Combine TTree with friend using only events from TEntryList

I have thousands of output TTree stored across thousands of files. In addition, I have thousands of friend trees and TEntryList stored across thousands of files.

For speedy access, I would like to write out a single TTree containing all the branches from the primary trees and the friend trees, only containing events from the TEntryList.

The examples I have found recommend just not doing this: I should just access the primary trees, add the friends, and apply the entry lists. However, access is extremely slow using this approach due to the multiplicity of files; I would like to have a single file with a smaller TTree.

ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

Hi @mwilkins,

I invite @pcanal as the ROOT I/O expert for this.

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