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chore: update compatibility guide for primitive to string casts #4012

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parthchandra merged 1 commit into from
Apr 21, 2026
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chore: update compatibility guide for primitive to string casts #4012

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12 changes: 12 additions & 0 deletions docs/source/user-guide/latest/compatibility.md
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Expand Up @@ -154,6 +154,18 @@ suffixes (e.g. `Europe/Moscow`), and the full Spark timestamp year range
(-290308 to 294247). Note that `CAST(string AS DATE)` is only compatible for years between
262143 BC and 262142 AD due to an underlying library limitation.

### Decimal with Negative Scale to String

Casting a `DecimalType` with a negative scale to `StringType` is marked as incompatible when
`spark.sql.legacy.allowNegativeScaleOfDecimal` is `false` (the default). When that config is
disabled, Spark cannot create negative-scale decimals, so Comet falls back to avoid running
native execution on unexpected inputs.

When `spark.sql.legacy.allowNegativeScaleOfDecimal=true`, the cast is compatible. Comet matches
Spark's behavior of using Java `BigDecimal.toString()` semantics, which produces scientific
notation (e.g. a value of 12300 stored as `Decimal(7,-2)` with unscaled value 123 is rendered
as `"1.23E+4"`).

### Legacy Mode

<!--BEGIN:CAST_LEGACY_TABLE-->
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