一次往返执行多个SQL命令

Question

I am building an application and I want to batch multiple queries into a single round-trip to the database. For example, lets say a single page needs to display a list of users, a list of groups and a list of permissions.

So I have stored procs (or just simple sql commands like "select * from Users"), and I want to execute three of them. However, to populate this one page I have to make 3 round trips.

Now I could write a single stored proc ("getUsersTeamsAndPermissions") or execute a single SQL command "select * from Users;exec getTeams;select * from Permissions".

But I was wondering if there was a better way to specify to do 3 operations in a single round trip. Benefits include being easier to unit test, and allowing the database engine to parrallelize the queries.

I'm using C# 3.5 and SQL Server 2008.

Answer 1

Something like this . The example is probably not very good as it doesn't properly dispose objects but you get the idea. Here's a cleaned up version:

using (var connection = new SqlConnection(ConnectionString))
using (var command = connection.CreateCommand())
{
    connection.Open();
    command.CommandText = "select id from test1; select id from test2";
    using (var reader = command.ExecuteReader())
    {
        do
        {
            while (reader.Read())
            {
                Console.WriteLine(reader.GetInt32(0));
            }
            Console.WriteLine("--next command--");
        } while (reader.NextResult());

    }
}

Answer 2

The single multi-part command and the stored procedure options that you mention are the two options. You can't do them in such a way that they are "parallelized" on the db. However, both of those options does result in a single round trip , so you're good there. There's no way to send them more efficiently. In sql server 2005 onwards, a multi-part command that is fully parameterized is very efficient.

Edit : adding information on why cram into a single call.

Although you don't want to care too much about reducing calls, there can be legitimate reasons for this.

• I once was limited to a crummy ODBC driver against a mainframe, and there was a 1.2 second overhead on each call! I'm serious. There were times when I crammed a little extra into my db calls. Not pretty.
• You also might find yourself in a situation where you have to configure your sql queries somewhere, and you can't just make 3 calls: it has to be one. It shouldn't be that way, bad design, but it is. You do what you gotta do!
• Sometimes of course it can be very good to encapsulate multiple steps in a stored procedure. Usually not for saving round trips though, but for tighter transactions, getting ID for new records, constraining for permissions, providing encapsulation, blah blah blah.

Answer 3

Making one round-trip vs three will be more eficient indeed. The question is wether it is worth the trouble. The entire ADO.Net and C# 3.5 toolset and framework opposes what you try to do. TableAdapters, Linq2SQL, EF, all these like to deal with simple one-call==one-resultset semantics. So you may loose some serious productivity by trying to beat the Framework into submission.

I would say that unless you have some serious measurements showing that you need to reduce the number of roundtrips, abstain. If you do end up requiring this, then use a stored procedure to at least give an API kind of semantics.

But if your query really is what you posted (ie. select all users, all teams and all permissions) then you obviosuly have much bigger fish to fry before reducing the round-trips... reduce the resultsets first.

Answer 4

I this this link might be helpful.

Consider using at least the same connection-openning; according to what it says here , openning a connection is almost the top-leader of performance cost in Entity-Framework.

Answer 5

Build a temp-table? Insert all results into the temp table and then select * from @temp-table

as in,

@temptable=....
select @temptable.field=mytable.field from mytable
select @temptable.field2=mytable2.field2 from mytable2

etc... Only one trip to the database, though I'm not sure it is actually more efficient.

Answer 6

Firstly, 3 round trips isn't really a big deal. If you were talking about 300 round trips then that would be another matter, but for just 3 round trips I would conderer this to definitley be a case of premature optimisation.

That said, the way I'd do this would probably be to executed the 3 stored procuedres using SQL:

exec dbo.p_myproc_1 @param_1 = @in_param_1, @param_2 = @in_param_2
exec dbo.p_myproc_2
exec dbo.p_myproc_3

You can then iterate through the returned results sets as you would if you directly executed multiple rowsets.