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From Eric Newton <eric.new...@gmail.com>
Subject Re: Definition of Column Qualifier in User Manual
Date Thu, 23 Feb 2012 20:39:40 GMT
It's a way of grouping columns so that they go into different index files.
 This can lead to better compression and faster query times.

-Eric

On Thu, Feb 23, 2012 at 3:22 PM, David Medinets <david.medinets@gmail.com>wrote:

> Please elaborate. What is a locality group? Why are they important?
> Please cite examples.
>
> On Thu, Feb 23, 2012 at 3:04 PM, Keith Turner <keith@deenlo.com> wrote:
> > One thing to consider when deciding if you want to use a column family
> > or qualifier is locality groups.  Column families can be placed in
> > locality groups.
> >
> > On Thu, Feb 23, 2012 at 2:37 PM, David Medinets
> > <david.medinets@gmail.com> wrote:
> >> In this kind of situation, I've seen marker classes used so the intent
> >> of the parameter is clear. Marker classes also make it feasible to
> >> overload method signatures to allow for missing parameters.
> >>
> >> While you have explained using the empty string as a column qualifier,
> >> you haven't explained the column qualifier itself. I won't be
> >> misdirected so easily :)
> >>
> >> On Thu, Feb 23, 2012 at 11:55 AM, Adam Fuchs <adam.p.fuchs@ugov.gov>
> wrote:
> >>> When we don't have an entry for an element of the key or the value we
> >>> typically use the empty string (or zero-length byte array). This is
> >>> technically different from null, but it seems to work well for all the
> use
> >>> cases we've encountered so far. Here are some examples:
> >>>
> >>>     Mutation m = new Mutation("david");
> >>>     m.put("age","","23, no, really");
> >>>
> >>>     Mutation m = new Mutation("david");
> >>>     m.put("age","23, honest", "");
> >>>
> >>>     byte[] empty = new byte[0];
> >>>     Mutation m = new Mutation(new Text("david"));
> >>>     m.put("age","23, honest",new Value(empty));
> >>>
> >>> The first example uses a zero-length column qualifier and populates the
> >>> value. The second and third examples result in exactly the same
> entries in
> >>> Accumulo, with a populated column qualifier and a zero-length value.
> >>>
> >>> Cheers,
> >>> Adam
> >>>
> >>>
> >>> On Wed, Feb 22, 2012 at 10:17 PM, David Medinets <
> david.medinets@gmail.com>
> >>> wrote:
> >>>>
> >>>> Perhaps I've overlooked it, but I don't see a definition of Column
> >>>> Qualifier in the User Manual. Is there a best practice recommendation
> >>>> when the data is simply row,column,value (i.e., david,age,23). Can the
> >>>> qualifier or value be null?
> >>>
> >>>
>

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