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https://issues.apache.org/jira/browse/PIG-953?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12753828#action_12753828
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Dmitriy V. Ryaboy commented on PIG-953:
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bq. Pradeep: Pig only guarantees order with limit following order - for any
other relational operator following order there are no guarantees. Today it is
true that filter or a column pruning foreach would also preserve order but this
can change if needed in the future. There explicit code to ensure order-limit
combination works by preserving order - there is no such explicit check for
other operators (keeping it open for change in the future)
That actually tells me that an orderPreserving property on a LogicalOperator is
a really good idea.
That way we can set it to true on all operators that are at the moment
order-preserving (limit, filter, column-prining foreach), and not commit to
forever maintaining that contract. If filter starts changing order, the patch
will simply have to include a change to set orderPreserving to false in
POFilter, and everything will work automagically.
> Enable merge join in pig to work with loaders and store functions which can
> internally index sorted data
> ---------------------------------------------------------------------------------------------------------
>
> Key: PIG-953
> URL: https://issues.apache.org/jira/browse/PIG-953
> Project: Pig
> Issue Type: Improvement
> Affects Versions: 0.3.0
> Reporter: Pradeep Kamath
> Assignee: Pradeep Kamath
> Attachments: PIG-953.patch
>
>
> Currently merge join implementation in pig includes construction of an index
> on sorted data and use of that index to seek into the "right input" to
> efficiently perform the join operation. Some loaders (notably the zebra
> loader) internally implement an index on sorted data and can perform this
> seek efficiently using their index. So the use of the index needs to be
> abstracted in such a way that when the loader supports indexing, pig uses it
> (indirectly through the loader) and does not construct an index.
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