Fix issues with JSON and float/numeric

Decimal accuracy and behavior has been improved when extracting floating
point and/or decimal values from JSON strings using the
:meth:`_sql.sqltypes.JSON.Comparator.as_float` method, when the numeric
value inside of the JSON string has many significant digits; previously,
MySQL backends would truncate values with many significant digits and SQL
Server backends would raise an exception due to a DECIMAL cast with
insufficient significant digits.   Both backends now use a FLOAT-compatible
approach that does not hardcode significant digits for floating point
values. For precision numerics, a new method
:meth:`_sql.sqltypes.JSON.Comparator.as_numeric` has been added which
accepts arguments for precision and scale, and will return values as Python
``Decimal`` objects with no floating point conversion assuming the DBAPI
supports it (all but pysqlite).

Fixes: #5788
Change-Id: I6eb51fe172a389548dd6e3c65efec9f1f538012e

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/requirements.py b/lib/sqlalchemy/testing/requirements.py
index 7ff3fcf38..30b42cbf3 100644
--- a/lib/sqlalchemy/testing/requirements.py
+++ b/lib/sqlalchemy/testing/requirements.py
@@ -871,6 +871,14 @@ class SuiteRequirements(Requirements):
         return exclusions.closed()
 
     @property
+    def cast_precision_numerics_many_significant_digits(self):
+        """same as precision_numerics_many_significant_digits but within the
+        context of a CAST statement (hello MySQL)
+
+        """
+        return self.precision_numerics_many_significant_digits
+
+    @property
     def implicit_decimal_binds(self):
         """target backend will return a selected Decimal as a Decimal, not
         a string.