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From Alan Gates <ga...@yahoo-inc.com>
Subject Re: Issues with group as an alias
Date Fri, 13 Jun 2008 14:26:18 GMT
All,

I too will vote for III, with the caveat that we don't give names to 
multi-field grouping keys.  We need to make sure we support AS to allow 
the user to name their grouping keys if they want.

So far, the vote totals are:
I: 1
II: 0
III: 3
IV: 0
V: 0

I'd like to make a decision and move forward by mid next week.  If you 
haven't voted and you'd like to, please do so now.  If you feel 
passionately about one of the options that is loosing, please make your 
arguments now.

Alan.

Alan Gates wrote:
> Currently in Pig Latin, anytime a (CO)GROUP statement is used, the 
> field (or set of fields) that are grouped on are given the alias 
> 'group'.  This has a couple of issues:
>
> 1)  It's confusing.  'group' is now a keyword and an alias.
> 2)  We don't currently allow 'group' as an alias in an AS.  It is 
> strange to have an alias that can only be assigned by the language and 
> never by the user.
>
> Possible solutions:
>
> I) Status quo.  We could fix it so that group is allowed to be 
> assigned as an alias in AS.
>
> Pros:  Backward compatibility
> Cons: a) will make the parser more complicated
>      b) see 1) above.
>
>
> II) Don't give an implicit alias to the group key(s).  If users want 
> an alias, they can assign it using AS.
>
> Pros:  Simplicity
> Cons:  We do assign aliases to grouped bags.  That is, if we have C = 
> GROUP B by $0 the resulting schema of C is (group, B).  So if we don't 
> assign an alias to the group key, we now have a schema ($0, B).  This 
> seems strange.  And worse yet, if users want to alias the group 
> key(s), they'll be forced to alias all the grouped bags as well.
>
> III) Carry the alias (if any) that the field had before.  So if we had 
> a script like:
>
> A = load 'myfile' as (x, y, z);
> B = group A by x;
>
> The the schema of B would be (x, A).  This is quite natural for 
> grouping of single columns.  But it turns nasty when you group on 
> multiple columns.  Do we then append the names to together?  So if you 
> have
>
> B = group A by x, y;
>
> is the resulting schema (x_y, A)?  Ugh.
>
> In this case there is also the question of what to do in the case of 
> cogroups, where the key may be named differently in different relations.
>
> A = load 'myfile' as (x, y, z);
> B = load 'myotherfile' as (t, u, v);
> C = cogroup A by x, B by t;
>
> Is the resulting schema (x, A, B) or (t, A, B) or are both valid?  
> This could be resolved by either saying first one always wins, or 
> allowing either.
>
> Pros:  Very natural for the users, their fields maintain names through 
> the query.
> Cons:  Quickly gets burdensome in the case of multi-key groups.
>
> IV) Assign a non-keyword alias to the group key, like grp or groupkey 
> or grpkey (or some other suitable choice).
> Pros:  Least disruptive change.  Users only have to go through their 
> scripts and find places where they use the group alias and change it 
> to grp (or whatever).
> Cons:  Still leaves us with a situation where we are assigning a name 
> to a field arbtrarily, leaving users confused as to how their fields 
> got named that.
>
> V) Remove GROUP as a keyword.  It is just short for COGROUP of one 
> relation anyway.
>
> Pros:  Smaller syntax in a language is always good.
> Cons:  Will break a lot of scripts, and confuse a lot of users who 
> only think in terms of GROUP and JOIN and never use COGROUP explicitly.
>
> One could also conceive of combinations of these.  For example, we 
> always assign a name like grpkey to the group key(s), and in the 
> single key case we also carry forward the alias that the field already 
> had, if any.
>
> Thoughts?  Other possibilities?
>
> Alan.

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