Server-side forward-only cursor breaks @@IDENTITY

问题

Here is a minimal repro example.

Database:

CREATE TABLE temp (x int IDENTITY(1, 1), y int);

Code (using VBA and ADO):

Public Sub repro()
    Dim cn As New Connection
    Dim rs1 As New Recordset
    Dim cmd As New Command
    Dim rs2 As New Recordset

    cn.Open "Provider=SQLNCLI11;Server=myServer;Database=myDatabase;Trusted_Connection=Yes"

    rs1.Open "SELECT 1", cn, adOpenForwardOnly      ' [X] '

    cmd.ActiveConnection = cn
    cmd.CommandText = "INSERT INTO temp (y) VALUES (1) "
    cmd.Execute

    rs2.Open "SELECT @@IDENTITY", cn, adOpenStatic
    Debug.Print rs2(0).value

    rs2.Close
    rs1.Close                                       ' [X] '
    cn.Close
End Sub

Expected result: The Debug.Print line outputs an integer to the debug window.

Actual result: The Debug.Print line outputs Null to the debug window.

Notes:

As soon as I remove the lines marked as [X], the code works as expected (the last inserted identity value is written to the debug window).
This is a minimal repro example: I know that server-side cursors are "evil". I know that in this particular case adding SELECT SCOPE_IDENTITY() to the INSERT batch is the correct way to get the newly inserted ID. This is just a minimal example to reproduce the issue with the least code possible. I stumbled upon this issue while modifying legacy code.
Tested with both SQL Server Native Client 11.0 and the "classic" MDAC SQL Server ODBC driver. Tested with SQL Server 2005 and 2012. Doesn't make a difference.

My question: Is this behavior by design, or did I stumble upon an SQL Server bug? If the former, where is it documented?

EDIT: Comparing the two options (with and without [X]) with an SQL Server Profiler trace, there is one significant difference: When the [X] lines are included, the connection is apparently dropped and reopened (Audit Logout - Audit Login) between cmd.Execute and rs2.Open. I guess that explains why @@IDENTITY no longer works.

That leaves the following question: Why does ADO (or the SQLNCLI11 driver?) close and reopen the connection in one case but not in the other case? And where is this documented?

回答1:

MARS is a nicer alternative to the default behavior which does actually allow multiple recordsets.

What happens is:

SELECT 1 acts as the active recordset for the connection & remains open
When you then execute the insert the provider knows it has an active recordset and tries to be helpful by creating a new connection to execute the statement without interfering with anything
This ephemeral connection executes the insert then tidies up by perfoming a logout - destroying state associated with it
select @@identity again uses an ephemeral connection where @@identity for the previous statement is out of scope, hence NULL.

回答2:

I noticed you are doing two selects on the same connection. Have you tried enabling "Multiple Active Result Sets" by adding "MultipleActiveResultSets=True" to your connection string?

Enabling Multiple Active Result Sets on MSDN

来源：https://stackoverflow.com/questions/48499514/server-side-forward-only-cursor-breaks-identity

标签

sql-server

vba

oledb

ado

identity