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https://issues.apache.org/jira/browse/PIG-1353?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ashutosh Chauhan updated PIG-1353:
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Release Note:
With this patch, it is now possible to perform [left|right|full] outer joins on
two tables as well as inner joins on more then two tables in Pig in map-side if
data is sorted and loaders implement required interfaces. Primary algorithm is
based on sort-merge join.
Following preconditions should be met in order to use this feature:
1) No other operations can be done between load and join statements.
2) Data must be sorted on join keys in ASC order.
3) Nulls are considered smaller then everything. So, if data contains null
keys, they should occur before anything else.
4) Left-most loader must implement {CollectableLoader} interface as well as
{OrderedLoadFunc}.
5) All other loaders must implement {IndexableLoadFunc}.
6) Type information must be provided in schema for all the loaders.
Note that Zebra loader satisfies all of these conditions, so can be used out of
box.
Similar conditions apply to map-side cogroups (PIG-1309) as well.
Example:
A = load 'data1' using org.apache.hadoop.zebra.pig.TableLoader('id:int',
'sorted');
B = load 'data2' using org.apache.hadoop.zebra.pig.TableLoader('id:int',
'sorted');
C = join A by id left, B by id using 'merge';
.....
was:
With this patch, it is now possible to perform [left|right|full] outer joins on
two tables as well as inner joins on more then two tables in Pig in map-side if
data is sorted and one of the loader implements {{CollectableLoader}}
interface. Primary algorithm is based on sort-merge join.
Additional implementation details:
1) No other operations can be done between load and join statements.
2) Data must be sorted in ASC order.
3) Nulls are considered smaller then everything. So, if data contains null
keys, they should occur before anything else.
4) Left-most loader must implement CollectableLoader interface as well as
OrderedLoadFunc.
5) All other loaders must implement IndexableLoadFunc.
Note that Zebra loader satisfies all of these conditions, so can be used out of
box.
Similiar conditions apply to map-side cogroups (PIG-1309) as well.
Example:
A = load 'data1' using org.apache.hadoop.zebra.pig.TableLoader('', 'sorted');
B = load 'data2' using org.apache.hadoop.zebra.pig.TableLoader('', 'sorted');
C = join A by id left, B by id using 'merge';
.....
> Map-side outer joins
> --------------------
>
> Key: PIG-1353
> URL: https://issues.apache.org/jira/browse/PIG-1353
> Project: Pig
> Issue Type: Improvement
> Components: impl
> Reporter: Ashutosh Chauhan
> Assignee: Ashutosh Chauhan
> Fix For: 0.8.0
>
> Attachments: pig-1353.patch, pig-1353.patch
>
>
> Pig already has couple of map-side join implementations: Merge Join and
> Fragmented-Replicate Join. But both of them are pretty restrictive. Merge
> Join can only join two tables and that too can only do inner join. FR Join
> can join multiple relations, but it can also only do inner and left outer
> joins. Further it restricts the sizes of side relations. It will be nice if
> we can do map side joins on multiple tables as well do inner, left outer,
> right outer and full outer joins.
> Lot of groundwork for this has already been done in PIG-1309. Remaining will
> be tracked in this jira.
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