PERFORMANCE: The RandomSampleLoader should be changed to allow it subsume
another loader
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Key: PIG-820
URL: https://issues.apache.org/jira/browse/PIG-820
Project: Pig
Issue Type: Improvement
Components: impl
Affects Versions: 0.3.0
Reporter: Alan Gates
Assignee: Alan Gates
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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