GitHub user gczsjdy opened a pull request:

    Add aggreagate expression nGrams

    ## What changes were proposed in this pull request?
    This is the implementation of Hive's `ngrams`, which is a popular 
statistical and data mining function.
    N-grams are  subsequences of length N drawn from a longer sequence. The 
purpose of the `ngrams` aggregate expression is to find the k most frequent 
n-grams from one or more sequences.
    This expression has the pattern of : ngrams(children: 
Array<Array<String>>(or Array<String>), n: Int, k: Int, accuracy: Int), all 
parameters are of `Expresion` type, I use the above way to indicate the type 
after evaluation. Among the parameters, 
    children indicates the 'longer sequence' we collect n-grams from;
    n & k indicate the top k n-grams;
    accuracy is related to the memory used for frequency estimation, more 
memory will give more accurate frequency counts.
    ## How was this patch tested?
    Unit tests are in `NGramsQuerySuite` 

You can merge this pull request into a Git repository by running:

    $ git pull UdafNGram

Alternatively you can review and apply these changes as the patch at:

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #17359


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