Question

I am running a simple select query thusly:

Private Function ReturnTableQuery(ByVal SQL As String) As DataTable

    Dim rs As DataTable = New DataTable
    Dim Adapter As NpgsqlDataAdapter = New NpgsqlDataAdapter

    Try
        If conn Is Nothing Then
            ConnectDatabase()
        End If
        If conn.State <> ConnectionState.Open Then
            ConnectDatabase()
        End If
        Adapter.SelectCommand = New NpgsqlCommand(SQL, conn)
        Adapter.SelectCommand.CommandTimeout = 10
        Adapter.Fill(rs)
    Catch ex As Exception
        PreserveStackTrace(ex)
        Throw ex
    End Try

    Return rs

End Function

The SQL command is:

Select id, application, datetimestamp, status, data, attemptcount from queue where application='reportengine' and status=4 and datetimestamp <= now()  order by datetimestamp limit 1

I sometimes get 0 rows returned.

If I run the exact same query in pgAdmin while it is failing in my program, it returns a row as expected.

If I close and re-open the connection it works, but I can’t determine if there is anything wrong with the connection beforehand.

I could just re-open the connection every time, but I would rather not recreate the connection as often as I would need to.

I am also getting intermittent errors like “unknown server response” which I am catching and re-opening the connection.

Any ideas why the connection is so fragile and is there an inexpensive way to check the actual connection status?

Thx,
Brad

Answer 1

Are you sharing your connection between multiple threads? Npgsql, as other data providers, is not thread safe. I would rank this is the most probable cause of your problems. You should open a connection, use it and then close it. That’s the most scalable pattern when using a connection pool. I hope it helps.