Lorand Bendig created PIG-3642:
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Summary: Direct HDFS access for small jobs (fetch)
Key: PIG-3642
URL: https://issues.apache.org/jira/browse/PIG-3642
Project: Pig
Issue Type: Improvement
Reporter: Lorand Bendig
Assignee: Lorand Bendig
Fix For: 0.13.0
With this patch I'd like to add the possibility to directly read data from HDFS
instead of launching MR jobs in case of simple (map-only) tasks. Hive already
has this feature (fetch). This patch shares some similarities with the local
mode of Pig 0.6. Here, fetching kicks off when the following holds for a script:
* it contains only LIMIT, FILTER, UNION (if no split is generated), STREAM,
(nested) FOREACH with expression operators, custom UDFs..etc
* no scalar aliases
* no SampleLoader
* single leaf job
* DUMP (no STORE)
The feature is enabled by default and can be toggled with:
* -N or -no_fetch
* set opt.fetch true/false;
There's no STORE support because I wanted to make it explicit that this
"optimization" is for launching small/simple scripts during development, rather
than querying and filtering large number of rows on the client machine.
However, a threshold could be given on the input size (an estimation) to
determine whether to prefer fetch over MR jobs, similar to what Hive's
'{{hive.fetch.task.conversion.threshold}}' does. (through Pig's
LoadMetadata#getStatistic ?)
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