Re: What is a relation?
I think the question isn't about feature or problem, or how hard it
is to implement. It's about what level we want the language at. The
advantage of making a relation equivalent to a bag, as Ted and others
point out, is ease comprehension on the part of the users. They are
not required to maintain an artificial distinction between what's
happening in parallel and what's happening on a single node. To put
Pradeep's point a slightly different way, by hiding this distinction
we make it harder for the users to understand how pig will process
their scripts. Moving Pig Latin further from execution will mean
that users will, at times, make less optimal choices in writing their
scripts because they may not realize that counting a bag at the top
level has a very different cost than counting a bag nested in a
foreach. So the choice is between a higher level abstraction that is
easier to think about (e.g. Python) or a lower level abstraction that
forces the user to think more like the machine and thus hopefully
make better choices (e.g. C). It sounds like most of the community
is voting for the higher abstraction.
To respond to Pradeep's statement about the filter, that
B = filter A.bg by x < 100;
which, if I understood correctly, we would be saying that we're
filtering out records of bg where bg.x is < 100, should be legal if
we say all relations are bag. I think this is incorrect. The filter
in this statement is acting on A, not A.bg. The correct way to write
the above statement would be:
B = foreach A {
A1 = filter bg.x < 100;
generate A1;
}
I believe this holds whatever we say about relations being bags. So
the semantic is that a relational operator always applies only to the
relation/bag it is applied to. In order to access elements of a
relation/bag, the foreach operator is provided.
Alan.
On Dec 11, 2008, at 12:09 PM, Ted Dunning wrote:
All of what you say sounds like a feature to me rather than a problem.
Yes, the implementor needs to do it right, but that kind of goes
with the
territory.
On Thu, Dec 11, 2008 at 11:32 AM, Pradeep Kamath inc.com>wrote:
I find it somewhat inconsistent that we treat both relations and bags
the same.
SIZE(A) where A is real bag will be different in implementation than
SIZE(A) where A is a relation - For the former, all the data is
already
in a container and one can just inspect the size. For the latter, you
have to do a group ALL-COUNT - this would be very confusing from a
backend implementation point of view.
If we do treat relations and bags as equivalent, then all statements
which currently work on relations should work on bags (say in my
input
data). Here is an example:
A = load 'bla' as (bg:{t:(x:int, y, z)}, str:chararray);
B = filter A.bg by x < 100; -- Directly access the bag "bg" inside A
(which is supposed to be bag too) and filter on on it - likewise
other
operations possible on relations should work).
Also A = load 'bla'; B = COUNT(A); will have to be supported
(implicitly
by a map reduce boundary doing a group ALL -COUNT). This will be done
under the covers and it may not be obvious to a user that and
explicit
group ALL - COUNT and a direct COUNT(A) are the same.
Thanks,
Pradeep
-Original Message-
From: Olga Natkovich [mailto:ol...@yahoo-inc.com]
Sent: Thursday, December 11, 2008 11:12 AM
To: pig-dev@hadoop.apache.org
Subject: RE: What is a relation?
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.
Olga
-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 ,
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 pas
Re: What is a relation?
You are correct.
This is a problem with the language. The same function means different
things in different contexts with (essentially) the same kind of input.
Do you propose to fix the user confusion caused by inconsistent language
constructs (aka a language defect) by an even less consistent
implementation?
That seems backwards to me. To fix a language defect, the language should
be changed so that users get a consistent world view. Then implementations
should implement that.
There a bunch of different ways to fix this. Here are two. Note that
neither has the benefit of more than a few seconds of thought and that
thought was from a person who is not an expert (by any stretch).
proposal 1) COUNT and all other scalar producing functions always return a
relation with a single row and column that contains the long of interest.
All operations that require a long will transparently unwrap the desired
value from such singleton relations transparently.
proposal 2) COUNT always returns a scalar value, but all scalar values are
transparently treated as singleton relations when necessary.
On Thu, Dec 11, 2008 at 12:15 PM, Santhosh Srinivasan wrote:
> A = load 'input';
> B = COUNT(A);
> C = group A by $0;
> D = foreach C { X = COUNT(A); generate X:};
>
> Is B a relation?
>
--
Ted Dunning, CTO
DeepDyve
4600 Bohannon Drive, Suite 220
Menlo Park, CA 94025
www.deepdyve.com
650-324-0110, ext. 738
858-414-0013 (m)
RE: What is a relation?
In the existing implementation, Pig has the following dilemma.
1. A subset of the relational operators are allowed inside a foreach
E.g: filter, distinct, order by
2. Non relational operators that are allowed inside a foreach are not
allowed outside a foreach
E.g: Projections: A = B.$0;, Assignments for scalars: X = COUNT(D);,
etc.
Lets assume that a relation is a bag (and vice versa too).
Scenario 1:
---
In the future if there are plans to allow all operators that exist
inside a foreach outside and vice versa, we will have the following
problem:
A = load 'input';
B = COUNT(A);
C = group A by $0;
D = foreach C { X = COUNT(A); generate X:};
Is B a relation?
Yes - is B a bag that contains tuples of longs?
No - is B a scalar of type long?
Scenario 2:
---
If there are no plans to allow operators inside a foreach outside (and
not vice versa).
It makes good sense to treat relations as bags and vice versa but there
are some open questions:
1. Do storage functions indicate that the stored data is a bag?
Likewise, do load functions treat the stored data as bags?
2. Will there be an equivalence of the operators wherein bags can
replace relations in all operators that support relational operator
inputs?
E.g: Pradeep alluded to the use of a bag column inside a relation with
other relational operators.
A = load 'input' as (x: int, b: {t:(a: int)});
B = filter A.b by a > 10;
Conclusion
---
The equivalence of bags and relations is influenced by the long term
plan of what will be legal in the language and not necessarily
influenced by the current state of the language. In the short term, it
makes sense to treat relations as bags (and vice versa in some cases).
In the long term, relations should be treated as its own type and define
legal operations on this type.
Santhosh
-Original Message-
From: Ted Dunning [mailto:ted.dunn...@gmail.com]
Sent: Thursday, December 11, 2008 12:10 PM
To: pig-dev@hadoop.apache.org
Subject: Re: What is a relation?
All of what you say sounds like a feature to me rather than a problem.
Yes, the implementor needs to do it right, but that kind of goes with
the
territory.
On Thu, Dec 11, 2008 at 11:32 AM, Pradeep Kamath
wrote:
> I find it somewhat inconsistent that we treat both relations and bags
> the same.
>
> SIZE(A) where A is real bag will be different in implementation than
> SIZE(A) where A is a relation - For the former, all the data is
already
> in a container and one can just inspect the size. For the latter, you
> have to do a group ALL-COUNT - this would be very confusing from a
> backend implementation point of view.
>
> If we do treat relations and bags as equivalent, then all statements
> which currently work on relations should work on bags (say in my input
> data). Here is an example:
> A = load 'bla' as (bg:{t:(x:int, y, z)}, str:chararray);
> B = filter A.bg by x < 100; -- Directly access the bag "bg" inside A
> (which is supposed to be bag too) and filter on on it - likewise other
> operations possible on relations should work).
>
> Also A = load 'bla'; B = COUNT(A); will have to be supported
(implicitly
> by a map reduce boundary doing a group ALL -COUNT). This will be done
> under the covers and it may not be obvious to a user that and explicit
> group ALL - COUNT and a direct COUNT(A) are the same.
>
>
> Thanks,
> Pradeep
>
> -Original Message-
> From: Olga Natkovich [mailto:ol...@yahoo-inc.com]
> Sent: Thursday, December 11, 2008 11:12 AM
> To: pig-dev@hadoop.apache.org
> Subject: RE: What is a relation?
>
> 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.
>
> Olga
>
> > -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 ,
> > where A is a bag. This does not necessarily mean that a
> > relation is a b
Re: What is a relation?
All of what you say sounds like a feature to me rather than a problem.
Yes, the implementor needs to do it right, but that kind of goes with the
territory.
On Thu, Dec 11, 2008 at 11:32 AM, Pradeep Kamath wrote:
> I find it somewhat inconsistent that we treat both relations and bags
> the same.
>
> SIZE(A) where A is real bag will be different in implementation than
> SIZE(A) where A is a relation - For the former, all the data is already
> in a container and one can just inspect the size. For the latter, you
> have to do a group ALL-COUNT - this would be very confusing from a
> backend implementation point of view.
>
> If we do treat relations and bags as equivalent, then all statements
> which currently work on relations should work on bags (say in my input
> data). Here is an example:
> A = load 'bla' as (bg:{t:(x:int, y, z)}, str:chararray);
> B = filter A.bg by x < 100; -- Directly access the bag "bg" inside A
> (which is supposed to be bag too) and filter on on it - likewise other
> operations possible on relations should work).
>
> Also A = load 'bla'; B = COUNT(A); will have to be supported (implicitly
> by a map reduce boundary doing a group ALL -COUNT). This will be done
> under the covers and it may not be obvious to a user that and explicit
> group ALL - COUNT and a direct COUNT(A) are the same.
>
>
> Thanks,
> Pradeep
>
> -Original Message-
> From: Olga Natkovich [mailto:ol...@yahoo-inc.com]
> Sent: Thursday, December 11, 2008 11:12 AM
> To: pig-dev@hadoop.apache.org
> Subject: RE: What is a relation?
>
> 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.
>
> Olga
>
> > -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 ,
> > 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
RE: What is a relation?
I find it somewhat inconsistent that we treat both relations and bags
the same.
SIZE(A) where A is real bag will be different in implementation than
SIZE(A) where A is a relation - For the former, all the data is already
in a container and one can just inspect the size. For the latter, you
have to do a group ALL-COUNT - this would be very confusing from a
backend implementation point of view.
If we do treat relations and bags as equivalent, then all statements
which currently work on relations should work on bags (say in my input
data). Here is an example:
A = load 'bla' as (bg:{t:(x:int, y, z)}, str:chararray);
B = filter A.bg by x < 100; -- Directly access the bag "bg" inside A
(which is supposed to be bag too) and filter on on it - likewise other
operations possible on relations should work).
Also A = load 'bla'; B = COUNT(A); will have to be supported (implicitly
by a map reduce boundary doing a group ALL -COUNT). This will be done
under the covers and it may not be obvious to a user that and explicit
group ALL - COUNT and a direct COUNT(A) are the same.
Thanks,
Pradeep
-Original Message-
From: Olga Natkovich [mailto:ol...@yahoo-inc.com]
Sent: Thursday, December 11, 2008 11:12 AM
To: pig-dev@hadoop.apache.org
Subject: RE: What is a relation?
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.
Olga
> -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 ,
> 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.
>
Re: What is a relation?
+1
I always vote for consistency for the user. The difference between bags and
relations is a really subtle point for most users.
PIG should, frankly, be free to handle tiny computations as a bag even for
the outer loop. There is no absolute requirement that a program be executed
as map-reduce. Due to the functional nature of the language, PIG could even
speculatively try to execute every computation with small inputs as a bag
and abort if the computation takes more than, say, 2 seconds. That is still
small in comparison to the startup time of an MR program and the win would
be really big for cases where it works. There are bound to be gobs of other
tricks that would take more than 10 seconds to come up with.
On Thu, Dec 11, 2008 at 11:11 AM, Olga Natkovich wrote:
> 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.
>
> Olga
>
> > -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 ,
> > 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.
> >
>
--
Ted Dunning, CTO
DeepDyve
4600 Bohannon Drive, Suite 220
Menlo Park, CA 94025
www.deepdyve.com
650-324-0110, ext. 738
858-414-0013 (m)
RE: What is a relation?
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.
Olga
> -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 ,
> 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.
>
Re: What is a relation?
Here is an example that I have given a while ago in JIRA Pig-158 :-
A = LOAD 'fil1' ;
B = A.($0,$1) ;
STORE B ;
which is similar to your top-level projection example.
I believe there is no distinction between so-called relations and bags in
our context.
"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." <== As I proposed before,
whether to run across many nodes or not should have nothing to do with
top-level or inner-level. The factor which comes into play should rather be
"job size" which is heuristically calculated.
To give users some power to control whether to run across nodes or not, we
may later on introduce a hint keyword instead. This keeps the language
simple but yet powerful if needed.
Pi
On Sat, Dec 6, 2008 at 1:04 PM, Alan Gates <[EMAIL PROTECTED]> wrote:
> 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 , 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.
>
