How to read only parts of a Tree

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When running on D3PDs your analysis will be much faster, when you only read the information from disk that you really need. If you use:

      GetEntries(entry);

for each event all Branches will be read, resulting an a large I/O usage and a slow analysis. You can instead use:

       b_MET_RefFinal_sumet->GetEntry(entry); // Get Branch MET_RefFinal_sumet

The drawback is, that a branch that was not read with GetEntry will contain 0, so you have to carefully check, that you retrieve all the branches you use.