here are my modifications to interface the Grid file catalog
(AliEn in this case) with TChain data sets. Some changes are just
one-liner changes to make some base class things virtual or to set
some members. With these changes one can do:

// connect
TGrid::Connect("alien://");

// query
TGridResult* result = gGrid->Query("/alice/cern.ch/user/p/peters/analysis/miniesd/","*.root","","");

or

TGridResult* result = gGrid->Query("/alice/cern.ch/user/p/peters/analysis/miniesd/","*.root","","-l 50");  // to query maximum 50 files

result->Print(""); // or result->Print("l") or result->Print("all"); => Tells you also the complete size of the TDSet (the ALICE one is 68 GB)

// build chain for ALICE files
TChain* mychain = new TChain("esdTree","AliceSession");

// Get a list of FileInfo Objects
// -> I cannot use directly a list of FileInfo objects in TGridResult,
//    because TGridResult can be also a list of jobs etc ....

TList* list = result->GetFileInfoList()

// add them to a chain
mychain->AddFileInfoList(list);    // adds all
mychain->AddFileInfoList(list,10); // adds only the first 10

// to use PROOF then, you need to do
mychain->Lookup();                 // open's all files via TAlienFile over
                                      the redirector and replaces the
                                      alien URL with the physical location
                                      of the file on the cluster
                                      (including the access token)

mychain->SetProof();
mychain->Draw("ESD.fTrigger");


git-svn-id: http://root.cern.ch/svn/root/trunk@12825 27541ba8-7e3a-0410-8455-c3a389f83636