Pradeep Kamath commented on PIG-820:

Some review comments:
In SampleOptimizer.java, 
      LoadFunc lf = 
 should be changed to 
      LoadFunc lf = 
This is so that we correctly handle loaders which do not have default 
constuctor. FuncSpec encapsulates both the classname and constructor arguments 
and hence would handle both loaders which only have default constructor and 
those which only have constructor with args.

fs = new FileSpec(predFs.getFileName(), new FuncSpec(predFs.getFuncName()));
should be changed to
      fs = new FileSpec(predFs.getFileName(), predFs.getFuncSpec());

Likewise, the constructor of RandomSampleLoader should take a FuncSpec object 
as its first argument to represent the loader classname and constructor args. 
So this will require callers who create RandomSampleLoader to create it with 
correct funcspec objects.

> PERFORMANCE:  The RandomSampleLoader should be changed to allow it subsume 
> another loader
> -----------------------------------------------------------------------------------------
>                 Key: PIG-820
>                 URL: https://issues.apache.org/jira/browse/PIG-820
>             Project: Pig
>          Issue Type: Improvement
>          Components: impl
>    Affects Versions: 0.3.0, 0.4.0
>            Reporter: Alan Gates
>            Assignee: Ashutosh Chauhan
>             Fix For: 0.4.0
>         Attachments: pig-820.patch, pig-820_v2.patch, pig-820_v3.patch
> Currently a sampling job requires that data already be stored in 
> BinaryStorage format, since RandomSampleLoader extends BinaryStorage.  For 
> order by this
> has mostly been acceptable, because users tend to use order by at the end of 
> their script where other MR jobs have already operated on the data and thus it
> is already being stored in BinaryStorage.  For pig scripts that just did an 
> order by, an entire MR job is required to read the data and write it out
> in BinaryStorage format.
> As we begin work on join algorithms that will require sampling, this 
> requirement to read the entire input and write it back out will not be 
> acceptable.
> Join is often the first operation of a script, and thus is much more likely 
> to trigger this useless up front translation job.
> Instead RandomSampleLoader can be changed to subsume an existing loader, 
> using the user specified loader to read the tuples while handling the skipping
> between tuples itself.  This will require the subsumed loader to implement a 
> Samplable Interface, that will look something like:
> {code}
> public interface SamplableLoader extends LoadFunc {
>     /**
>      * Skip ahead in the input stream.
>      * @param n number of bytes to skip
>      * @return number of bytes actually skipped.  The return semantics are
>      * exactly the same as {...@link java.io.InpuStream#skip(long)}
>      */
>     public long skip(long n) throws IOException;
>     /**
>      * Get the current position in the stream.
>      * @return position in the stream.
>      */
>     public long getPosition() throws IOException;
> }
> {code}
> The MRCompiler would then check if the loader being used to load data 
> implemented the SamplableLoader interface.  If so, rather than create an 
> initial MR
> job to do the translation it would create the sampling job, having 
> RandomSampleLoader use the user specified loader.

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