Eric Gaudet updated PIG-795:

    Attachment: sample2.diff

This patch implements the SAMPLE command. It basically add a random sample mode 
to the LIMIT class. 

The syntax is like LIMIT: "a = SAMPLE x", where x is an integer and 0<=x<=100. 
Each row will be selected if rand()<(x/100).


    a = LOAD 'mybigdata'
    b = SAMPLE 5

will select 5% of the data.

> Command that selects a random sample of the rows, similar to LIMIT
> ------------------------------------------------------------------
>                 Key: PIG-795
>                 URL: https://issues.apache.org/jira/browse/PIG-795
>             Project: Pig
>          Issue Type: New Feature
>          Components: impl
>            Reporter: Eric Gaudet
>            Priority: Trivial
>         Attachments: sample2.diff
> When working with very large data sets (imagine that!), running a pig script 
> can take time. It may be useful to run on a small subset of the data in some 
> situations (eg: debugging / testing, or to get fast results even if less 
> accurate.) 
> The command "LIMIT N" selects the first N rows of the data, but these are not 
> necessarily randomzed. A command "SAMPLE X" would retain the row only with 
> the probability x%.
> Note: it is possible to implement this feature with FILTER BY and an UDF, but 
> so is LIMIT, and limit is built-in.

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