I think we should consider Bag and relations to be the same so that we
can handle processing in the outer script as well as inside of nested
foreach the same and make it easier to extend the set of operators
allowed inside of foreach block.


> -----Original Message-----
> From: Alan Gates [mailto:ga...@yahoo-inc.com] 
> Sent: Friday, December 05, 2008 6:04 PM
> To: pig-dev@hadoop.apache.org
> Subject: What is a relation?
> All,
> A question on types in pig.  When you say:
> A = load 'myfile';
> what exactly is A?  For the moment let us call A a relation, 
> since it is a set of records, and we can pass it to a 
> relational operator, such as FILTER, ORDER, etc.
> To clarify the question, is a relation equivalent to a bag?  
> In some ways it seems to be in our current semantics.  
> Certainly you can turn a relation into a bag:
> A = load 'myfile';
> B = group A all;
> The schema of the relation B at this point is <group, A>, 
> where A is a bag.  This does not necessarily mean that a 
> relation is a bag, because an operation had to occur to turn 
> the relation into a bag (the group all).
> But bags can be turned into relations, and then treated again 
> as if they were bags:
> C = foreach B {
>         C1 = filter A by $0 > 0;
>         generate COUNT(C1);
> }
> Here the bag A created in the previous grouping step is being 
> treated as it were a relation and passed to a relational 
> operator, and the resulting relation (C1) treated as a bag to 
> be passed COUNT.  So at a very minimum it seems that a bag is 
> a type of a relation, even if not all relations are bags.
> But, if top level (non-nested) relations are bags, why isn't 
> it legal to do:
> A = load 'myfile';
> B = A.$0;
> The second statement would be legal nested inside a foreach, 
> but is not legal at the top level.
> We have been aware of this discrepancy for a while, and lived 
> with it.  But I believe it is time to resolve it.  We've 
> noticed that some parts of pig assume an equivalence between 
> bag and relation (e.g. the  
> typechecker) and other parts do not (e.g. the syntax example 
> above).   
> This inconsistency is confusing to users and developers 
> alike.  As Pig Latin matures we need to strive to make it a 
> logically coherent and complete language.
> So, thoughts on how it ought to be?
> The advantage I see for saying a relation is equivalent to a 
> bag is simplicity of the language.  There is no need to 
> introduce another data type.  And it allows full relational 
> operations to occur at both the top level and nested inside foreach.
> But this simplicity also seems me the downside.  Are we 
> decoupling the user so far from the underlying implementation 
> that he will not be able to see side effects of his actions?  
> A top level relation is assumably spread across many chunks 
> and any operation on it will require one or more map reduce 
> jobs, whereas a relation nested in a  
> foreach is contained on one node.   This also makes pig much more  
> complex, because while it may hide this level of detail from 
> the user, it clearly has to understand the difference between 
> top level and nested operations and handle both cases.
> Alan.

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