Retain tree name through TChain with RNTupleImporter

[bernhardmgruber]

Explain what you would like to see improved and how.

The other day someone helpful suggested this way of creating an artifically enlarged ntuple file for testing:

TChain chain;
for (int i = 0; i < 25; i++) chain.Add("B2HHH~zstd.root?#DecayTree"); // TTree file
ROOT::Experimental::RNTupleImporter::Create(&chain, "B2HHH~zstd~25x.ntuple")->Import();

Afterwards however, I was quite lost because the tree inside the result file was no longer called "DecayTree", but "ROOT::Experimental::RNTuple". I understand that if you mix multiple files with a TChain there is no unique name to pick. But if there was a unique name, I think it would be great to carry it over to the resulting file!

ROOT version

ROOT Version: 6.29/01
Built for linuxx8664gcc on Jun 22 2023, 16:45:00
From heads/master@v6-29-01-1712-g1d65f2ecf4

Installation method

built from source

Operating system

Linux

Additional context

No response

[vepadulano]

Hi @bernhardmgruber , thank you for reporting this!

I understand the problem, we need to make sure we give a meaningful name to the RNTuple. Regarding your specific example above, I have a doubt about the design. Since the input is a TChain with no name, there is no evident unique name to give to the imported RNTuple. If the TChain had a name then that would be the correct name to assign. But in this case, we could:

1. Get the name of the first tree in the first file of the chain, and assign that irrespective of the names of the other trees in the chain.
2. Check that all trees of the chain have the same name (the first one), and assign that. If this is not the case, fallback to assigning the first name anyway ?
3. Invent a new name for the imported RNTuple which will always be applied in case the user is importing from an un-named TChain. This could be anything from something simple like MyRNTuple to something more unique and complicated like R__IMPORTED_RNTUPLE.

Thoughts? Also inviting @pcanal and @jblomer to comment

[enirolf]

Hello! I think in the case where the provided TChain does not have a name, picking the name of the first tree in the chain would be a sensible choice (provided this is well-documented, of course).

In any case, if a different name is desired, one can always call RNTupleImporter::SetNTupleName to change the name of the target RNTuple.

[bernhardmgruber]

If the TChain had a name then that would be the correct name to assign.

Yes.
(I didn't know TChain's could have a name. I thought they were conceptually just concatenations of trees)

1. Get the name of the first tree in the first file of the chain, and assign that irrespective of the names of the other trees in the chain.

That sounds too deliberate to me.

2. Check that all trees of the chain have the same name (the first one), and assign that.

I think this would be the only sensible solution. But I don't kow how big TChains can get and whether such a check would become expensive at some point.

If this is not the case, fallback to assigning the first name anyway ?

Again, I think this is too deliberate.

3. Invent a new name for the imported RNTuple which will always be applied in case the user is importing from an un-named TChain. This could be anything from something simple like MyRNTuple to something more unique and complicated like R__IMPORTED_RNTUPLE.

That is the the status quo, as observed by me. The name picked is ROOT::Experimental::RNTuple.

[vepadulano]

But I don't kow how big TChains can get and whether such a check would become expensive at some point.

Yes, sorry I should have given a bit more context. A TChain could also have O(10^3) files, in extreme cases O(10^4).

Personally, I agree with @enirolf as I still think getting the first name is not that bad of an idea. It's surely not super elegant, but I believe it is at least better than giving it an arbitrary name like it's done currently. Maybe a compromise could be:

1. Check how many files the TChain has.
2. If it's less than THRESHOLD, then run the check that all files have the same tree name. The THRESHOLD number should probably be decided after a few performance benchmarks. If the files don't have all the same tree, I still think we should give the first tree name (and maybe issue a warning that we're doing so).
3. If it's more, just take the first tree name (with a warning).

[bernhardmgruber]

Alright, than maybe really just take the first tree's name. The algorithm for chosing a name involving a threshold may also be surprising to some users. Imagine relying on the TChan picking the first tree's name and then adding a bunch of more files and suddenly the name changes :)

[enirolf]

One more thing to add, it is also possible to create a named TChain and have ROOT only the trees with that name:

TChain chain("DecayTree");
for (int i = 0; i < 25; i++) chain.Add("B2HHH~zstd.root"); // TTree file

This will of course only work when all trees you want to chain have the same name so we should still do something for the other cases (or the case you described in the original post), but just wanted to point out that this is an option (and in this case, the importer will use the original name).

[pcanal]

One more thing to add, it is also possible to create a named TChain

Indeed. And this is the most common use case.

and have ROOT only the trees with that name:

Technically this is inaccurate. TChain::Add will default to using the chain name but it can be overriden:

TChain chain("DecayTree");
for (int i = 0; i < 25; i++) chain.Add("B2HHH~zstd.root"); // TTree file
for (int i = 0; i < 25; i++) chain.Add("B2HHH~zstd-2024.root?#BetterTreeName"); // TTree file

[pcanal]

I think that we need to add a signature of ROOT::Experimental::RNTupleImporter::Create that let the user over-ride the default name (what ever it is).

For the default behavior I see only 2 sensible behavior:

• Use the user provided name, if none, use the TChain name, if not set, use the first TTree and leave it at that.
  or
• Use the user provided name, if none, use the TChain name, if not set, use the first TTree. In the later case (i.e. we guessed the name) throw an exception if encountering a different name.

Note that the 2nd behavior might actually be the prefer one ... depending on what we do if/when we encounter a TTree which a different structure. (I.e. we reject TTree with a different structure, we ought to reject the name. If we accept TTree with a different structure, we ought to ignore the name).

hadd for example let some differences slides (like added branches being ignored) and use the first file as the decider of the structure.

[bernhardmgruber]

For completeness, RNTupleImporter already allows to supply a tree name:

auto imp = ROOT::Experimental::RNTupleImporter::Create(&chain, "B2HHH~zstd~10x.ntuple");
imp->SetNTupleName(“DecayTree”);
imp->Import();

My issue is just about an improvement of user experience.

[github-actions]

Hi @enirolf, @vepadulano,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖

[github-actions]

Hi @enirolf, @vepadulano,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖

[github-actions]

Hi @enirolf, @vepadulano,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖

[github-actions]

Hi @enirolf, @vepadulano,

It appears this issue is closed, but wasn't yet added to a project. Please add upcoming versions that will include the fix, or 'not applicable' otherwise.

Sincerely,
🤖