So "best" means fastest?

We don't know how big your table is and if memory is an issue.

In programming you can almost always trade memory with time!

... like pulling n big chunks of the table in sliding windows.

For instance I suppose (see your footnote) two SQL queries for ID and SN are faster but your map solution occupies less memory ( the second level hash refs are reused scalars)

So .... It really depends...

> * I should probably benchmark that

That's a bingo! ;)