Question

Unfortunately, SQL Server can not a handle a parameter list which exceeds 2100 parameters. I have a couple of queries which when run in a batch mode exceed this limit. To still get the results I want and quickly cycle through the data set I have decided to make use of Skip(i).Take(2000) in a for loop.

What I was not prepared for was that both Union and Concat require an IEnumerable to be more than just instantiated to work. Sure it’s an empty, but I must be missing something fundamental about their use. At any rate, to solve the issue I ended up using a List and AddRange. I am making use of NHibernate, but don’t think that factors into why Union and Concat aren’t working.

var machineResults = new List<Machine>();

for (int i = 0; i < machines.Count(); i += 2000)
{
                   // I would have expected Union or Concat to work here
    machineResults.AddRange(GetSession().CreateQuery(
       @"select distinct m
        from Machine m
        where m in (:MachinesList)") // there's more criteria than this
       .SetParameterList("MachinesList",
                          machines.Skip(i).Take(2000).ToList())
       .List<Machine>());                
}

return machineResults;

So what am I missing about Union and Concat? Why don’t they work in the above statement? I understand why AddRange works, but what if I was concerned about the uniqueness of my results and wanted to use Union?

Answer 1

Union and Concat are pure methods. They do not modify the object they are run on, instead they return a copy.

var a = new [] {1, 2, 3, 4};
var b = new [] {5, 6, 7, 8};

var c = a.concat(b);

After this code has run

a => [1, 2, 3, 4]
b => [5, 6, 7, 8]
c => [1, 2, 3, 4,5, 6, 7, 8]

Meaning if you had machineResults.concat(x), you’re not actually modifying the machineResults collection. Instead you’d have to have machineResults = machineResults.concat(x).