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Date: Thu, 23 Feb 2012 15:22:42 -0500
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Subject: Re: Definition of Column Qualifier in User Manual
From: David Medinets <david.medinets@gmail.com>
To: accumulo-user@incubator.apache.org
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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. =A0Column 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> wro=
te:
>>> 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:
>>>
>>> =A0 =A0 Mutation m =3D new Mutation("david");
>>> =A0 =A0 m.put("age","","23, no, really");
>>>
>>> =A0 =A0 Mutation m =3D new Mutation("david");
>>> =A0 =A0 m.put("age","23, honest", "");
>>>
>>> =A0 =A0 byte[] empty =3D new byte[0];
>>> =A0 =A0 Mutation m =3D new Mutation(new Text("david"));
>>> =A0 =A0 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?
>>>
>>>