> I am doing a comparison of speed of accessing data > in a root file and in a custom database file in our company. I had exactly the same problem. Trees save themselves using ZIP compression to save disk space. Maybe you could try turning ZIP compression off, this will cut uncompressing time significantly BUT... disk space and disk access time might be a problem since ROOT's tree format is not designed to save memory but to handle complex objects, ALSO check file sizes, the ROOT file might be significantly larger. My solution was NOT to use trees to save the data, instead, I designed a tick-data format which is very efficient and uses on-the-fly uncompressing so disk access takes very little time. The trick is to play around with the algorithm so un-compressing time is shorter than disk access time. There is no need to worry about compression time since tick files are mostly read-only. In my experience this has turned out to be quite faster than a SQL database.
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