[python-win32] MS Access: silent overflow failure

[Robert Brewer]

Mark Hammond wrote:
> Robert Brewer wrote:
> > The small demo script below makes ADO calls to a Microsoft 
> > Access (Jet)
> > database via win32com. But it fails in a potentially dangerous way;
> > "SELECT int * int;" can return None with no warning if the result is
> > large enough. This can also occur if one or both of the 
> > operands is a column reference.
> > 
> > The same SQL run as a Query inside the Access GUI works as expected.
> > Manually changing one of the ints to a Single by appending 
> > ".0" works as expected.
> > 
> > Any ideas about where I need to look next to find the cause 
> > of, and then fix, this behavior?
> 
> I'd suggest using VB to see if the behaviour is different 
> than with Python.  If it is and we can narrow a test case to 
> a very small snippet, then we can probably sort out why.  If 
> VB works the same, it is likely to be a "feature" of ADO as 
> implemented, and not much we can do.

Below is a VB 4 demo (wow, that takes me back! :).
The return value on overflow is a zero-length string.


Robert Brewer
System Architect
Amor Ministries
fumanchu at amor.org


' demo_access_overflow.bas

Attribute VB_Name = "Module1"
Option Explicit

Const adOpenForwardOnly = 0
Const adLockReadOnly = 1
Const dbname = "test.mdb"

Sub create_db()
    Dim cat As New ADOX.Catalog
    cat.Create "PROVIDER=MICROSOFT.JET.OLEDB.4.0;DATA SOURCE=" & dbname
    cat.ActiveConnection.Close
End Sub

Function fetch(query, conn) As String
    Dim res As New ADODB.Recordset
    res.Open query, conn, adOpenForwardOnly, adLockReadOnly, -1
    Dim Data As Variant
    Data = res.GetRows(1, 0)
    fetch = Data(0, 0)
    res.Close
End Function

Sub Main()
    create_db
    
    Dim conn As New ADODB.Connection
    conn.Open "PROVIDER=Microsoft.Jet.OLEDB.4.0;Data Source='" & dbname
& "'", "", "", -1
    
    ' This works fine.
    Dim Data As String
    Data = fetch("SELECT 86400;", conn)
    If Data <> 86400 Then MsgBox (Data)
    
    ' This fails because data is ""
    Data = fetch("SELECT 62647 * 86400;", conn)
    If Data <> CStr(CSng(62647) * CSng(86400)) Then
        MsgBox (Data & " (len " & Len(Data) & ")")
    End If
    conn.Close
    Kill dbname
End Sub