SQLColAttribute and SQLDescribeCol return incorrect precision/scale for DECIMAL/NUMERIC types

[kevinjamesus86]

DuckDB's ODBC driver incorrectly reports precision and scale information for DECIMAL/NUMERIC columns when using SQLColAttribute and SQLDescribeCol functions.

Current Behavior

SQLColAttribute issues:

• SQL_DESC_PRECISION: Always returns 38 (maximum precision) instead of the actual column precision
• SQL_DESC_SCALE: Always returns 38 instead of the actual column scale
• According to https://learn.microsoft.com/en-us/sql/odbc/reference/syntax/sqlcolattribute-function, SQL_DESC_PRECISION should return "the applicable precision" for numeric data types

SQLDescribeCol issues:

• ColumnSize parameter: Returns precision + scale instead of just the precision
• According to https://learn.microsoft.com/en-us/sql/odbc/reference/appendixes/column-size, for SQL_DECIMAL and SQL_NUMERIC types, ColumnSize should be "the defined number of digits" (i.e., the precision only)
• DecimalDigits parameter: Correctly returns the scale ✓

Examples

Column Declaration	SQLColAttribute Returns(Precision/Scale)	SQLDescribeCol Returns(ColumnSize/DecimalDigits)	Expected Values(Precision/Scale)
DECIMAL(5,2)	38/38	7/2	5/2
DECIMAL(8,4)	38/38	12/4	8/4
DECIMAL(10,9)	38/38	19/9	10/9
DECIMAL(30,9)	38/38	39/9	30/9
DECIMAL(38,0)	38/38	38/0	38/0
DECIMAL(38,37)	38/38	75/37	38/37
NUMERIC(5,2)	38/38	7/2	5/2
NUMERIC(8,4)	38/38	12/4	8/4

Takeaway

• SQLColAttribute always returns 38 for both precision and scale
• SQLDescribeCol returns precision + scale for ColumnSize (e.g., 5+2=7 for DECIMAL(5,2))

Expected Behavior

• SQLColAttribute with SQL_DESC_PRECISION: Should return the actual precision (e.g., 5 for DECIMAL(5,2))
• SQLColAttribute with SQL_DESC_SCALE: Should return the actual scale (e.g., 2 for DECIMAL(5,2))
• SQLDescribeCol ColumnSize: Should return only the precision (e.g., 5 for DECIMAL(5,2))
• SQLDescribeCol DecimalDigits: Should return the scale (currently working correctly)

Fork + possible fix n' tests

main...thedatamates:duckdb-odbc:kg/decimals-info

[staticlibs]

Thanks for the report and for the code change! This is a great clean up, improvements to api_info (temporal precision and display size) are highly welcome!

I wonder if it would be possible for you to create a PR with these changes? There may be a couple of nits there, but in general it looks very good. Or, if it is more convenient, I can take the changes from the branch and create the PR myself?

I believe similar cleanup is also required for the output of the SQLColumns call - we can do it as a follow-up.

[kevinjamesus86]

@staticlibs I spent some time on that branch over the weekend and have a few follow-up fixes that bring GetColumnSize and GetDisplaySize into better alignment with the ODBC documentation. I'll clean those up and get them into a PR, then I'm happy to address any feedback you might have. Thanks for taking a look at these!

[staticlibs]

Closing as fixed by #177.