[PIG-847] Setting twoLevelAccessRequired field in a bag schema should not be required to access fields in the tuples of the bag

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.3.0
• Fix Version/s: 0.9.0
• Hadoop Flags: Reviewed
• Release Note: In outputSchema method of UDF, user no longer need to set two level access flag for a bag schema. Pig only do vertical slice of a bag. User can refer to a particular field of a bag, but there is no way to refer a particular row of a bag.
• Assignee: Daniel Dai
• Reporter: Pradeep Kamath
• Created: 12/Jun/09 22:33
• Updated: 04/Aug/11 00:34
• Resolved: 08/Feb/11 18:22

Description

Currently Pig interprets the result type of a relation as a bag. However the schema of the relation directly contains the schema describing the fields in the tuples for the relation. However when a udf wants to return a bag or if there is a bag in input data or if the user creates a bag constant, the schema of the bag has one field schema which is that of the tuple. The Tuple's schema has the types of the fields. To be able to access the fields from the bag directly in such a case by using something like <bagname>.<fieldname> or <bag>.<fieldposition>, the schema of the bag should have the twoLevelAccess set to true so that pig's type system can get traverse the tuple schema and get to the field in question. This is confusing - we should try and see if we can avoid needing this extra flag. A possible solution is to treat bags the same way - whether they represent relations or real bags. Another way is to introduce a special "relation" datatype for the result type of a relation and bag type would be used only for true bags. In this case, we would always need bag schema to have a tuple schema which would describe the fields.

Issue Links

• blocks: PIG-998 revisit frontend logic and pig-latin semantics