I do not know if that is how all indexes work. I am not really a card-carrying database manager or programmer. I just play one in my spare time. The price I pay is not remembering how to write syntax when I need to do something. That to me, is a higher price than slow subsetting. If the syntax is not easy I would rather just use the traditional vector scan methods that one sees in conventional data.frame subset commands.
Notwithstanding my idiosyncratic needs, I thank you very much for your explanation. Farrel On Thu, Jan 19, 2012 at 18:26, Steve Lianoglou < [email protected]> wrote: > On Thu, Jan 19, 2012 at 6:09 PM, Farrel Buchinsky <[email protected]> > wrote: > > Oy gevalt!.Am I correct to believe that the technique is rearranging the > > data.table so that J can accept the input as pertaining to a secondary > key? > > That seems as if it is too much work for me and my computer. I will > rather > > stick to the vector scan methods for now. > > Not the entire data.table, just the key columns. > > Depending on how many queries you're going to make against the 2nd key > only, the pay off for your troubles could be anywhere from zero to > mucho. Of course if you simply don't have the RAM to make the idx > data.table in the first place, then that's that. > > That's how all indexes work though, no? In a database for instance, if > you have a compound key/index over two or more columns, the index will > only help queries that work any prefix (or whole) part of the key, and > not just any subset elements of it (as you want to do here), right? > > HTH, > -steve > > -- > Steve Lianoglou > Graduate Student: Computational Systems Biology > | Memorial Sloan-Kettering Cancer Center > | Weill Medical College of Cornell University > Contact Info: http://cbio.mskcc.org/~lianos/contact >
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