After this PR, the casting always uses plain string representation.
### Why are the changes needed?
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1. The current behavior doesn't compliant to the ANSI SQL standard.
<img width="918" alt="image"
src="https://user-images.githubusercontent.com/1097932/178395756-baecbe90-7a5f-4b4c-b63c-9f1fdf656107.png";>
<img width="603" alt="image"
src="https://user-images.githubusercontent.com/1097932/178395567-fa5b6877-ff08-48b5-b715-243c954d6bbc.png";>
2. It is different from databases like PostgreSQL/Oracle/MS SQL server/etc..
3. The current behavior may surprise users since it only happens when the
adjusted exponent is less than -6. The following query will return `false` by
default (when ANSI SQL mode is off) since the `0.0000000123` is converted as
`1.23E-8`:
```sql
select '0.0000000123' in (0.0000000123, '0.123');
```
### Does this PR introduce _any_ user-facing change?
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Yes, after changes, Spark SQL always uses plain string representation on
casting Decimal to String. To restore the legacy behavior, which uses
scientific notation if the adjusted exponent is less than -6, set
`spark.sql.legacy.castDecimalToString.enabled` to `true`.
### How was this patch tested?
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Unit test
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