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From Christopher <ctubb...@apache.org>
Subject Re: scanner question in regards to columns loaded
Date Sun, 26 Jan 2014 22:44:24 GMT
Ah, I see. Well, you could do that with a custom filter (iterator),
but otherwise, no, not unless you had some other special per-term
entry to query (rather than per-term/document pair). The design of
this kind of table though, seems focused on finding documents which
contain the given terms, though, not listing all terms seen. If you
need that additional feature and don't want to write a custom filter,
you could achieve that by putting a special entry in its own row for
each term, in addition to the entries per-term/document pair, as in:

RowID                       ColumnFamily     Column Qualifier     Value
<term1>                    term                   -                            -
<term1>=<doc_id2>   index                  count                     5

Then, you could list terms by querying the "term" column family
without getting duplicates. And, you could get decent performance with
this scan if you put the "term" column family and the "index" column
family in separate locality groups. You could even make this entry an
aggregated count for all documents (see documentation for combiners),
in case you want corpus-wide term frequencies (for something like
TF-IDF computations).

--
Christopher L Tubbs II
http://gravatar.com/ctubbsii


On Sun, Jan 26, 2014 at 7:55 AM, Jamie Johnson <jej2003@gmail.com> wrote:
> I mean if a user asked for all terms that started with "term" is there a way
> to get term1 and term2 just once while scanning or would I get each twice,
> once for each docid and need to filter client side?
>
> On Jan 26, 2014 1:33 AM, "Christopher" <ctubbsii@apache.org> wrote:
>>
>> If you use the Range constructor that takes two arguments, then yes,
>> you'd get two entries. However, "count" would come before "doc_id",
>> though, because the qualifier is part of the Key, and therefore, part
>> of the sort order. There's also a Range constructor that allows you to
>> specify whether you want the startKey and endKey to be inclusive or
>> exclusive.
>>
>> I don't know of a specific document that outlines various strategies
>> that I can link to. Perhaps I'll put one together, when I get some
>> spare time, if nobody else does. I think most people do a lot of
>> experimentation to figure out which strategies work best.
>>
>> I'm not entirely sure what you mean about "getting an iterator over
>> all terms without duplicates". I'm assuming you don't mean duplicate
>> versions of a single entry, which is handled by the
>> VersioningIterator, which should be on new tables by default, and set
>> to retain the recent 1 version, to support updates. With the scheme I
>> suggested, your table would look something like the following,
>> instead:
>>
>> RowID                       ColumnFamily     Column Qualifier     Value
>> <term1>=<doc_id1>   index                  count                    
10
>> <term1>=<doc_id2>   index                  count                    
5
>> <term2>=<doc_id3>   index                  count                    
3
>> <term3>=<doc_id1>   index                  count                    
12
>>
>> With this scheme, you'd have only a single entry (a count) for each
>> row, and a single row for each term/document combination, so you
>> wouldn't have any duplicate counts for any given term/document. If
>> that's what you mean by duplicates...
>>
>>
>> --
>> Christopher L Tubbs II
>> http://gravatar.com/ctubbsii
>>
>>
>> On Sat, Jan 25, 2014 at 12:19 AM, Jamie Johnson <jej2003@gmail.com> wrote:
>> > Thanks for the reply Chris.  Say I had the following
>> >
>> > RowID     ColumnFamily     Column Qualifier     Value
>> > term         Occurrence~1     doc_id                    1
>> > term         Occurrence~1     count                      10
>> > term2       Occurrence~2      doc_id                     2
>> > term2       Occurrence~2      count                      1
>> >
>> > creating a scanner with start key new Key(new Text("term"), new
>> > Text("Occurrence~1")) and end key new Key(new Text("term"), new
>> > Text("Occurrence~1")) I would get an iterator with two entries, the
>> > first
>> > key would be doc_id and the second would be count.  Is that accurate?
>> >
>> > In regards to the other strategies is there anywhere that some of these
>> > are
>> > captured?  Also in the your example, how would you go about getting an
>> > iterator over all terms without duplicates?  Again thanks
>> >
>> >
>> > On Fri, Jan 24, 2014 at 11:34 PM, Christopher <ctubbsii@apache.org>
>> > wrote:
>> >>
>> >> It's not quite clear what you mean by "load", but I think you mean
>> >> "iterate over"?
>> >>
>> >> A simplified explanation is this:
>> >>
>> >> When you scan an Accumulo table, you are streaming each entry
>> >> (Key/Value pair), one at a time, through your client code. They are
>> >> only held in memory if you do that yourself in your client code. A row
>> >> in Accumulo is the set of entries that share a particular value of the
>> >> Row portion of the Key. They are logically grouped, but are not
>> >> grouped in memory unless you do that.
>> >>
>> >> One additional note is regarding your index schema of a row being a
>> >> search term and columns being documents. You will likely have issues
>> >> with this strategy, as the number of documents for high frequency
>> >> terms grows, because tablets do not split in the middle of a row. With
>> >> your schema, a row could get too large to manage on a single tablet
>> >> server. A slight variation, like concatenating the search term with a
>> >> document identifier in the row (term=doc1, term=doc2, ....) would
>> >> allow the high frequency terms to split into multiple tablets if they
>> >> get too large. There are better strategies, but that's just one simple
>> >> option.
>> >>
>> >>
>> >> --
>> >> Christopher L Tubbs II
>> >> http://gravatar.com/ctubbsii
>> >>
>> >>
>> >> On Fri, Jan 24, 2014 at 10:23 PM, Jamie Johnson <jej2003@gmail.com>
>> >> wrote:
>> >> > If I have a row that as the key is a particular term and a set of
>> >> > columns
>> >> > that stores the documents that the term appears in if I load the row
>> >> > is
>> >> > the
>> >> > contents of all of the columns also loaded?  Is there a way to page
>> >> > over
>> >> > the
>> >> > columns such that only N columns are in memory at any point?  In this
>> >> > particular case the documents are all in a particular column family
>> >> > (say
>> >> > docs) and the column qualifier is created dynamically, for arguments
>> >> > sake we
>> >> > can say they are UUIDs.
>> >
>> >

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