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https://issues.apache.org/jira/browse/PIG-1460?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12888090#action_12888090
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Daniel Dai commented on PIG-1460:
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Hi, Corinne,
Can you update Pig UDF Manual? After section "LoadPushDown", add a subsection
(pushProjection() is a method of LoadPushDown (also help to polish my writing):
pushProjection(): This method tells LoadFunc which fields is used in Pig, so
LoadFunc is able to optimize not to load unneeded fields. pushProjection takes
a RequiredFieldList. Each item inside RequiredFieldList indicates a required
field Pig needs. Pig will use column index RequiredField.index to communicate
with the LoadFunc about the fields Pig needs. If the field is a map, Pig will
optionally pass RequiredField.subFields which contains a list of keys Pig needs
for that map. For example, if we need two keys "key1", "key2" for the map, we
will have a subFields for that map which contains two RequiredField. The alias
field for 1st RequiredField is "key1", alias field for 2nd RequiredField is
"key2". RequiredField.type is reserved for future use. LoadFunc will use
RequiredFieldResponse.requiredFieldRequestHonored to indicate whether the
pushProjection request is honored.
> UDF manual and javadocs should make clear how to use RequiredFieldList
> ----------------------------------------------------------------------
>
> Key: PIG-1460
> URL: https://issues.apache.org/jira/browse/PIG-1460
> Project: Pig
> Issue Type: Bug
> Components: documentation
> Affects Versions: 0.7.0
> Reporter: Alan Gates
> Assignee: Daniel Dai
> Priority: Minor
> Fix For: 0.8.0
>
> Attachments: PIG-1460-1.patch
>
>
> The UDF manual mentions that load function writers need to handle
> RequiredFieldList passed to LoadPushDown.pushProjection, but it does not
> specify how the writer should interpret the contents of that list. The
> javadoc is similarly vague.
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