[PIG-1309] Sort Merge Cogroup

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.7.0, 0.8.0
• Component/s: impl

Release Note

With this patch, it is now possible to perform map-side cogroup if data is sorted and loader implements certain interfaces. Primary algorithm is based on sort-merge join with additional restrictions.

Following preconditions must be met to use this feature:
1) No other operations can be done between load and cogroup statements.
2) Data must be sorted on join keys for all tables 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 outer joins (using merge) (PIG-1353) 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 = COGROUP A by id, B by id using 'merge';

• Assignee: Ashutosh Chauhan
• Reporter: Ashutosh Chauhan
• Created: 19/Mar/10 18:46
• Updated: 17/Dec/10 22:43
• Resolved: 09/Jul/10 17:49

Description

In never ending quest to make Pig go faster, we want to parallelize as many relational operations as possible. Its already possible to do Group-by( PIG-984 ) and Joins( PIG-845 , PIG-554 ) purely in map-side in Pig. This jira is to add map-side implementation of Cogroup in Pig. Details to follow.