[PIG-2353] RANK function like in SQL - ASF JIRA

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Pig includes a new RANK operator:
RANK <relation> ( BY <column> (ASC|DES)? (DENSE)? )?
This operator prepends a consecutive integer to each tuple in the relation starting from 1.
If the BY clause is present, RANK sorts the relation before ranking it, otherwise it uses the order in which it receives the relation (e.g. the order in which the relation is stored if RANK is performed right after a LOAD).
The DENSE modifier produces a dense rank, which has no gaps in it regardless of ties.

RANK is now a reserved keyword and is *not* backward compatible.
Please review your scripts to avoid usage of RANK as a relation name.

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Pig includes a new RANK operator: RANK <relation> ( BY <column> (ASC|DES)? (DENSE)? )? This operator prepends a consecutive integer to each tuple in the relation starting from 1. If the BY clause is present, RANK sorts the relation before ranking it, otherwise it uses the order in which it receives the relation (e.g. the order in which the relation is stored if RANK is performed right after a LOAD). The DENSE modifier produces a dense rank, which has no gaps in it regardless of ties. RANK is now a reserved keyword and is *not* backward compatible. Please review your scripts to avoid usage of RANK as a relation name.

Implement a function that given a (sorted) bag adds to each tuple a unique, increasing identifier without gaps, like what RANK does for SQL.

This is a candidate project for Google summer of code 2012. More information about the program can be found at https://cwiki.apache.org/confluence/display/PIG/GSoc2012