[datatable-help] using J() to select for a value that is in something other than the first key

[Farrel Buchinsky]

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 <
mailinglist.honeypot at gmail.com> wrote:

> On Thu, Jan 19, 2012 at 6:09 PM, Farrel Buchinsky <fjbuch at gmail.com>
> 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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