Fix DECIMAL parameter cast size for exponent-notation and negative values

DecimalParameter.calculate_decimal_cast_string inferred DECIMAL(precision,
scale) by string-splitting str(value) on ".", which miscounts two forms
that str(Decimal) legitimately produces:

- exponent notation (Decimal("1500").normalize() -> "1.5E+3"): the "E+3"
  was counted as fractional digits, giving DECIMAL(5,4) for 1500, which
  overflows (an error in ANSI mode, NULL in legacy mode).
- a leading minus sign: counted as an extra integer digit, so a negative
  38-digit value produced DECIMAL(39,0), exceeding Databricks' max DECIMAL
  precision of 38 and failing the cast.

Derive precision/scale from Decimal.as_tuple() (digits + exponent), ignoring
sign and display format. Existing cast-string tests are unchanged; added
regression cases for exponent notation and negative values.

Signed-off-by: Madan Kumar <winklemad@outlook.com>

Author: winklemad

src/databricks/sql/parameters/native.py

@@ -651,23 +651,20 @@ def calculate_decimal_cast_string(self, input: decimal.Decimal) -> str:
             Output:  DECIMAL(8,4)
         """
 
-        string_decimal = str(input)
-
-        if string_decimal.startswith("0."):
-            # This decimal is less than 1
-            overall = after = len(string_decimal) - 2
-        elif "." not in string_decimal:
-            # This decimal has no fractional component
-            overall = len(string_decimal)
-            after = 0
+        # Derive precision/scale from the exact numeric value rather than str(),
+        # whose sign and exponent notation (e.g. "1.5E+3", "-12.34") would be
+        # miscounted as extra digits.
+        _, digits, exponent = input.as_tuple()
+        if exponent >= 0:
+            # Integer value: `digits` followed by `exponent` trailing zeros.
+            scale = 0
+            precision = len(digits) + exponent
         else:
-            # This decimal has both whole and fractional parts
-            parts = string_decimal.split(".")
-            parts_lengths = [len(i) for i in parts]
-            before, after = parts_lengths[:2]
-            overall = before + after
+            scale = -exponent
+            # A value < 1 still needs `scale` digits of precision.
+            precision = max(len(digits), scale)
 
-        return self.CAST_EXPR.format(overall, after)
+        return self.CAST_EXPR.format(precision, scale)
 
 
 def dbsql_parameter_from_int(value: int, name: Optional[str] = None):

tests/unit/test_parameters.py

@@ -100,6 +100,12 @@ def test_parameters_enabled(self, test_input, expected):
         (Decimal("123456789.123456789"), "DECIMAL(18,9)"),
         (Decimal("12345678912345678912345678912345678912"), "DECIMAL(38,0)"),
         (Decimal("1234.56"), "DECIMAL(6,2)"),
+        # Exponent notation (e.g. from Decimal.normalize()) and negative values
+        # must be sized from the numeric value, not str().
+        (Decimal("1500").normalize(), "DECIMAL(4,0)"),
+        (Decimal("1e5"), "DECIMAL(6,0)"),
+        (Decimal("-" + "9" * 38), "DECIMAL(38,0)"),
+        (Decimal("-12.34"), "DECIMAL(4,2)"),
     ),
 )
 def test_calculate_decimal_cast_string(value, expected):