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From Alan Gates <ga...@yahoo-inc.com>
Subject Re: Getting query information while loading data
Date Tue, 12 Feb 2008 20:28:37 GMT
Comments at the end.

Charlie Groves wrote:
> On Feb 4, 2008, at 1:42 PM, Alan Gates wrote:
>> Charlie Groves wrote:
>>> On Jan 18, 2008, at 11:45 AM, Alan Gates wrote:
>>>> Our thinking of how to provide field metadata (name and eventually 
>>>> types) for pig queries was to allow several options:
>>>>    1) AS in the LOAD, as you can currently do for names.
>>>>    2) using an outside metadata service, where we would tell it the 
>>>> file name and it would tell us the metadata.
>>>>    3) Support self describing data formats such as JSON.
>>>> You're suggestion for a very simple schema provided in the first 
>>>> line of the file falls under category 3.  The trick here is that we 
>>>> need to be able to read that metadata about the fields at parse 
>>>> time (because we'd like to be able to do type checking and such).  
>>>> So in addition to the load function itself needing to examine the 
>>>> tuples, we need a way for the load function to read just enough of 
>>>> the file to tell the front end (on the client box, not on the 
>>>> map-reduce backend) the schema.  Maybe the best way to implement 
>>>> this is to have an interface that the load function would implement 
>>>> that lets the parser know that the load function can discover the 
>>>> metadata for it, and then the parser could call that load function 
>>>> before proceeding to type checking.
>>>> We're also interested in being able to tell the load function the 
>>>> fields needed in the query.  Even if you don't have field per file 
>>>> storage (aka columnar storage) it's useful to be able to 
>>>> immediately project out fields you know the query won't care about, 
>>>> as you can avoid translation costs and memory storage.
>>>> It's not clear to me that we need another interface to implement 
>>>> this.  We could just add a method "void neededColumns(Schema s)" to 
>>>> PigLoader.  As a post parsing step the parser would then visit the 
>>>> plan, as you suggest, and submit a schema to the PigLoader 
>>>> function.  It would be up to the specific loader implementation to 
>>>> decide whether to make use of the provided schema or not.
>>> I don't see the use for the first new function in addition to the 
>>> second.  If a schema is required by the query, the loader must be 
>>> able to produce data matching that schema.  If the loader can figure 
>>> out an internal schema, it can make that check that you describe in 
>>> function 1 in addition to structuring its data correctly as in 
>>> function 2.  If it can't determine its internal schema until it 
>>> loads data, then it can do neither and we have to wait until runtime 
>>> to see if it succeeds.  What about making the call "Schema 
>>> neededColumns(Schema s) throws IOException"?  The returned Schema is 
>>> the actual Schema that will be loaded which must be a superset of 
>>> the incoming Schema.  If the loader is unable to create the needed 
>>> schema, an IOException is thrown.
>> I'm not sure I understand what you're proposing.  I was trying to say 
>> that we need two separate things from the load function:
>> 1) A way to discover the schema of the data at parse time for type 
>> checking and query correctness checking (e.g. the user asked for 
>> field 5, is there a field 5?)  This is needed for metadata option 3, 
>> where the metadata is described by the data (as in JSON) or where the 
>> metadata is located in a file associated with the data.  We want to 
>> detect these kinds of errors before we submit to the backend (i.e. 
>> Hadoop) so that we can give the earliest possible error feedback.
>> 2) A way to indicate to the load function the schema it needs to 
>> load, as a way to support columnar storage schemes (such as you 
>> propose) or pushing projection down into the load.
>> Were you saying that you didn't think one of those is necessary, or 
>> are you saying that you think we can accomplish both with one 
>> function being adding to the load function?
> I'm saying that both can be accomplished with one new function on the 
> load func: Schema neededColumns(Schema s) throws IOException.  s is 
> the schema derived from the query, and the load func can use it to 
> satisfy your first requirement.  If it can check its underlying data, 
> it can then compare it to the schema in s and throw an IOException if 
> it can't satisfy that.  s can also be used to satisfy your second 
> requirement as it indicates to the load func what it's expected to load.
> The returned Schema is the form that the actual data returned by the 
> load func will take.  It must be a superset of the passed in Schema, 
> and really just exists to allow the load func to say it isn't going to 
> prune any of the data away at load time and just return everything 
> that it finds.  For load funcs that don't know the structure of their 
> data until they actually read it, they can return the * schema and 
> just wait until runtime to see if things blow up just like things work 
> currently.
> I think this makes more sense as a single function because the two 
> requirements are essentially the same operation.  To load enough of 
> the data to check a given schema against what's actually in the store 
> is almost the same work to determine what it'll actually load for 
> requirement two.
> Make more sense?
Let's work through a use case with the following script:

a = load 'mydata' using myloader();
b = filter a by $1 matches '.mysite.com';
c = group b by $0;
d = foreach c generate $0, SUM($1.$5);
store d into 'summeddata';

A post process step would figure out that the data loaded from 'myfile' 
needs to have at least 6 columns, column 2 needs to be a string, and 
column 6 needs to be int, long, float, or double.  It would then compose 
a schema with those slots filled in and call neededColumns, passing in 
that schema.  If myloader was of a type that it could push the 
projection down into the load, it would store this information for use 
later when actually loading data.  If myloader was loading some type of 
self describing data it would need in this same function call, to 
discover the schema of the data it is loading.  It would then check this 
against the passed in schema to assure it makes sense.  In addition, it 
would create an output schema that describes the data, and return that 
from neededSchema.  In the case where the data was not self describing, 
it would simply return a star schema (why not the schema passed in, 
since the data should match that or we'll get an error?).  Is that correct?

It still feels to me like you have one function doing two things.  One 
way or another, something needs to check that the schema derived from 
the post process analysis of the script matches the schema derived by 
investigating the data itself, and in your proposal that can be done in 
this function.

One way or another I think we agree on the needed functionality, 
interface concerns are probably secondary.   I need to update my type 
design doc to address how types are converted (at load time or lazily) 
and how the load function exposes that functionality.  I'll add this to 
the doc at the same time.


> Charlie

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