如何访问sql.Rows的未转换driver.Value切片

Question

My goal is to get to the raw driver.Value values as deserialized by a sql driver in its implementation of driver.Rows.Next(). I want to handle the conversion from the values returned by the driver to the needed target types, instead of relying on the automatic conversions built in to Rows.Scan. Note this question does not ask your opinion on whether Rows.Scan "should" be used. I don't want to use it, and I am asking if there is any way to avoid it.

A meaningful answer does not use Rows.Scan at all. The dynamic approach illustrated in Working with Unknown Columns is awful: It invokes all the overhead of Scan and destroys the type information of the source columns, instead shredding the actual driver.Values into SqlBytes.

The following hack works, but relies on the internal implementation detail that sql.Rows.Next() populates the internal field lastcols with exactly the unconverted values which I want:

vpRows := reflect.ValueOf(rows)                    // rows is a *sql.Rows
vRows := reflect.Indirect(vpRows)                  // now we have the sql.Rows struct
mem := vRows.FieldByName("lastcols")               // unexported field lastcols
unsafeLastCols := unsafe.Pointer(mem.UnsafeAddr()) // Evil
plastCols := (*[]driver.Value)(unsafeLastCols)     // But effective

for rows.Next() {
    rowVals := *plastCols
    fmt.Println(rowVals)
}

Answer 1

The normal solution is to implement your own sql.Scanner. But this does use rows.Scan, so it violates your mysterious requirement not to use rows.Scan.

If you truly must avoid rows.Scan, you'll need to write your own driver implementation (possibly wrapping an existing driver) which provides access to the driver.Value values without rows.Scan.