The following bug has been logged online:

Bug reference: 2818
Logged by: Brien R. Givens
Email address: ic3b3rg (AT) gmail (DOT) com
PostgreSQL version: 8.2
Operating system: Win XP
Description: ADO Field.Attributes reports NULL on NOT NULL fields
Details:

In ADO, the nullability of a field can be tested against its Attributes
property. Another option is to examine the Schema. As shown below, the
Attributes property of a NOT NULL field indicates it is nullable while the
Schema reports that it is not.

ADO Code:

Set conn = Server.CreateObject("ADODB.Connection")
conn.Open "Driver={PostgreSQL
ANSI};Server=localhost;Database=clinic;UID=clinic; PWD=c0nner99"

conn.Execute "DROP TABLE test"
conn.Execute "CREATE TABLE test (f1 INT NOT NULL)"

Set rs = Server.CreateObject("ADODB.recordset")
rs.Open "test", conn, adOpenStatic, adLockReadOnly, adCmdTableDirect

Response.Write ((rs(0).Attributes And adFldIsNullable) = adFldIsNullable) &
"<br>"
rs.Close

Set rs = conn.OpenSchema(adSchemaColumns,Array(Empty,Empty, "test","f1"))
Response.Write rs("IS_NULLABLE")

rs.Close
conn.Close


-- OUTPUT --

True
False

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match