Mathematica 9.0 problem: ParallelTable with SQLExecute

Question

I have run into the following problem: when I try to run the following code in Mathematica 9 I get SQL connection error on all kernels. The code is:

ParallelTable[SQLExecute[conn, "Select 2+2"], {4}]

And the error message: SQLConnection::conn: Connection is not opened properly.

The connection itself works fine - I tested it. Also the above code used to work in Mathematica 7. On 8 a similar but more complex SQL request did not work.

I would appreciate any suggestions what I could do to solve this (tried distribute Definition on conn and also tried creating a list of connections and selecting one according to $KernelId).

Answer 1

As it seems to solve the problem I'm moving my comment to an answer:

To make this work you need to open an independent connection from every parallel kernel. It is important the the call to OpenSQLConnection is evaluated on the parallel kernels, so generating a list of connection on the master and distributing these to the parallel kernels will not work. To generate the connections on the parallel kernels the most straightforward solution is to ParallelEvaluate something like conn=OpenSQLConnection[...]. This should work even if the parallel kernels run on different machines and platforms (as long as they can connect to the database). It is the database that will need to handle potential problems from simultaneous access from the parallel kernels, so you'll need a database system that supports concurrent access (which most but not all such systems do).