RE: [ROOT] Comparison of speed of accessing data

From: Rene Brun (Rene.Brun@cern.ch)
Date: Sat Nov 23 2002 - 15:48:53 MET


Hi Valery,

I don't think it is a good idea to encourage people to use C structs
instead of classes. This is not the way to go.
libTable is an optional library in the Root distribution.
Future versions of Root will provide more and more  services (eg PROOF)
for Trees, one of our mainstream developments.

Rene Brun

On Sat, 23 Nov 2002, Faine, Valeri wrote:

>  
> >    Valeri mentioned about TTable class.
> [Valeri Fine] 
>   TTable is a self-defined array of C-structures.
> 
>   It occupies a continuous area in memory and in file.
>   The table object can not be read by piece. It is always read in as
> whole thing.
>   In our applications (STAR/ATLAS) we have what we call "an event". Each
> event may contain several different tables organized heirachically. Each
> event is stored separately and can be read back in the random order or
> sequentially. To look up the table a separate TTableSorter class is
> provided also (see: http://root.cern.ch/root/htmldoc/TTableSorter.html )
>   Web site has several simple examples of how this class can be used
> http://www.usatlas.bnl.gov/computing/software/db/htmldoc/examples/ 
> 
> To try them one may download these examples and replace there the name
> of the share library. One needs to replace the statement:
> gSystem->Load("libRootKernel");
> with 
> gSystem->Load("libTable");
>  there.
> 
> 
> >    Maybe that is another thing to try.
> >    Anyone knows the fundamental difference between TTable and TTree
> >    in terms of how they organize the data on disk ??
> [Valeri Fine] 
> TTable is store row-wise, TTree is stored column-wise.
> The memory and disk space for TTable is allocated at ones as one piece.
> 
> [Valeri Fine] 
> The picture http://root.cern.ch/root/html/gif/tree_layout.gif gives some
> info about TTree layout.
> > 
> > --HP
> > 
> > 
> 



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