SUM did not return the correct DecimalSize()

[didip]

Example SQL:

select sum(1 + 2) as col

Called this function to get precision and scale:

precision, scale, ok := columnType.DecimalSize()

The returned precision is 42. I don't understand where did 42 come from. It's supposed to be 1, right?

[methane]

It returns precision and scale of column type, not a value in specific row.
In this case, it represents max digits in the DECIMAL type that SUM returns. (I don't know what database and version you are using.)

See also: https://pkg.go.dev/database/sql/driver#RowsColumnTypePrecisionScale